Are imperial units natural? (and some useful rules of thumb)

One of the claims sometimes made by defenders of imperial weights and measures is that they are “natural”. The metric system (they may say) is all very well for science and technical matters, but for everyday life imperial units like the foot conform to the human scale and are more “natural”,  unlike the arbitrary metric unit, the metre. We examine this argument.

In the first place, if a unit of measurement is to be useful it must necessarily be arbitrary –  that is, it must have the same assigned value anywhere and everywhere. People’s feet are not all the same length, and in any case not many people even know how long their foot is! That is why the 1824 Weights and Measures Act standardised imperial units (like the foot and the mile) so that they would be the same in every region of the country, thus reducing the opportunity for traders to cheat their customers.

Even if the imperial foot was based approximately on the length of a real person’s foot, that person must have had very big feet. (A typical adult man’s foot, taking a size 9 shoe, would be about 260 mm long, whereas an imperial foot is 305 mm.)

Similarly, there is nothing particularly “natural” about the pint or the gallon. If there were, it would be difficult to explain why what is “natural” for Americans is smaller than what is “natural” for the British and Irish – or even Canadians. (An US gallon is 3.785 litres, whereas an imperial gallon is 20% larger at 4.546 litres).

Of course the truth is that when people say that measurement units are “natural”, what they mean is that they are what they are used to. Just in the same way as it is “natural” to speak English – except in most of the world.

To people brought up in exclusively metric countries, litres, metres and kilograms also seem “natural”. Some continental European correspondents have provided the following examples of how working in metric is “natural” for them, enabling them to devise various rules of thumb.


CK (German) writes:

“To me, metric is much more “natural”, i.e. linked to the body, the planet or nature in general. Just have a look at the statistics below. However, all figures are approximations, as nature does not produce things in uniform sizes:

Normal walking speed for a human on flat ground: 6 km/h [actually, this is fairly brisk – Ed]
This equates to 1 km every 10 min or 100 m every 1 min. Metric allows me to estimate distance by walking and checking the time. Or conversely, I know how long it takes me to walk between two points on a map.
For rough terrain, [or leisurely walking] halve the figures above (1 km every 20 min).

Average speed of a car on a fast road (motorway): 100 km/h
This makes a 550 km trip 5.5 h long.

Average speed of a car on a normal road: 60 km/h
This makes a 30 km km trip 0.5 h long.

Normal pace (double-step) of average humans: 1 m

Average height of men: 1.75 m
Average height of women: 1.65 m

Circumference of the earth: 40 000 km

Thickness of a fingernail: 1 mm
Width of a finger: 1 cm
Width of a hand: 10 cm

Average annual precipitation (rain, snow, hail, etc.) in most countries with moderate climate (i.e. most of Europe, North America): 1 m = 1 000 mm

1 cm of snow = 1 mm of water = 1 L/m² = 1 kg (Can’t be easier.)

1 liter of water and most common liquids: 1 kg
1 cubic meter of water and most common liquids: 1 t
1 ml of of water and most common liquids: 1 g = 1 cm³

Volume of water humans should drink per day: 2 L

Normal weight range for newborns: 2.5 kg to 4.5 kg
Average weight for newborns: 3.5 kg

Weight that normal people can lift without severe difficulties: 50 kg
Average weight for women: 65 kg
Average weight for men: 75 kg
Normal body weight for humans: Height in cm minus 100 = weight in kg

Weight of an average, small car 1000 kg = 1 t

Water freezes/ice melts: 0 °C
Water boils: 100 °C
Comfortable temperature, room temperature: 20 °C
Human body temperature: 37 °C
Fever: Temperature above 38 °C
Skin contact starts to get painful at 50 °C

Current potentially deadly for humans: 50 mA

Safe limit for radioactivity in food: less than 500 Bq/kg

Standard light bulb: 500 lm, 100 W

Illumination required for an office: 500 lx
Illumination outside on a cloudy day: 10 000 lx
Illumination outside on a sunny day: 100 000 lx

Water pressure for household pipes 200 kPa
Pressure rises 10 kPa for each meter under water.

One hectare: 100 m x 100 m

Distance to the moon: 300 000 km
Distance to the sun: 150 million km

You will notice that all the figure are nice, round and easy to remember. The list is endless. You could continue with temperature drop, or pressure per 100 m elevation, mixture of flour and water for bread’  etc.

In my experience, people who grew in metric countries are much better at estimating und understanding weights and measures than people brought up in dual-measure countries (i.e. Britain, Canada).

Try to ask a person in Britain: What is an acre? How much does it cost to fill a swimming pool? How much does this letter weigh? You will hardly get reasonable answers. I have experienced so many intelligent, well-educated, numerate people here who frequently or at least occasionally completely screw up a measurements and display a sometimes complete lack of comprehension. It is easy for me to have a “relationship” to a kilogram, I know how much it is. For a person, who is confronted with ounces, pounds, stones, tons, grams and kilograms, it is not so easy. They confuse things, they don’t have a firm foundation. I have seen a pharmacist completely helpless in trying to remember an average baby’s weight, a nurse unable to calculate 10 % weight loss – and there are many more examples. I don’t have the same experiences in metric countries.

To me, metric is more natural. The oldest measurement unit we know about is the ell. Surprisingly, the oldest known ell is pretty much exactly 50 cm. If this ell was a natural unit, then so is the meter.”


And here is a similar comment from WB, who is Danish:

“As you have already guessed, I can assure you that my countrymen have no difficulty in judging distances, sizes, weights, speed etc. using the metric system. I can further assure you that we consider metric units completely “natural” (if that’s the term we are debating here). As you say, what you are familiar with (like your mother tongue) feels “natural”.

Apart from that, it is nonsense to suggest that imperial units are more “natural” than metric units. The size of a foot obviously varies from person to person (yesterday, I stood next to a young man whose foot was about 50% longer than mine!). Therefore the unit “foot” is completely arbitrary and the same goes for lots of other units (like inch or fathom, say).

As for metric units – it would be only too easy to give examples of their “naturalness”, if that’s what you want. E.g. a metre is the distance from the ground to my belly buttton (yes, absolutely spot on). Two metres is the height I can comfortably reach when I raise my arm above my head. And a cm is the width of my little finger. As for weight (like a kilo) and volume (like a litre) you acquire a feel for these quite “naturally” when you grow up with them (although in actual fact it takes no more than a few minutes if you put your mind to it). The fact that a ton(ne) is 1000 kilos is also quite easy to grasp for a normal person, and the same goes for a cubic metre (1000 litres). What about 100 metres? Well, that is the shortest sprint distance run by atheletes and nobody here or abroad seems to have the slightest difficulty with that. “Natural”, wouldn’t you say? And what’s a comfortable temperature, both indoors and outdoors? That’s a nice round 20 degrees. And 100 degrees is the temperature at which water starts to boil, of course. I find that very natural.

Many units used to be more or less loosely connected with agriculture (not only here but everywhere), because most people worked in agriculture and lived in the country. Hence an acre = what a span of oxen can plough in half a day (I think is the definition). Not very accurate, of course, since this will vary with the terrain, the soil, the weather, the strength of the animals, the equipment and the skill of the ploughman. The relevance to the way of life today is non-existent and into the bargain, the size and subdivision of this funny unit are so awkward that it is almost impossible to use for calculations if you are not a maths professor. All in all a totally useless unit that could hardly be described as “natural”, certainly not today. No wonder that the vast majority of people in this country have no idea what an acre is. Give me a square metre or a hectare any day – they never vary, are dead easy to visualise and dead easy to use for calculations. These advantages are so enormous that I would say that these units feel very “natural”.

I shall never forget that when I was at secondary school and later at business school, as part of our training we had to do “English” invoices, i.e. not only in awkward money like £sd but also for quantities like tons, hundredweights, quarts, pounds and ounces (consignments of coal for instance). None of this seemed in the slightest “natural” to any of us. To say that it was a nightmare and that we hated it, is an understatement. Imagine our relief when we heard that the UK was going both metric and decimal and that we could forget about imperial units and “English” invoices! At least we in Denmark were then spared further punishment – unlike people in this country who are still suffering from having to live with imperial units 40 years later! If that’s “natural”, give me “unnatural”, please!”


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6 Responses to Are imperial units natural? (and some useful rules of thumb)

  1. Phil Hall says:

    A very good article. Hopefully people will read it and realise that the unit sizes in metric are quite useful after all.
    In addition to the above I think that the millimetre is very helpful too. It's a bit fine for many applications but that is no bad thing. By expressing dimensions of things in mm (no more than a few metres in length), everything is in whole numbers only. Very useful in drawings, documents and working practices generally. By contrast there is no scope for this in imperial, where the smallest unit is the inch.

  2. David says:

    > To people brought up in exclusively metric countries, litres, metres and kilograms also seem “natural�.

    ..not just foreigners. Never forget that the metric system is, at least until we enter the adult world and all kinds of random units also get hurled at us, also the "native" and natural system of measurement to all of us who learned it at school!

    Of course there are all kinds of natural-feeling metric measurements: my pinkie finger is about 1 cm across, my hand about 10 cm, the span of my hand (thumb and pinkie outstretched) about 20 cm. Handy for estimating measurements if you don't have a tape measure. But, of course, such measurements vary from person to person, making any claims of innate naturalness of such measurements (either in the metric system or in any of the various and varying archaic systems) nothing but pure nonsense.

    Where the metric system does have a natural elegance is in the relationships between all of the basic units. The fact that a litre of water weighs a kilogram is a work of genius. And all of the other basic units also have simple one-to-one relationships: 1 watt = 1 joule/second, and so on.

  3. Roddy Urquhart says:

    The "imperial is natural" argument is appealing but on afterthought very disturbing...after all as humans we vary a lot in size.

    As somebody with a size 27.5 cm foot (size 9 UK, size 9.5 US some of the US standards are not so standard) I am somebody that "imperialists" would regard as having an "unnatural" foot size. The imperial foot is 30.5 cm so a UK size 12. But what percentage of the UK population has the elite size of 12? Most of us are disqualified.

    Anybody who argues that imperial is natural should, for the sake of consistency, argue that the vast majority of us have sub-natural or defective feet sizes. This smacks of "foot-apartheid". A standard, natural foot size could only be achieved by Nazi-style cloning.

    We should face the fact that foot sizes vary naturally and that there is no "natural foot size". Let's enjoy our natural diversity and use a sensible metric set of units as a standard way to measure that.

  4. Han Maenen says:

    I live in The Netherlands. I read the part of the article about making invoices. My father went to secondary school and then to business school before the Second World War. And he also had to go through the torment of making up invoices in Lsd/ He had exactly the same feelings about it - he deeply hated having to go through this. Making up such invoices was taught up to the sixties. With the spread of metric and the demise of Lsd money this teaching came to an end.

  5. Han Maenen says:

    I remember a day in the sixties when I arrived at school and entered a classroom. On the blackboard I saw proof that there had been a torture session the evening before during an evening business course. It was covered in sums about English invoices! I also remember that once we had a slight brush with it; a sum in which the price of a party of goods was expressed per 50.8 kg. My classmates would of course not know what that was, but I immediately recognized the good old hundredweight in disguise!

  6. Martin Vlietstra says:

    From a simplistic point of view, units based on the human form are natural, but like most simplistic models, are subject to limitations. These limitations include, but are not limited to deciding whose human form should be used - yours or mine? If you are the king, but I am a peasant, then naturally you can pull rank, but if I am a merchant from one country and you are a merchant from another, there are obvious problems. Another problem area is when the size and the utility of the object being measured is confused - for example, the "hide" of land used in the Domesday book was a measure, not of the size of the piece of land, but of its economic worth.

    Another example is the stone - a unit of measure now all but phased out, but as recently as 1936 Smithfield’s Market used two different stones - animals on the hoof were weighed in stones of 14 pounds while carcases were weighted in stones of 8 pounds, the difference of 6 pounds being the supposed weight of blood, offal, head etc. This was a classic case of the "stone" doubling up as both a weight and an economic indicator.

    A further example is the gallon. Prior to the introduction of the imperial gallon in 1824, England had the "wine gallon", the "milk gallon" and the "ale gallon", each different in size and each used for various fiscal purposes. There was no "water gallon", which put engineers at a disadvantage - which gallon should they use? Little wonder that some twenty years before the introduction of the metre, the engineer James Watt called for a universal decimal-based system of measure. This qaundry summarises metrication in the United Kingdom - British scientist have been in the forefront of developing the metric system, but the British Government seems it be doing is best to sabotage it.


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