
- John F. Kennedy

I love a challenge. I go out of my way to do Hard Sums in my head, in the hope of one day matching the Mathematical Ninja’s prowess. I run long distances. I juggle childcare with a tutoring business. I work with children, as a choice. Difficult things? I love them.

But they’re no way to run an education system. I genuinely can’t believe I have to write a blog post explaining to the Prime Minister why teaching the imperial system is one of the worst things he could do for schools in this country - and this, remember, is the man who employed Michael Gove as Education Secretary for four years.

Quick, how many fluid ounces in a gallon? Oh - you’re right, I didn’t specify whether I meant in imperial or US (‘British’) units - obviously, 128 US fluid ounces make a US gallon, and 160 imperial fluid ounces make an imperial gallon. Obviously, I don’t mean a US dry gallon, which is something else entirely - an eighth of a Winchester bushel, to be precise (not to be confused with other sorts of bushel).

OK, let’s say we meant an imperial gallon (similar, but not the same, as an ale gallon). How many cubic inches is that? Anyone? No? I’ll tell you, then - it’s 277.42 cubic inches (to two decimal places). That’s the lovely thing about imperial units, they break down so nicely into simple factors.

Compare that with metric: there is only one sort of litre. It’s defined as 1,000 cm³. Incidentally, a litre of water has a mass of (pretty much exactly) one kilogram. What’s a centimetre? Well, it’s pretty much exactly a billionth of the distance from the north pole to the equator. It’s all linked together (apart from time, which we ought to decimalise, but that’s a different battle).

The only thing routinely weighed in pounds and ounces these days is a baby, and even then midwives have a handy chart for converting the metric measurement on the scales into old-fashioned units. Feet and inches? Human heights - although it’s awkward to get the accuracy you need, and you’ll often hear people say they’re five feet four-and-a-half… if only we had a unit of length that was about half an inch!

I’ll leave it to @standupmaths to handle the imperial measurement system in detail:

As for ounces… well. There are ten different ounces listed on the Wikipedia site for ounce. None of them are the weight of a fluid ounce of water. Even the name, ounce, is from the Latin ‘uncia’ which is related to ‘one’ but means ‘a twelfth’; an ounce is, of course, a sixteenth of a pound, while a fluid ounce is a twentieth of a pint.

If we switch back to imperial units, we’ll be joining an impressive list of countries that do likewise: the USA, obviously, but also the industrial powerhouses of Liberia and Myanmar. And nobody else.

Why does nobody else use it? I can give you 277.42 reasons. One of these is, it’s expensive: this report suggests that just using metric in the Australian building industry increased profits by around 10%.

Perhaps this is an indication that Cameron puts ideology before growth? You might think that. I couldn’t possibly comment.