Myths, misinformation and fallacies (1) – Are imperial units “natural”?

The claim is often made by last-ditch defenders of miles, feet, pints and acres that “Imperial units are natural whereas metric units are artificial”.   In the first of an occasional series of articles on “myths, misinformation and fallacies” used by opponents of completing metrication, we examine this claim.

A list of such myths is summarised in a webpage on UKMA’s main website at this link entitled “Briefing note for UKMA representatives”.  This note arose from a discussion at UKMA’s 2012 annual conference about the abysmal standard of debate heard on local radio phone-in programmes.  The original intention was (and remains) to help UKMA members and supporters to make the case in radio and television interviews, in newspaper correspondence and online.

In the coming months we shall be discussing particular arguments from this list and opening them up to readers of MetricViews.  This week we look at the claim that:

“Imperial units are natural whereas metric units are artificial”

This claim is a favourite with defenders of imperial units.  It is argued that the foot approximates to the size of the real foot at the end of your leg, and therefore it is a “natural” unit that people can relate to.  Thus it is claimed that it is easy to estimate short dimensions – e.g. the width of a room – in feet.  Similarly with an inch, which corresponds (allegedly) to the width of a thumb.  Likewise a “hand” … and so on.

George Orwell is often prayed in aid, as his satire on life in a totalitarian state in “1984” included the grumble by the old man whom Winston Smith meets in the proles’ pub:  “A ’alf litre ain’t enough. It don’t satisfy. And a ’ole litre’s too much. It starts my bladder running.”

Another somewhat far-fetched claim is that it is easy to estimate by hand the weight of, say, a bag of potatoes in “natural” units (pounds), but less easy in “artificial” kilograms.

Yet another claimed example of a “natural” unit is the acre.  This is (apparently) easily visualised since it is roughly the area of land that a horse can plough in a day.


None of these claims can withstand even the most cursory examination.

If a measurement unit is to be of any practical use in (e.g. in shopping, building design or regulating speed limits) it has to be precisely defined – and defined in terms of some objective standard which can be referred to.  Yet, as soon as it is precisely defined, it ceases to be “natural” and becomes arbitrary (or “artificial”).  This is to ignore the obvious fact that adult human feet are of varying lengths, generally in the range 220 – 290 mm, and rarely as long as 304.8 mm (the official definition). Similarly with thumbs, hands etc.

Orwell was of course parodying – not endorsing – the old man’s preposterous belief  that he could tell the difference between a pint (568 mL) and half a litre (500 mL)of beer when poured into a pint mug. Later in this passage, he narrates that “Winston went to the bar and came back with two more half-litres. The old man appeared to have forgotten his prejudice against drinking a full litre.” (To get the full context, you can read chapter 8 of the novel online at this link.

(For a discussion of the advantages of using lined beer glasses rather than brim measure “pint” glasses that deliver short measure, see this link).

Horses are rarely used in the UK for ploughing these days (except as a competition sport), so sights such as in the photograph below will be unfamiliar to most British people (it was actually taken in Slovakia, which – inexplicably – does not use the acre).

So one wonders how many people can actually visualise the acre – or indeed have any idea how big it is.  (Answer: it is a furlong multiplied by a chain.  This then gives the conveniently round number of 4840 square yards – or 43 560 square feet if you prefer).

Of course the claim to “naturalness” is nonsense.  Imperial units are only “natural” in the sense that it is “natural” to speak English.  In other words, it is what you are brought up with, what you learn from your parents, siblings and peers.  In countries where the metric system is well established it is equally “natural” to use metres, kilograms and litres.

Ironically, a case can be made that metric units actually are “natural” in the sense that, with the exception of the kilogram, they are defined in terms of constants of nature. For example, the metre is defined as the distance travelled by light in a vacuum in a tiny fraction (1/299 792 458) of a second.  And a second is defined in terms of a particular energy level transition frequency of caesium atoms.

Even more ironic is that, since the Weights and Measures Act 1985, the surviving imperial units are defined in terms of metric (SI) units.  Thus in reality an imperial foot is the distance travelled by light in a vacuum in 1/983571056 of a second.

19 thoughts on “Myths, misinformation and fallacies (1) – Are imperial units “natural”?”

  1. The Wikipedia article on the foot ( ) catalogues over 50 different feet that have been used in Europe in the last two centuries. Imagine the problems of specifying railway tracks across national borders with that variety of feet!

    All of these, apart from the English foot are now history and even in the UK, the foot is only used officially on roads signs – standard railway guage for example being 1435 mm.


  2. It can be useful sometimes to be able to relate units of measurement to one’s body parts certainly but we can do that with metric units as well.

    For example my favourite is the fact that the width of my hand is about a decimetre (100 mm) So by gripping or placing my hands on some objects I can get a rought idea of sizes in mm. The decimetre is useful for estimating volume too, because a litre is one cubic decimetre.

    I’d be interested in similar ideas from other readers.


  3. The briefing paper for UKMA representatives is quite good. One minor point you may wish to add to section “C.” I think your anti-metric forces believe goods in the US are labeled ONLY in Imperial or Customary. You rightly point out that Customary has some differences. However, US labeling law generally requires consumer goods to be labeled in US Customary and metric. For some products, it usual for the Customary to be the “round” amount, for other products, the metric. (An exception exists for goods weighed at point-of-sale, or sold in random weights.)

    Secondly, a very technical point on Q (aviation); you may decide it is too complex to include. Aircraft fly at “pressure feet” not real feet. Altitude is measured by pressure with a barometric altimeter. The conversion between altitude and pressure depends on a number of assumed parameters which actually vary day-to-day. The actual altitude of the aircraft departs from the label on the altimeter as those parameters vary. The US, ICAO, and ISO Standard Atmospheres (the model) all agree over all altitudes used by commercial aviation and the models are defined in metric. The labels on the altimeter are converted in the end by noting 1 ft = 0.3048 m.


  4. As it happens, it’s far more convenient to specify most human-scale measurements (in carpentry, plumbing etc.) in millimeters than in inches, or even eighths or tenths of an inch, because the latter aren’t accurate enough. I grew up with both systems and I’ve never understood the people who say inches are “natural”. All they mean is they’re used to them.


  5. A decimal system such as the metric system is more natural to calculate in as we tend to calculate using decimals in maths; calculators and spreadsheet software generally work in decimals (I know its possible to set Excel to calculate in all the various imperial divisions but it’s difficult); finally the vast majority of people have 10 fingers!


  6. The units of the metric system are obviously easier to use than imperial units, and are arguably far more “natural” …

    1 metre is 1/10 000 000 of the distance from the Earth’s equator to the North Pole.
    1 kilogram is the mass of 1 litre (1 dm³) of water.
    0 °C is the temperature at which water freezes.
    100 °C is the temperature at which water boils.

    There are also some useful coincidences that result from these definitions …
    A simple pendulum, 1 metre in length, takes approx. 1 second to swing from one side to the other.
    Accelaration due to gravity is approx. 10 m/s²
    Normal atmospheric pressure is approx. 100 000 N/m² (1000 hPa)


  7. Both John Wilkins and the founding fathers of the metric system sought a natural and universal standard of length to facilitate and encourage adoption of their novel systems of measures. Wilkins recommended the one-second pendulum. The French, after considering the pendulum, chose in 1791 the distance from pole to equator along a meridian. In this they followed a precedent set at sea.

    From ancient times, sailors relied on astronomical observations to determine latitude at sea. The system inherited from the Babylonians in the second century BC, which divided a circle into 6 x 60 degrees each of 60 minutes, gave 90 degrees of latitude from pole to equator. The nautical mile was defined to be as close as practicable to the average length of one minute of latitude, or 1/5400 of the distance from pole to equator. Later, when the metre was first defined, only the fraction was different: 1/10 000 000.

    As with the one-second pendulum, this definition of the metre was soon found wanting, but by then the metric system was sufficiently well established to survive its demise. For over 160 years, the metre was defined by the distance between two marks on a metal bar, as indeed was the imperial yard. Fortunately, the definition of the metre adopted in 1983 that is mentioned in the article complements perfectly the surveying and navigation systems upon which we rely today and which require an unvarying and extremely accurate determination of the speed of light.


  8. Regarding the question posed in the article “So one wonders how many people can actually visualise the acre”…

    I wonder whether the estate agents who advertise properties actually know what they mean when stating garden sizes as, for example, “approx. 0.3 acres” for the poshest of their offerings. How do they measure the sizes? Do they summon the assistance of a horse and plough?

    I for one couldn’t estimate the size of a garden, or any other area, in acres but I know the size of a metre and can easily estimate an area in square metres. Knowing that a hectare is an area of 100 m x 100 m, one would have thought that most people, especially those who have run a 100 metre race at school, would have no difficulty visualising this.

    Of course, no-one would dare talk of hectares in the leafy suburbs where gardens are “approx. 0.3 acres”, even if they knew what a hectare was!


  9. @Phil H

    The 10 cm hand reference is interesting. I also find that my arm measures 50 cm from elbow to tip of middle finger. Maybe thats just me!


  10. The argument about whether imperial units are ‘natural’ or not is purely academic. Over 95% of the world’s population uses the metric system as part of their day-to-day lives, and probably almost 100% of the world has some contact with the metric system as part of their work if not in their daily lives. That is not going to change, and therefore it doesn’t matter which is more ‘natural’.


  11. The following is a quote from The Daily Telegraph Book of the Weather, “The beauty of the metric system means that one millimetre of rain is also one litre per square metre.” Not only is metric natural it is beautiful as well. It is a pity the the Telegraph does not adopt this in their papers!


  12. ‘Thus in reality an imperial foot is the distance travelled by light in a vacuum in 1/91 376 741 of a second.’

    I’m not sure how you calculated that, but that’s incorrect. Light travels 1 foot (304.8 mm) in 1/983571056 seconds. In the time you stated, light travels about 3,280.8 mm.

    [Quite right. I have corrected it. Doesn’t affect the basic point though – Erithacus]


  13. I understand that the definition of length using a pendulum failed when an attempt was made to reconstruct the imperial yard after the fire of 1834 (?). Legislation had decreed that should the official yard be lost, and then it was to be reconstructed using a specified pendulum.


  14. If you go to any shop that sells shoes, you will find shoes in different sizes because there is no one-size-fits-all pair of shoes for all feet. If the foot as a unit of length is to be any use, it cannot be based on the length of any person’s foot otherwise we will have a bewildering number of definitions for a foot and would cause all kinds of problems; it must be based on a standard size. The question then arises, whose foot will serve as the basis for the definition of this unit of length? Whichever definition is used, it will be as arbitrary as any other definition of a measurement unit. Let’s not forget that all measurement units are arbitrary and man-made. The debate about which ones are “natural” and which ones are not is academic. Why should measurement units based on the human body be regarded as more natural than ones based on something else (e.g. the metre)?


  15. @Brian
    Off topic, but where did that half inch come from? This may be the explanation.
    George Stephenson was asked to design the Liverpool and Manchester Railway, partly because of his work developing railways in NE England, where he used a track gauge of 4′-8″. This was gauge for used for the L&M. The locomotives were designed and built by his son, Robert Stephenson. But when they were delivered, it was found they stuck on curves. What was to be done? A conference between father and son agreed that it would be easier to widen the gauge than modify the locomotives, and half inch was added.


  16. @ Brian
    I know that the standard rail gauge was originally set at 4′ 8-1/2”. However, this is one of the few cases where the original imperial size is compatible with a metric size: 1435 mm (the 0.1 mm has been dropped). As far as I am concerned, for metric countries this gauge is a metric standard.


  17. Before we get the usual Roman chariots and horse’s rumps theories from the BWMA, it may be worth reading at least the Wikipedia article on rail gauge:

    Some of the useful tidbits I picked from the article:
    *Non-standard gauge preceded standard gauge. Nearly every mine had its own gauge and this persisted at least from around 1800-1840. (Perhaps the mines had non-standard horses with non-standard rumps.) 1435 mm gauge began as one of many non-standards and won largely because a couple of rail lines of this gauge were interconnected and very successful.
    *There is debate from credible historic observers whether Stockton & Darlington was initially 1422 or 1435 mm.
    *Tolerances exist. The US rating for 60 mph track is that measured gauge must lie between 1422 – 1460 mm; otherwise speed must be lowered or track repaired. This is a relatively large and non-symmetric tolerance around the nominal 1435 mm.
    *1435 mm track largely won out and accounts for about 60% of worldwide rail, but many non-standard sizes persist as national standards or in special applications. Various clumsy solutions exist to handle transitions between different gauge rail lines.


  18. @Brian
    I guess you consider 4 daft units and 8 1/2 stupid units to be more ‘natural’ and more ‘rational’ and ‘easier to remember’ than a simple 1435 mm as used all over the world, including UK.


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