Myths about imperial

Despite the end of the British Empire and the demise of the pith helmet, quite a few British people are nostalgic about imperial units. However, many people are not really proficient in using them and often are ignorant about their history and usage.

Imperial army

Imperial is “natural”
A unit needs to be based on an invariant standard. People’s body sizes vary and there is not a natural size for say the foot. Read more

Imperial was invented in Britain
Imperial units were imposed by various foreign invaders Read more

A blood donor gives a “pint” of blood
A unit of blood in the UK is 470 ml (almost half a litre) while the imperial pint is 568 ml (about 21% more). Read more

Metric has adopted the second from imperial units
The second is much older than either metric or imperial. Read more

Imperial is duodecimal
There is no consistent number system in imperial. Read more

Imperial has the advantage of being divisible by 3
The majority of imperial units are not divisible by 3. Read more

A pot of jam weighs a pound
While the contents of a new jar may be 454 g (one pound) the jar and contents will weigh more. Read more

Imperial is “natural”

This myth is commonly stated in the British media and presumably only refers to the foot. What part of the body is a pound? What naturally corresponds to a pint?

However, a little thought shows that this is an empty justification for using imperial. A unit needs to have a standard size otherwise there would be complete chaos. Human feet are not a standard size and so there is no natural size for the unit. The fact that human feet vary led to many different standard feet in Europe in the past.

In fact very few people have feet that are as large as the imperial foot. You would need a British size 12 ½ foot! The vast majority of people have smaller feet. The average human foot is 24 cm versus 30.48 cm for the imperial foot.

If an imperial foot is “natural” then by implication most people have unnaturally small feet! If in doubt try measuring your own feet.


Imperial was invented in Britain

Some people regard it as patriotic to use imperial, because it was ‘invented in Britain’. However, most of these units originated elsewhere in Europe and were imposed by invaders. English versions of these units were imposed on Scots by the Act of Union. Consider the following:

  • Acre came in with the Anglo-Saxon invasion and was based on the word for a strip field. It is a similar word to the modern German Acker or Dutch akker, both words for cultivated fields.
  • Foot came in with the Roman invasion – Furlong meaning “furrow long” came in with Anglo-Saxon invaders too.
  • Mile was a unit that came in with the Roman invasion. The word “mile” is derived from the Latin millia, meaning a thousand. The Romans introduced the mile, which was originally defined as the length of 1000 paces where each pace was two steps.
  • Ounce came in with the Roman invasion and the current abbreviation is based on the old Italian word onza. It is derived from the Latin uncia, meaning one twelfth part. The Roman libra, which is Latin for pound, consisted of 12 ounces, hence the name.
  • Pint is a word probably coming from the Romans (pincta)
  • Pound came in originally with the Romans who had a libra pondo consisting of 12 ounces. The imperial 16 ounce avoirdupois pound came in from France. Its abbreviation, lb, is derived from “libra”, the Latin word for pound. The term “avoirdupois” comes from Old French and literally means goods of weight.
  • Yard was originally an Anglo-Saxon unit based on their word gyrd for stick.

Of course over time the units of invaders were modified. So the logical Roman mile of 1 000 left-right paces was replaced by one of 1 760 yards. The yard was re-defined as 3 feet by King Henry I.

The idea that it is patriotic to use imperial units (which came in from Italy, France and Germany), but unpatriotic to use metric ones (which were first suggested in England and first used in France) is quite absurd.


A blood donor gives a “pint” of blood

People often speak of donating “pints of blood”. This is surprising when the medical profession speaks of litres. In fact the “unit of blood” is 470 ml.

The unit of blood is much closer to half a litre (500 ml) than the imperial pint (568 ml). The imperial pint is about 21% bigger than the unit of blood.

Whilst we’re on the subject you may like to consider donating blood. Please visit the National Blood Service web site for more information.


Metric has adopted the second from imperial units

An imperial supporter once claimed this in a national newspaper. Of course, both metric and imperial use seconds. The second is a base unit in the international system (SI).

A number of ancient civilisations divided the day into 12 hours and the Egyptians were the first to divide the night into 12 hours. The Babylonians came up with the idea of dividing intervals by 60. So both metric and imperial owe our concept of time to ancient civilisations.

The second is now defined by the natural beats of a highly stable caesium 133 atomic clock.


Imperial is duodecimal

It is often claimed that imperial is based on twelves (duodecimal) and that this has advantages over decimal. On the contrary it could be argued that it makes more sense to use the same number system for measurement, money and other practical uses. But is imperial duodecimal?

Consider length:

One foot = 12 inches
One yard = 3 feet
One rod = 5.5 yards
One chain = 4 rods
One furlong = 10 chains
One mile = 8 furlongs

Only the foot uses a base of 12.

Consider volume:

One pint = 20 fluid ounces
One gallon = 8 pints

No duodecimal units.

Consider avoirdupois weight:

One pound = 16 ounces
One stone = 14 pounds
One hundredweight = 8 stones
One imperial ton = 20 hundredweight

Again no duodecimal units.

A disadvantage of imperial is that it does not have any consistent number system but uses a hodgepodge of bases.


Imperial has the advantage of being divisible by 3

Advocates of imperial criticise metric because it is not divisible by 3 into a neat number. For example (sticking to 5 significant figures)

1/3 m = 33.333 cm
1/3 kg = 333.33 g
1/3 L = 333.33 mL

So one third of a foot is 4 inches but the majority of imperial units do not readily divide by 3 either.

1/3 mile = 2 furlongs 6 chains 2 rods 3 yards 2 feet
1/3 pound = 5.3333 ounces
1/3 pint = 6.6667 fluid ounces
1/3 gallon = 2 pints 13.33 fluid ounces

Where division by 3 is very important is in rotation and angles. This is why the use of degrees of an arc is in the international system (SI).


A pot of jam weighs a pound

It is often stated that it is easy to imagine a pound by thinking of the weight of a pot of jam. Of course the most common jam pot contents are 454 grams – about a pound but the glass weighs too. To get an accurate feel for a pound would require the contents to be poured into the hand!