Imperial in 2015

It is almost 50 years since the announcement in Parliament that the UK would be ‘going metric’, and 15 years since significant progress last occurred. Although most Imperial units have passed into history, a few have survived in common use. Here, we offer a Guide to those surviving units for readers familiar only with metric. And for Americans, who may feel at home with Imperial measures, we provide some warnings.

When countries convert to the metric system, it is usual to find ‘legacy’ units in use alongside metric for a period of several years. In the UK, several years have become fifty. This article aims to draw attention to situations where Imperial measures may still be encountered.

We hope that those who find the Guide useful will include:

  • Visitors to the UK
  • Foreign readers of UK newspapers and magazines
  • Listeners to the BBC World Service
  • Viewers of UK TV programmes abroad through satellite, cable or the internet

It is worth noting that over 95% of the population of the world now live in countries using the metric system as their primary system of measurement.

In the UK, over half the population has been taught in metric throughout their school careers, and has had to pick up Imperial “on the go”. Some of these may also find parts of the Guide helpful.

When there is a significant difference between a surviving Imperial unit and its American counterpart, the Guide provides a warning.

Approximate conversions are given for those who, wisely, prefer to “think metric”, and at the end of the Guide we provide more accurate conversion factors for reference.

Readers’ comments and suggestions are, of course, welcome


The MV Guide to Imperial units in 2015

The Guide looks in turn at each of the areas where Imperial units are likely to be encountered in the UK, provides information on the units, and concludes with a comment.

Length, height and altitude
Imperial units:                           foot and inch
Abbreviations/symbols:           ft  or  ‘   and   in  or  “
Approximate equivalents:       1 foot is about 300 mm
.                                                     1 inch is about 25 mm
These units are used in conversation for human body height, and also appear on road traffic signs that show height, width and length restrictions (it is proposed that new restriction signs should always include metric measures; older signs sometimes do not). There are twelve inches in a foot, so those who are unfamiliar with duodecimal arithmetic are advised to convert measurements to either decimal feet or inches before attempting calculations.

Body mass  (customarily ‘weight’)
Imperial units:                           stone and pound
Abbreviations:                            st  and  lb
Approximate equivalents:       1 stone is 6.35 kg
.                                                     1 pound is about 450 g
These are often preferred to metric units for human body weight, although, as with height, metric units are used for medical purposes. There are 14 pounds in a stone, so if calculation is required involving stones and pounds it may be sensible to convert all to pounds first.

Draught beer and cider
Imperial unit:                             pint
Abbreviation/symbol:              pt
Approximate equivalent:         1 pint is about 570 mL
Though appearing to be a measurement unit, the pint has now become a size number like those for clothes. As a primary unit, it may only be used for selling draught beer and cider, and then only in the following sizes: 1/3, ½, 2/3 and multiples of half a pint. “Going for a pint” has come to mean going to a pub or bar, where wine, spirits and soft drinks are actually priced in metric.

Land and floor area
Imperial units:                           acre and square foot
Abbreviation:                             sq ft (the acre has no recognized abbreviation)
Approximate equivalents:       1 acre is about 4000 m²
.                                                     1 sq ft is about 0.093 m²
The acre is favoured when describing residential properties on large plots. It has an advantage that few are sure how large it is. The square foot is used, similarly, to describe offices available for letting. Residential sales often use an undefined unit, “the bedroom”, so prospective purchasers are advised to check the EPC which gives the floor area in square metres.

Distance and speed
Imperial units:                          mile, yard and miles per hour
Abbreviations:                           m, yd and mph
Approximate equivalents:       1 mile is about 1.6 km
.                                                     1 yard is about 900 mm
.                                                     50 mph is about 80 km/h
Miles, yards and numbers showing speed limits in miles per hour appear on road traffic signs, where “m” may mean mile or metre depending on context. It is common to hear miles in conversation to describe distance, and the Met Office uses mph for wind speeds in forecasts. There are 1760 yards in a mile, so mental arithmetic involving a mixture of yards and miles is best avoided.

Fuel consumption
Imperial unit:                             miles per gallon
Abbreviation:                             mpg
Approximate equivalent:         mpg x fuel consumption in L/100 km = 282
This is primarily a measure of comparative performance – it is unsatisfactory for calculating fuel consumption, since fuel is sold by the litre.

Energy, power and refrigeration
Imperial units:                           British thermal unit, horsepower
Abbreviation:                             Btu, hp
Approximate equivalents:       1 Btu is about 1 kJ
.                                                     1 hp is about 750 W
The watt has become almost universal for describing the power of heating equipment. Paradoxically, US influence results in the output of refrigeration plant and air conditioning sometimes being described in Btu/h. And, in common with the US, the horse remains a popular point of reference in Britain for vehicle performance.

Warnings for Americans

Do not be surprised if you have never heard of the stone – it is used only in the UK.
For conversion to pounds, see above.

Outside literature and museums, ‘ton’ and ‘tonne’ have become interchangeable in the UK and mean 1000 kg, not to be confused with the US ton of 907 kg.

The Imperial quart is history. The Imperial pint is 568 mL. The US pint is 473 mL. If you measure your consumption of draught beer by the pint, you will get drunk faster in the UK. And if you measure it by the unit, you will just get confused, as a unit in the UK in 10 mL of pure alcohol whereas it is about 15 mL in the US.

Americans normally encounter the yard only on the football pitch. Be prepared to find it on road traffic signs, warning you of imminent danger ahead.

Fuel consumption
The Imperial gallon is 20% larger than the US gallon. Rest assured, the variation in fuel consumption when measured in mpg between identical vehicles in the UK and the US is due to the quantity not the quality of the fuel.

Celsius and its predecessor centigrade have been around for so long that the word “Celsius” is sometimes dropped when quoting temperatures. If you hear “30 degrees” think twice – it is likely you will need a sun hat not an overcoat.


Conversion factors printed in bold are exact and others are given to four significant figures.

Length, height, altitude and distance
1 mile              = 1.609 km                               1 km                = 0.621 4 mile
1 yard              = 0.914 4 m                            1 m                  = 1.094 yard
1 foot               = 0.304 8 m                                                    = 3.281 feet
1 inch              = 25.4 mm                                                        = 39.37 inches

1 acre              = 4047 m²                                1 hectare (ha)  = 2.471 acres
.                        = 0.404 7 ha
1 sq ft              = 0.092 9 m²                           1 m²                  = 10.76 sq ft

1 pint               = 0.568 261 2 L                    1 L                    = 1.760 pint

1 pound           = 0.453 592 37 kg               1 kg                 = 2.205 lb
1 stone             = 6.350 kg                                                        = 0.1575 stone

Fuel consumption
1 mpg              = 0.3540 km/L                        1 km/L           = 2.825 mpg

1 Btu                = 1.055 kJ                                 1 kJ                  = 0.9478 Btu
1 hp                  = 745.7 W                                 1 kW                 = 1.341 hp

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24 Responses to Imperial in 2015

  1. Charlie says:

    if only there were a logical way we could get rid of the need for any kind of conversion factors except simple powers of 10...

    but seeing as this guide is, unfortunately, needed (especially as I'm 21) - this is really useful for units I never actually encountered at school such as the acre and whatever a BTU is supposed to be!

  2. John Steele says:

    Good guide.

    A few thoughts for consideration:

    Since you warn about the larger pint (of beer), it might be worth noting that the UK unit of alcohol is 10 mL of pure alcohol, while the US unit (which is less used) is 0.5 US fl oz, about 15 mL.

    In the area conversions, the conversion for 10 000 m² is correct but looks odd. You might want to note somehow that it is also 1 ha.

    It might be worth mentioning the Fahrenheit in summer, Celsius in winter syndrome, and a conversion.

    (John. Thanks for this. I have amended the Guide. Editor.)

  3. Jake says:

    While it is true to say that only a few Imperial measurement units have survived in common use, the all-pervading use of these units on road signs up and down the length of Britain means that a visitor could be forgiven for thinking that Britain had not embraced metric at all. Although new bridge height and width restrictions are now required to include metric units, Imperial units must still be displayed too, with a sign being either dual metric/Imperial or with two separate signs, one for metric and one for Imperial. Even that progress, i.e. the inclusion of restrictions in metric units, has only come about as a result of bridge strikes and the perceived need for 'foreign' drivers to have metric information, not because metric units are the normal units of measurement taught and used in maths and science in UK schools, i.e. learnt by the youngsters who will become British drivers, and thus the country's primary units of measurement. There are still plenty of old road signs in place with nothing but feet and inches on them and these are destined to remain in place until replacement, which in many cases may be years away. How can it be in a modern, forward-looking country like Britain in the 21st century that the metric measurement units taught in schools are not used as a matter of course on road signs?

  4. Arty says:

    Interesting as this blog may be from an historical perspective, I otherwise believe that its effects are more negative than positive. It may be useful for visitors to the UK, but as a “guide” for the resident population it only serves to provide a crutch and as such is a barrier to metrication, as are dual unit markings relating to signage and goods.

    My view is that metrication, as per decimalisation of UK currency, should have been total and absolute from a given date. As for the present, for the over half the population educated in metric units, conversion between the archaic and the current should be irrelevant. For those of us who were adolescents or adults in 1965, we were well aware what was coming over the hill at us (remember the “m” logo of the metrication board?) and should have had the self motivation and reliance to educate ourselves in the new system.

    “There’s none as blind as those that don’t want to see”.

  5. Tim Bentley says:

    It's also worth mentioning milk. Standard milk is mostly sold in pints except in independent convenience stores where they moved to litres many years ago. In supermarkets, they tend to sell all their skimmed, organic, soya, goats milk in litres but semi-skimmed and whole milk in pints. This makes price comparison very difficult. It is worth noting that wholesale milk has been sold by farmers in litres for about 40 years.

  6. John Frewen-Lord says:

    I've made this comment before, but I wonder if the British population, in general, are not way ahead of the politicians in terms of metrication in the UK. I've recently been renovating a buy-to-let house, with a number of hired in workmen and trades, as well as my own hands-on efforts. All the trades and workmen, almost without exception (occasional relapse) used metric measurements when setting out work and talking measurements in general. Likewise, ALL materials (not a single exception) have been bought in exclusively metric quantities.

    The new kitchen was a particularly relevant example. Kitchens have of course been metric for some years, but this was brought home when I and two hired in workmen were setting it all out. The units came in modules of 300, 400, 500 and 600 mm, and all conversations were based on that - e.g. "Would the 600 space for the washing machine work best at the very end, or shall we put a 300 unit at the end, and the washing machine space next to that?".

    But are (uncalled for) expectations for imperial units part of the problem? I had to visit one of my tenants where the property had a repair I had to take care of. The wife showed me their new baby, premature, and the baby had recently been released from hospital. I asked how much it weighed. Six pounds 3 ounces, said the wife. Interestingly, the husband immediately chimed in with, "Actually, it was 2.852 kilos when we left the hospital two days ago. ******* [wife's name] converted it to imperial. Don't know whether she got it right." "Kilos is fine for me," I replied. "Oh, I wouldn't have expected that," said the wife.

    I do believe that a majority of people in the UK can understand, and are probably ready for, the completion of the conversion to metric. I think the politicians, in the (mistaken?) belief that we are not, are prolonging the unnecessary retention of imperial units in British life (road signs being the glaring example). To that extent, I have to agree with Arty, in that the conversion factors shown in this MV article are doing more harm than good. Time to get it all over with, and forget imperial.

  7. philh says:

    Although well written and useful for the intended audience, the guide shows just how incompatible metric and imperial units are and how awkward it can be to convert.

    We don't need this nonsense and in spite of what some claim, the metric system enables a much better understanding of measurement, provided it is used exclusively throughout everyday applications.

  8. BrianAC says:

    @John Frewen-Lord. Just about spot on.

    I would say though that the media are far more to blame than the politicians, although the prime minister saying on TV that he thinks we should use Imperial, for sure does not help. This alone perpetuates the assumption that 'we' all expect and understand Imperial.

    I also have said before, that I know and understand more about the Imperial system from reading these pages than I have ever known, needed or used at any time in my life, private or professional. That includes working for American companies. That does not please me one bit. So what does it do for those that resist change? They simply convert from metric to Imperial. If these conversions were not known, and indeed NOT TAUGHT (in schools), then life would be so much easier for all.

    We are however, still looking for a solution to what is now so entrenched in our lives that neither side knows what to do.

    Even if the roads went 100% metric today, it would not make a lot of difference, if the media continued to spout Imperial in every programme. Everything would be in miles for another 100 years, may be even a century, decades for sure. Funny how we can use decades, centuries, millennia, even 3G, 2k, 4k (new TV's) in everyday life but 'people do not understand' the metric system! Herein is the problem, 'system'. It seems 'Jo public' cannot accept their babies being weighed in the same units as potatoes and the car. Petrol heads cannot accept their engines measured in the same units as an electric fire or the central heating boiler, too easy to see how much power they are using (180 kW car engine = 10 central heating boilers on full power,) how many would enjoy 241 km/h up the M1 with that fact in their head? It makes the engine look smaller too.

    The very fact that supermarkets still 'bottle' milk in pints to sell as litres, and estate agents advertise in square feet and acres to sell in square metres and hectares, car manufacturers build-in mpg, when the Imperial gallon is used nowhere in the whole world, sets the stupidity of the situation. So ingrained, that change is always going to be painfully slow. Media and consumer groups still use price per pint for milk, (that one caught me out in a survey, they asked price per pint, I (of course) used the 'unit price' per litre, was I the stupid one? Read the question stupid boy!) And oddly even price per gallon for petrol. Roll on a nice hot summer so they can go back to degrees F (I still can't spell it, and no, I do not need anyone to educate me).

  9. Alex says:

    The pint is a problem that just keeps giving...

    Milk, outside of doorstep deliveries, should not be in pints but large retailers skirt around the law to sell milk in pints while still labelling them legally in metric.

    As to other drinks... if only draft beer is supposed to be legal for sale in pints then something is woefully wrong. Many establishments such as pubs, cafes and restaurants still sell soft drinks in pints or fluid ounces; I was sold a "pint of lemonade" as recently as Saturday in a Frankie and Benny's in Nottingham and recently had a soft drink at a burger bar in Solihull where, although sold as "S/M/L" the cups (likely imported from the US) were labelled in FL OZ.

    On the upside, if you see the word "yard" on a road sign then increasingly this could be taken as actually meaning "metre" in all but name.

  10. Jake says:

    Is the sale of lemonade (or any other fizzy drink) from a dispenser actually regulated? I have certainly been sold a 'pint'of shandy in the past. I have the impression that (again) this may be a grey area or one where inspectors would not take much interest, on the basis that if the customer gets what he or she is expecting, then no offence has been committed.

    The use of FL OZ on drinks containers is definitely a problem, in my view. The Imperial version of these units is no longer legal for trade and the US version (which is not the same as the defunct Imperial unit) is not legal unit for trade. I have often wondered how the establishments concerned get away with displaying these units on their containers without being challenged. (But then some of them seem to get away with quite a lot of other things too, as we know.)

    As for 'yards' meaning 'metres', well, we know they do not legally mean the same thing. But I am convinced that when speaking English some foreigners use the word 'yard' thinking it is the English for 'metre'. They could be forgiven for thinking so. Given the closeness between yards and metres and the fact that distances measured out by highway engineers in metres are signposted as the same number of yards (the total irony and an insult to Brits, in my mind, since metric has been taught in schools for forty years), there is absolutely no reason at all for not switching to displaying those numbers as metres (which is what they are).

  11. Tim Bentley says:

    With the price of milk at a record low this would be the perfect time to convert to litre sized containers; 500mL, 1L, 2L and 3L. Whilst not condoning down-sizing , assuming the prices of the slightly smaller litre containers remained the same as the roughly equivalent pint containers, the big supermarkets could perhaps pass on the increased profit to hard pressed dairy farmers. I don't believe the argument that switching to litre sized containers would decrease consumption. I think most consumers buy milk as and when they need to. There is also another advantage of using metric milk containers and that is because they fit in fridge doors better and being about 10 percent lighter are also easier and safer for elderly people to handle.

  12. BrianAC says:

    Further to the post by Tim Bentley 2015-03-02 at 19:32
    Industry would not need to make two different ranges of milk bottle sizes, and there would not need to be two different bottling plants either (assuming there will be a common link somewhere in the production chain). The same goes for the production of beer glasses (by the French), with a special range just for the UK. Not to mention duplication of all forms of measuring devices, one range for UK, another for USA (with the UK I notice being lumbered with US measurements which no-one can understand) and one for the rest of the world.
    To say this has no cost implications would be naive, UK, on its own, is now a relatively small market on a global scale, and economy of scale is everything.

  13. Alex says:

    Adding to the post by BrianAC a lot of this is a question of cost.

    We're often told by the nay-sayers that the cost of conversion is too high but nobody ever thinks about the ongoing cost of retaining two systems; nobody seems interested in quantifying the cost of continuing to manufacture a separate range of car speedos, bathroom and kitchen scales, measuring jugs, rulers and tape measures, conversion charts, shipping containers for retail, speed measuring devices (radar guns and speed cameras)... I'm sure there's a lot more. Granted some of this is now done by software but even that has costs and it all adds up. Even time spent in the classroom with primary school children or doctors/nurses having to switch between kg and stones/lb/oz for their patients has an underlying cost that nobody is counting and all of this is passed on to the consumer and the tax payer.

    And how many people have noticed that UK motor vehicle regulations no longer seem to require cars made for the UK market to have dual mile/km speedos? Or the amount of money spent on electronic speed limit signs that are physically incapable of displaying any more than two digits when otherwise a simple software upgrade could have made them metric-capable? Or the continuing practice of installing road signs using "m" to mean miles? One could be mistaken into believing that, in some cases, some groups are actively working to ensure that a final switch over will be as complicated and expensive as possible, actively creating reasons for consumers and businesses to have a cause for complaint.

  14. John Frewen-Lord says:


    I have posted one or more articles here on MV regarding the costs of both not converting (specifically education, lost international business, the extra time using imperial measurements, including a metric-vs-imperial question-and-answer article), as well as the cost of completing conversion (specifically road signs). I don't know off hand the links to these articles, but I expect a search would find them.

  15. Alex says:

    @John: I'll concede that I've seen several such articles here, the real problem is that when the mainstream press or politicians start to spout the all too common "it's too expensive" line the argument against this is seemingly ignored and the evidence is seemingly not presented (or perhaps is just not reported or given a fair airing because of vested interests or short term thinking).

  16. Martin Vlietstra says:

    While the use of fluid ounces might be ignored by W&M inspectors (US fluid ounces are slightly larger than British fluid ounces)m the use of pints is a legal minefield for the retailer – if they are using US fluid pints (16 US floz per US pint), then they are short-changing their customers – the UK pint (20 UK floz per UK pint) is some 20% larger than the US equivalent.

  17. derekp says:

    In the RTS lecture on BBC4 last night, Michael Dobbs said the BBC aims to double its global audience over the next 7 years to "half a billion". This could be made easier if the BBC changes it policy, or rather lack of a policy, on measurement units.

  18. Alex says:

    Well I think I've seen it all now... I needed some tap washers this week. Despite the fact that the building trade has been metric since the 1970s I do know that plumbing can be a grey area but I didn't know it was quite as bad.

    Checking on the B&Q and Wickes web sites you find washers specified in metric, but on the shop shelves the packaging are exclusively in fractions of inches.

    What really made my day though was the text on the Wickes website regarding the size. For instance, the 12 mm tap washer, in the product description, states "Actual size: 1/2in". Is 12 mm not a real size?

  19. Ezra Steinberg says:

    As a point of reference and historical interest here are a couple of stories about how conversion from Imperial to metric went initially in Canada (with a focus on Celsius vs Fahrenheit):

  20. John Frewen-Lord says:


    I was living in Canada at that time, and I remember the changeover so well. Personally, I welcomed it, although many didn't.

    There was an interesting comment made by Dave Philips in that interview - "Why learn something new when you don't have to?". That of course sums up why the voluntary approach to metric conversion, which Americans have rigidly stuck to, does not work. Like medicine, switching to the metric system may not taste good at the time, but if you are forced to take it, you come to realise that the benefits outweigh any discomfort over the long haul.

  21. BrianAC says:

    Searching for sensible kitchen items, I notice kitchen measuring jugs are now almost all in US measures, even in Argos.
    I wonder how many in UK realize the difference between US and UK fluid ounces and if they get 'funny results' by using the wrong ounces.
    The other one that comes up often is USC inch pounds, how many have wondered why that does not match the torque figures seen in UK data as foot pounds.
    Add to this the fast food outlets and US customary is probably used more in UK than Imperial! This is inevitable as UK officially not longer uses Imperial and thus, I believe, there can be no legal control.
    As 'they' all say, use whatever measures you want to, there is the end result.

  22. BrianAC says:

    I have just been reminded of a mountaineering article I read recently, the 'Archie Challenge.'
    For a long time now I have been wondering why all the climbing hype was always in feet, when just about everything to do with outside activities are metric.
    Well, at last it is here .
    At last someone has come up with something different. There are 130 peaks of over 1000m, in Scotland, two nice round numbers. The challenge was to climb them all in something like 4 weeks, for charity.
    True there has been the predictable lack of media interest in anything metric, but at least it is here in real life.
    I hope the Archies catch on!

  23. Brian: Since you persist in repeating this tired, irrelevant bit of pro-metric propaganda, I will assign relevance more appropriate to this topic. A search of the term 'Scottish Archies' on a popular search engine yielded a large number - TEN! - of hits, of which only six were relevant and of those one was a PR push by UKMA.

    So, outside of the UKMA, and this pro-metric advocate's own self-promotion, just what relevance does the term 'Archie' have for anypone? Where is it in common use? What [legitimate] news publication has given it coverage? Where has this classification been adopted in climbing associations? Who holds current records at this height? Weren't you the one criticising a petition with 312 sognatures? Yet we now see a petition with only seven signatures [mouse clicks, strenuous, time consuming, legally binding] and a 'classification' with only six hits. Were I to bring six mates to a pub and, over pints, concoct a new Imperial measure or classification, should I report it on the A.R.M. site, page, feed?

  24. BrianAC says:

    I find this post an offence against my right to express a personal opinion and promote an activity that I like.
    So, you say I am not allowed to express a liking for a certain activity because it does not have widespread media coverage. On what moral grounds of freedom do you base that?
    Media (FB,Twitter etc) coverage comes from people liking something and re-posting it, that then is not acceptable if it is something you do not agree with, and so long as it is not a private individual showing a preference for something you do not approve of.
    That is offensive. In any case I have no knowledge of me re-posting this anywhere. So what if I had? I don't see how that makes it 'propaganda'. It was a factual post of something that happened (well, I was not there, so maybe it did not happen, I don't know) I have no way of knowing for sure.
    I said in the post to which you refer that it had a 'predictable lack of media interest', so what are you adding to that? Does the lack of media interest make it illegal for me to talk about it? Personally I think not.
    Frankly I find you repulsive and insulting. This pro-metric advocate’s own self-promotion of an article in his own self interest is just that, like it or not. So what, is that illegal? It was a charity fund raising effort, is that offensive to you?
    The rest I do not understand and not trying to. I have no knowledge of any UKMA involvement in this, so cannot comment, and do not see what that has to do with this post. I have made no mention of this being in any climbing classification of any sort, nor even my wish that it could be. Personal freedom of wishful thinking, is that a crime also?
    I do not do pubs and I do not do pints, but you are free to suggest any new measures you like, they all start from one person's idea. Many that did were put to death, fortunately that does not happen these days.

    UKMA did post it so I guess it was within their rules.


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