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
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.
Imperial unit: miles per gallon
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.
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
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