cm versus mm

As the UK’s metric changeover drags on with no end in sight, we consider if the advantages of encouraging widespread use of the centimetre outweigh the risks.

UKMA recently received this message through the contact form on its website, :

“Dear Sirs,

More and more I am concerned to see the widespread use of cm as a measure of dimension, by manufacturers and even by the BBC for depth of snow for example, instead of mm, being the SI unit of length, which was agreed and adopted.

Unless something is done to address and halt this practice, I can see the mm disappearing from common use altogether, other than by engineers and scientists …”

The Secretary of UKMA replied as follows:

“Dear …

Thanks for your e-mail.

Anne Attlee (daughter-in-law of Clement Attlee, who was PM from 1945 to 1951) ran a campaign from the 1980s onwards entitled “Metric sense”. This was in response to populist pressures, then as now, holding up the UK’s metric changeover. “Metric sense” argued that the British require measures that are familiar in scale to their Imperial equivalents. In most instances, this was easy: yard-metre, pound-kilogram, ton-tonne. For energy, it was accepted that we would continue to use the kWh rather than change to the joule. And for the inch, “Metric sense” suggested the centimetre would make for an easier transition.

I recently encountered some of the resulting problems and wrote an article on about them which was posted on Metric Views in January:

As I mention in the article, I worked in the construction industry from 1965 until my retirement in 2000, covering the period of its metric changeover. A decision was taken early on to use multiples of 1000 whenever possible, and I do not recall a single instance during the transition period when this caused me a problem. Clearly, it was the right decision, and my recent experience of working with my sons in France reinforces this view. But I do not think it was intended that it should apply in general to the wider economy.

As far as I know, SI does not discourage the use of the prefix ‘c’, and it is indeed listed in Table 5 of the SI brochure. Another common example of its use is cL. The Met Office advises that mm should be used for rainfall and cm for snow. Centimetres are normally used for human height, for example at the doctor’s surgery and on fairground rides.

The UK was one of the first countries in the world to adopt SI. The problem we face now, if we are to prosper in a metric world, is to ensure people are more familiar with SI measures than with Imperial. Perhaps popular use of the centimetre, and the resulting risk of confusion, is a small price to pay.

Yous sincerely

Secretary UKMA”

Readers’ views are welcome on this trade off between, on the one hand, the ease of persuading a public familiar with the inch to use a unit similar in magnitude, namely the centimetre, and on the other hand the resulting risk of confusion and costly mistakes.

As there are now around 200 countries around the world that use the metric system as their primary system of measurement, we in the UK are surely not the first to face this dilemma. In particular, many Commonwealth countries that formerly used the inch have now “gone fully metric”. How did they proceed?

20 thoughts on “cm versus mm”

  1. While I agree with the reasons for favoring millimeters over centimeters in many instances (and readers familiar with the writings of the Australian champion of metric, Pat Naughtin, will be familiar with many of those arguments), I would also agree that making even some headway amongst the British populace with regards to metric usage is better than little or no progress at all (as seems to be the case thus far).

    So, if promoting the use of centimeters for height and similar lengths in everyday usage helps displace inches and feet, I am all for it.

    The prevalence of Imperial even today in everyday usage seems to be quite strong. My wife and I are watching a BBC detective series called “Scott and Bailey” (which we like much more than most American detective shows). I confess I am quite chagrined to see that all references to lengths, distance, etc. are given in the show’s dialog exclusively in Imperial.

    If that show properly reflects current everyday usage in the UK, then I fully favor bending any preferences for best practices regarding metric usage if it serves to displace Imperial in any way possible.

    As the old saying goes: “Don’t make perfect the enemy of the good.”


  2. I’ve just looked up a random television on Dimensions are given in centimetres and kilograms. Even the screen size is given in centimetres (followed in brackets by its size in inches).
    I think the fuss over whether we should stick to the preferred prefixes is symptomatic of a country that has not properly switched to metric and become comfortable with it. I’m comfortable with both centimetres and millimetres, but only tend to use millimetres for things that need to be accurately measured. So I give my height in a way that can be easily interpreted in either metres or centimetres (eg “one eighty two”), but I measure bicycle rim thicknesses in millimetres (because 0.1mm is the difference between usable and dangerous).


  3. Throughout the Alps, snow depths are very often, if not always, given in centimetres. I have no problem with this, indeed it would seem strange to me if these depths were expressed in millimetres, it would almost be like ‘over-precision’, because obviously the snow depth will not be ‘exactly’ so-many centimetres. But more importantly, I have always assumed that centimetres are used because skiers can easily relate the snow depth to their own height, which they will know in centimetres. Even if someone tends to think of their height as, say, 1.75 m rather than 175 cm, the relationship between personal height and snow depth is still perfectly obvious. People do not tend to think of personal height in millimetres, so that may explain why centimetres are used in snow reports.


  4. In Cainz Home, my local hardware store out here in the Japan Alps, I noticed timber marked “4.5mm x 19.0 x 12 feet”. I’m not making this up. Presumably this is timber imported from the US and the store muppets lack the skills and or confidence to convert.


  5. All of the prefixes should be encouraged where they apply as a means to eliminate as much decimal dust as possible, the use of counting words and large amounts of zeros. Centimetres are fine as long as the value sticks to whole numbers and the numbers are in a suitable range. There should never be a dimension of 33.8 cm, instead it must be 338 mm. 20 cm of snow is fine, but not 23.3 cm.

    The same is true for large numbers. We should get use to the sun to earth distance being 150 Gm, not 150 million km or 150 000 000 km. Light years need to be replaced by terametres, petametres, exametres, zettametres, yottametres, whichever applies. Andromeda is 24 Zm from the earth and the observable universe has a diameter of 880 Ym. All of the large masses in space can be measured using one of the existing prefixes. The total mass of the observable universe would however need to be expressed in scientific notation (3 x 10^55 g).

    The metric system (SI) is not taught correctly in schools. It is taught in the spirit of “Metric Sense” to resemble former systems like imperial and USC. No, it must be taught to take advantage of all of the prefixes in simplifying numbers. Only then can the SI be used to its full advantage.

    Teaching SI as just a different system whose format is no different from imperial or USC is what is holding back the completion of metrication. Using SI like one uses imperial or USC limits the advantages of using SI over these older collection of units. SI’s full potential is only realised when the system is taught and used to its fullest.


  6. @Daniel Jackson

    I wholeheartedly agree with Daniel. That said, we may need intermediate steps even if it is a bit of a hybrid with some vestiges of Imperial ways of expressing quantities “contaminating” best practice with SI in order to move the ball down the field.

    One abomination that should always be shunned, though, is an expression like 1 m 75 cm. Ugh! That really sticks a dagger into SI since it is barely disguised X ft Y in from Imperial usage.


  7. @ Daniel Jackson

    Even in countries that are not struggling to oust pre-metric units, I think you would have difficulty finding people who understand the SI prefixes for some of the orders of magnitude you mention. Many of those really are for very extremely high figures that the ordinary mortal does not normally need to concern him or herself with. The problem in the UK is not only the remnant pre-metric units that are still widely used, but also the invented units so beloved of the media: the lengths of football fields, the height of Nelson’s column (both of which were used by the BBC to describe length and height in its programme on the warship Queen Elizabeth), the size of Wales, the volumes of Olympic swimming pools. I don’t know what it is about the ordinary Briton that they cannot (or are not considered to be able to) grasp the height or length of an object when it is described in the metres most of them have been taught at school. My conclusion is that metric is taught as an abstract, not as something that relates to life outside the school gate. I believe it is essential to get the absolute basics of SI in place and in official use across the board, especially on road signs, before trying to use what I would call the ‘higher registers’. As a non-scientific person myself, I am perfectly happy to be told distances in thousands or millions of kilometres as the km is a very basic unit to which I relate from the metric world around me. Anything but miles – or lengths of football fields!


  8. @Daniel Jackson – I disagree with you regarding the usage of light years and other distance-time vectors in astronomy,as they serve a dual purpose imo, of conveying two important ideas – the sheer scale of the Universe and the counter intuitive idea that we are always looking at the past, as it takes even light time to travel distance x (even on human scales, we’re always seeing the past).

    @ Jake I do believe the Media Measurement System is a standard developed over enough booze to fill three Olympic size swimming pools 🙂 As an aside, I think they are taught to include such things in journalism schools to provide a frame of reference, to the reader, in case they hadn’t already grasped the fact that either something is big, whopping or humongous.

    Finally, as to the premise of the article, purism is the enemy of progress. And let’s face it, progress in the UK has gone from glacial to catatonic in the last two decades ever since the then Tory Government got a bashing over the standardisation of weights and measures at the retail level.

    I don’t know if the use of cm’s will speed things up at a governmental level (probably not, as metric isn’t seen as a vote winner) and there’s a certain stubborn streak about cleaving to the older way of doing things at a colloquial level, but it sure as hell can’t hurt.


  9. @Bodrules

    As far as Astronomy goes, there is a chance the light-year and parsec maybe in their last days or years. A few years back, problems in the Astrological unit for the Astronomy Community forced it to redefine the unit as an exact number of metres. It was defined exactly as 149 597 870 700 m since 2012.

    Now there is a call to bring astronomy into line with other sciences and adopt SI units exclusively.

    Metric Maven covered the topic in this blog:

    The strict use of metres with the proper prefix makes the numbers come out clean and organised. Intuitiveness is relative. It’s all based on what you learned as a child or have relearned as an adult.

    Whether or not you like the status quo or prefer counting units to proper usage of SI, SI proper is the only way SI should and must be taught. If you choose afterwards to delve into a different format, then that is your choice and a choice you will have to deal with. Purism is not the enemy of progress. Purism is the foundation of standards and standards is what makes science, engineering and economy run smooth and advance. Truth and Right are pure and corrupting them slows progress. Ludditism is the enemy of progress. Clinging to old, incoherent and inconsistent practices is what causes confusion and stifles progress. Technical progress only came about after the metric system came into existence, and has advanced as the SI has advanced. SI must remain pure and disciplined if it is to remain respected.

    SI is in a state of constant evolution. Am evolution towards full perfection, and not a devolution into chaos. That is the theme of the year (2018) for World Metrology.

    We can look forward to the day when SI will be used everywhere in its pure form and be the light that guides science, engineering and economy into the future.

    Happy World Metrology Day 2018. We look forward to World Metrology Day 2019 when the kilogram will finally be defined from a invariable natural constant.


  10. @ Jake

    I understand that the average person does not understand SI fully. I’m contending this is so due to it being improperly taught in the classroom. Teach it correctly and it will be understood.

    Don’t blame the BBC for the way measurements are used, blame instead the individual persons who write the scripts that are read over the air or the reporters that insist on using these types of descriptions rather than proper units. Institutions don’t make decisions, people do.

    The reason for the use of comparative measurements has everything to do with the decisions of managers caught between a rock and a hard place. They know if they use metric units, the Brexit thinking Luddites will wail and complain and threaten to cancel their subscriptions. If they use imperial units the younger Remain thinking forward thinkers will do the same. To the managers this appears to be a neutral solution. Give neither side a chance to complain that they are having the other side’s units shoved down their throats.

    This will eventually change as the older anti-metric generation dies out and the younger pro-metric generation fills the gap. Once they become the full majority, the remnant minority can be ignored. Right now it is pretty much a 50-50 split.

    The teaching of metric in the schools has to be an abstract. The classes that use measurements, such as science, engineering, economy, etc is where the application of the abstract is taught and where the students obtain their proper perspective in relating measuring units to the real world. If the students aren’t learning, the teaching is either incorrect or students aren’t taking the right classes. The present means of teaching SI in the same confused form that imperial was once taught is the major contributing factor in the misunderstanding of units and the result is utter confusion.


  11. I happen to be interested in Astronomy and prefer the metre and kg for most measurements. But I don’t think it’s a good to outlaw the light year or the astronomical unit (AU). Those units do make perfect sense because they are tied to natural phenomena in a way that is entirely relevant. I find much easier to comprehand interplanetary distances in AU and visualise the scale and structure of the Solar system. The light year immediately conveys scale and time etc of the Cosmos in away that matters.
    We should also recognise that masses are conveniently measured with reference to the Sun for Stellar phenomenon and Earth And Jupiter for planets. Again entirely relevant to study extra-solar planets etc, because they immediately convey their significance.
    I think this kind of departure from SI is acceptable because it is logical and the meaning is immediately obvious. This does not compare with the silly stuff we get in the media with things like busses and football fields.


  12. I was taught decimetres at infant school in the mid 70’s. As a replacement for the foot I suppose. Didn’t last.


  13. Even though we still have many measure-hybridization issues here in Canada (body measures and carpentry still in Imperial, some weights and measures persist in being stated in pounds or ounces), mm vs cm is not one of our issues. Each has their place and role in distance measures. Aside from certain seniors and the plain stubborn, we use mm for rain, cm for snow and have for four decades. Short measures are mostly in cm, longer in metres, with mm reserved for fine measures. We have never really embraced the deci-anything, distance-wise.

    I was driving in the US the other weekend and it reinforced to me how out-of-the-norm miles are to me as measures. I can think and envision multiples of metres and kms but can’t wrap my head around miles anymore…


  14. @Craig Chouibard says:
    “I can think and envision multiples of metres and kms but can’t wrap my head around miles anymore.”

    Such a clear data point on the salutary effect of metric road signs!

    I also recall a Canadian woman calling into an American radio show who was told something using degrees Fahrenheit. She quickly responded: “I’m sorry, but I have no idea about Fahrenheit. We always use Celsius here in Canada.”

    Ta-daaa!!! 🙂


  15. In construction in NZ, all dimensions are expressed in m, or mm. People may indeed understand what cm, dm are, but for effective communication, sticking to m and mm has considerable advantages. 3.5 is assumed to mean 3.5m. 351 is assmed to mean 351 mm. When you say “two metres thirty” it means 2030mm not 2m 30cm. Much easier to say ‘2030’ that ‘2m 3cm’. Similarly in mapping distances can be shown in km or m, and there is no confusion. As in many situation, numbers expressed with zeros at the end suggest that the zeros indicate a lack of precision.
    The idea that people find it easier to convert when the new subdivisions of units are familar is not I think useful. The advantages of always using m or mm that I have outlined above are an additional advantage of the metric system, which encourage its use.
    An example above talks of measuring height as 1.75m or 175cm. But actually if you gave that height as 1750 you would be using fewer characters and everybody would understand.


  16. @ frenzed
    Thank you for those interesting observations from/regarding New Zealand. It seems that the construction industry has developed its own particular way of using metric (and nothing wrong with that). I must say, I would not immediately understand “two metres thirty” as meaning 2030 mm, but if that is the way they do it in New Zealand in the building trade, then so be it. What would a builder understand if I said a child’s height is “one metre thirty”? 1.30 m or 1.03 m, I wonder?


  17. I am old enough to have been taught under imperial cgs then mks prior to si. I just cant understand why cm exist let alone are taught in schools. I am lucky enough to be fluent in imperial and SI. I worked in the aerospace industry manufacturing electronics. In the 80s We had a visiting teacher that was confused by our use of units, linear dimensions in thou (thousandths of an inch and thickness in microns. Historic plus design software was American. Later on other companies i worked for used the American unit “mil” a thousandth of an inch. Confusion with engineers short version of millimetres or the chemists millilitres, very rarely. Why is imperial alive and well in the electronics industry, well its back to that American connection and standards especially package standards. Recently (semiconductor
    Assembler) most dimensions are expressed in mm except for wire diameter, our old frend mil. Things are changing but not to centi somethings or kilos. On a ridiculous closing note the 1908 Norton twin (motorbike) had a Peugeot engine the threads were metric but used whitworth spanners because they used standard hexagon bar stock from uk. the uk was in the lead when it came to seting standards, are we now?


  18. When SI was introduced in UK in engineering drawings to BS308 dimensions were always in mm.
    centi and hecto do not fit into the changing powers by three that the rest of the system uses. Who has ever used centigram or hectogram? Who uses hectometres? So why use cm. Most of the comments on here relate to personal usage experience rather than a definitive answer, just as mine does.
    Body mass index uses kg and m not cm.
    Using cm just confuses the issue.
    If your child is 1 metre 100, if the 100 was cm it would be 2 metres, so you would say 2 metres, the 100 is mm so 1.1 metres.
    Sheet material is always quoted as 2440 mm x 1220 mm.
    A cm is only 10 mm which is still too small for many applications. In SI countries your kitchen units are all in mm.
    It would seem that there are a lot of people who do not understand the powers of 10 so, personally, I see no practical use for the cm, it just causes confusion.


  19. Consider teaching a young child how to use a ruler to measure length. It would hardly be practicable to use a millimetre for the first steps in measurement. It is just too small for the child to grasp the concept. I have at home a ruler that appears to be one originally issued for school use. On side is graduated in centimetres. Just that. Not even millimetre sub-divisions. (The other side, incidentally, lists kings and queens and their reign dates. This shows that history and maths can go together.) I have other rulers, also believed to have come from educational establishments, graduated in centimetres and millimetres. No inches.
    It is true, as Paul says, that goods for sale in DIY shops are nearly always described in millimetres nowadays. That’s fine; it relates to the way we work. That level of accuracy is important in most of the projects I perform, e.g. measuring glass for a window, aligning screw holes for shelving supports, calculating offset from the horizontal to set guttering on a slope, etc.

    If I want to measure furniture to establish whether it will fit in a space, centimetres are sufficiently accurate. If I want to plan the furniture layout for an entire room (e.g. for a house move) I make a scale drawing of the room and cardboard cut-outs representing the scaled size of the furniture. Centimetre accuracy is sufficient; for a scale of 1:20 a centimetre would be represented by half a millimetre – not easy to draw to this accuracy on a diagram.
    Some time ago I observed two council workers taking measurements in a park. They were shouting to each other things like “three point four”. Clearly accuracy to a tenth of a metre was sufficient there. Expressions like “three thousand four hundred” or “three metres 40” would have been more cumbersome and less succinct.
    So as we promote the usage of metric units, let us be thankful that we can use simple multiples of ten to convert between different sized units. The idea of changing powers by three makes sense when we deal with large multiples or small fractions of the base units, but surely we can be practical closer to the base units and allow some flexibility?


  20. @Paul Meddemmen
    Paul wrote: “Who has ever used centigram or hectogram? Who uses hectometres?”.

    On my first visit to Italy (1975), I noticed that pizza was priced in “L/etto” which I deduced waslira per “hectogram”. (When I vsited in 2001, prices were in €/kg). I have also noticed that the deli counter in many UK supermarkets often price some cold meats and cheeses “per 100 g” (but do not use the term hectogram). Location markers on On Dutch motorways that are spaced at 100 m intervals are called “hectometerborden” (See for illustrations; the text is in Dutch).


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