Think metric – don’t convert

The editor remembers this slogan from his time in the UK construction industry in the early 1970’s. Almost half a century later, Ronnie Cohen gives real-world examples of metric quantities that may help those who are not as familiar with metric units as they might wish.

Familiarity with metric units comes with usage. However, the continuing presence of imperial units in some areas discourages Britons from thinking clearly and consistently in a single measurement system.

“Think metric and don’t convert” is the central theme of UKMA’s ThinkMetric website, currently under review. In this article, we will look at how we can visualise metric quantities with real-world objects. For example, many Britons seem to have difficulty in making sense of their and others’ personal height and weight in metric units but seem to have no trouble understanding weather reports with temperatures in Celsius. Clearly, the predominant use of Celsius indicates that we can adapt to the use of metric units when given an opportunity to become familiar with them.


Object Approximate Length of Typical Item
Thickness of a compact disc 1 millimetre
Diameter of a shirt button 1 centimetre
Length of AA battery 5 centimetres
Diameter of a compact disc 12 centimetres
Size of A4 sheet of paper 20 x 30 centimetres
Height of a door knob from the ground 1 metre
Length of adult-sized bed 2 metres
Distance between start and finish line for a sprint race on athletics track 100 metres
Height of the Elisabeth Tower aka ‘Big Ben’ 92 metres
Central span of the Forth road bridge 1 kilometre
Length of Great Britain 1000 kilometres

Long distances are normally expressed in miles because that is what is used on British roads, even when the original source uses kilometres. Journalists fear that their audience will not understand distances expressed in kilometres. As long as distances and speeds are expressed in miles on British roads, Britons will continue to think in miles and shun relatively unfamiliar kilometres.

Mass or ‘weight’

Object Approximate Mass of Typical Item
Paper clip 1 gram
AA battery 25 grams
Bar of soap 100 grams
Small tub of margarine 250 grams
Regular bag of sugar 1 kilogram
Typical motor car tyre 7 kilograms

Britons normally express their personal weight in stones and pounds but use grams and kilograms for cooking recipes, weights in a gym, food products, DIY products and household goods. Yet when we express our personal weight in kilograms to other Britons, they will most likely ask us for conversions to stones and pounds. You can help them by refusing.


Speed Typical Example
1 m/s or 3.6 km/h Average human walking speed
20 km/h Easy cycling speed on a bicycle
50 km/h Cars on main roads in built-up areas
90 – 100 km/h Cars on motorways
200–300 km/h High speed train
360 km/h Racing car
1000 km/h Cruising speed of a passenger jet aircraft

The use of kilometres in the UK is relatively unfamiliar because of the use of miles for distance and speed on British roads. The continued use of imperial units on traffic signs and other public signage prevents Britons thinking consistently in metric units. That requires action from government to complete the changeover and banish imperial units from our roads.


Area Typical Example
1 square metre The area covered by a large umbrella
100 square metres Typical house or large flat
1 hectare Rugby pitch

The media rarely provide real-world examples of square metres and hectares, usually defaulting to imperial units.


Volume Typical Example
250 millilitres Mug of tea
750 millilitres Bottle of wine
9 litres Household bucket


And so?

Britons tend to use familiar measurements, typically those they have grown up with. The use of imperial units for personal weight and height is a typical example. But in addition to lack of familiarity, other causes of the UK’s measurement muddle may include inertia, habit, peer pressure, chauvinism (France invented it, but we do it better), hubris and wilful ignorance. Most countries around the world have made the change to a single, simple, logical and universal measurement system, in this process switching from the familiar to the unfamiliar. If we are as smart as we believe, then why can’t we? ‘Thinking metric’ will surely help.

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6 Responses to Think metric – don’t convert

  1. Martin Vlietstra says:

    A good article by Ronnie, but may I add two points:

    Firstly, driving using metric units on British motorways is not difficult. Driver location signs have been erected at 500 metre intervals along English (but not Scots, Welsh or N. Irish) motorways (See for more information). If you visit any Wikipedia article about English motorways, the location of each junction is catalogued in both metric and imperial units. If you check the location of your junction exit in kilometres before you start your journey, you can check your location from the next driver location sign.

    Secondly the suggestion that the French invented the metric system is a myth put out by the French and perpetuated by those Britons who wish to cling to their Luddite ways. If one looks at the early history of the metric system, one will see that apart from the period 1790 to 1800, British scientists took the lead in almost every aspect of the development of the metric system (only t be snookered by their own government), while the French Government, in providing the political basis for the metric system to exist, tried to air-brush British contributions from history. A very good letter in "The Philosophical Magazine" vol XXI, published in 1805 draws reader's attention to just this. In order to elaborate on this point, the International Prototype Kilogram which is due to be "pensioned off" next year was cast in England, while the Kibble Balance, an integral part of the new definition was also developed in England.

  2. Mary says:

    Martin's second paragraph above needs much wider publicity.
    " ... the French invented the metric system is a myth ... "
    I'm sure I when I was at school I was taught that the French invented the Metric System. This myth has sadly become an 'established fact' and is probably still taught in schools. If the national curriculum was carefully examined it might be discovered that students at the primary stage are told this. I'm not sure if the History of the Metric/SI System is covered in a few specialized syllabuses for GSCE & A- level exams - perhaps part of a module/unit.
    When teachers introduce students to using metric/SI units and symbols, I'm fairly confident in saying that the French connection will get mentioned without any Britons getting the credit.

  3. Charlie P says:

    It is not a myth Martin, it is a fact that the metric system was invented by the French. The system may have been developed and refined by others later in its life, and some of the concepts embodied into it may have been based on earlier works and ideas by others, but the first true working version of the metric system was that designed, developed and implemented in France at the end of the 18th century. All later evolutions of it were derived from that French original.

  4. Daniel Jackson says:

    Whether the French may or may not have created the metric system is a moot point. The French did create the system of prefixes and pushed it out on the world. When Napoleon reverted to Mesures Usuelles in 1812 it looked like the metric system was dead forever, but in 1837, the French decided to abandon Mesures Usuelles and in 1840 the metric system became official for all people for all time. After that it spread around the world.

    If the metric system had been created in England it would not have had prefixes and would look like the system John Wilkins proposed or what the English were using prior to the Norman invasion. There would have been a rebellion against the new system then as there is against the metric system today among some groups and a reversion back to the previous collection of units. They would not as the French did try it again a generation later.

    In addition to Martin's comment about Driver Location Signs, there are also a large amount of people using GPS systems. There is nothing stopping anyone from ignoring the road signs and setting their GPS to metric mode. What we don't know is how many of them already do. One way to end the use of imperial on GPS systems would be for a system upgrade in which imperial was removed from all GPS systems nationwide. With only metric on the GPS, everyone would get use to metric distances in a matter of a few days.

    The same is true for mass and height. Get rid of the instruments that still include imperial units. Don't sell tape measures and scales with dual or imperial units. Metric only. Eventually the home units will wear out and need to be replaced. It may take a few years but eventually this area will succumb too.

    In addition, there has to be better teaching and encouragement to use metric in the schools. Such that the students should put pressure on the family to use metric, not the other way around. The schools should introduce a program where the students identify non-metric use in the home and teach other family members still resistant the workings of SI with practical applications.

  5. Jake says:

    Whether the metric system was 'invented' in England or France probably turns on your definition of 'invented'. Certainly, the first practical proposals for a decimal-based system of weights and measures seem to have been made by Bishop Wilkins in Chester in 1668. The French then seem to have taken up this idea and developed such a system ('developed', not 'invented') and British scientists have made significant contributions over the years in between. Six metric units are named after British scientists, more than from any other country. So what does 'invented' mean and what does 'developed' mean? The fact that today's modern metric system is based on scientific principles which are continually being refined makes the argument over the actual original 'invention' of the system rather academic, in my view, if not obsolete. It is the world standard system of measurement. Even 'imperial' and American 'customary' units are defined by reference to the metric system, though that does not makes those units metric, of course. Egyptian cubits could also be defined by reference to modern metric, but I cannot see a future for those units except in reference to the building of the pyramids. The point is do we need a plethora of competing units in a modern world which is becoming increasingly globalised and 'one' whether you happen to like that or not. If we do leave the EU and want to strike out as a 'global player' in our own right, we are going to need more metric on an everyday basis because 95% of the world is metric. That is the case for the UK to upgrade its system of measurement in all respects and to become accustomed to using it.

  6. Martin Vlietstra says:

    @CharlieP: I agree that the metric system as we know it today derived from the metre and kilogram as defined by the French in the 1790's, but the French cannot clain credit for the preparatory work. In particular:

    * In 1620 Gunter developed and used a decimal based system for measuring length and area - there are 100 links in a chain and 10 chains in a furlong. Gunter also published sets of what we would call "trigonometric tables".

    * In 1662 Sir Christopher Wren proposed using Huygens' seconds pendulum as a standard for units of length. The French claim that ths proposal was made by Picard in 1771 (See

    * In 1668 Wilkins published a proposal for a system of weights and measures. I am not aware of any earlier proposals for an entire system - length and weight were always dealt with in isolation. The philosopher John Locke worked closely with him.

    * In 1670 Mouton published his proposal that the unit of length should be based on one degree of longitude measured on the equator.

    * In 1671 Picard proposed using a seconds pendulum as the standard for the unit of length.

    * In 1673 Huygens published his Horologium oscillatorium in which he described refinements to his pendulum and also discussed how it could be used as an international standard.

    In parallel to this, Ole Rømer (a Dane), worked closely with Picard between 1671 and 1681 and Locke spent the years 1675 to 1679 in France where he almost certainly communicated with Rømer. At the same time Locke introduced the concept of the "philosophical foot" which was one third of the length of the seconds pendulum and he also had a brass ruler made according to the philosophical foot. Shortly before he returned to England (accompanied by Rømer), he visited Picard.

    In short, there appears to have been considerable interchange of ideas between the English and the French philosophers of the day with the English having contributed most of basic ideas. The French actually implemented them after 1789 only because their pre-revolutionary system of weights and measures had become a complete mess.


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