Everyday rulers: the objects people actually measure with
Nobody says a hundred and five metres. They say the length of a football pitch, which is the same thing on a good day and a different thing on most others. The informal units people use are worth taking seriously, because they are what a comparison has to land on to be understood.
Why informal units exist
A metre is precise and unhelpful. Nobody carries a mental image of a metre; they carry an image of a door, a car, a bus. When someone says a thing is about the size of a fridge, the sentence works because both parties have stood next to a fridge, and it fails the moment the listener has not.
The catalogue is full of these objects, and they turn out to divide sharply into two groups: the ones whose size is fixed by a standard, and the ones whose size everybody assumes is fixed and is not.
The ones that are genuinely standardised
A4 paper, 29.7 cm on its long edge. The A series is defined so that every size has the same proportions and halving one gives the next. That is why the numbers are irrational-looking and why the system survives. A wine bottle at 30 cm against a sheet of A4 is a ratio of 1.01, which is the kind of near-tie that only turns up when both objects are standardised.
A credit card, 8.56 cm on its long edge. Fixed to a tenth of a millimetre by international standard so that every reader and wallet in the world accepts it. It is the most rigidly specified object in the catalogue and the best small reference available. Against a chicken at 0.35 m the ratio is 4.09, and the card is the half of that comparison you can trust to the millimetre.
An AA battery, 5.05 cm. Fixed by standard, including the contacts. Battery compartments are built a millimetre or two longer, which is why a slightly loose fit is normal. A size 5 football at 0.22 m against it gives 4.36, though the ball is the softer figure of the two: the laws fix the ball's circumference rather than its diameter.
A shipping container, 6.058 m. The twenty-foot box is the unit the entire shipping industry counts in. Every port statistic anywhere is quoted in multiples of this one object. Against a Formula 1 car at 5.6 m the ratio is 1.08, and both numbers are the product of rules rather than of design.
A LEGO brick, 3.18 cm on its long side. The system tolerance is a fraction of a millimetre, which is what allows bricks moulded forty years apart to clutch together. Against a medium chicken egg at 5.5 cm, ratio 1.73, the brick is the precise half and the egg is not.
Eggs are graded by weight, not size. The 5.5 cm used here is a typical height for a medium egg, not a specification, and the grades overlap more than most people assume. An informal unit that sounds standardised and is not.
The ones everyone assumes are fixed
A bus. The modern London double-decker is 10.9 m long, about half as long again as the Routemaster it replaced. Since the Routemaster is the shape most people still picture when they hear double-decker, the unit and the image have drifted apart. A T. rex at 12 m against the modern bus is 1.10, and against the older one it would be closer to 1.5.
A horse. Measured in hands at the withers by riders and nose to tail by everyone else. The catalogue uses the nose-to-tail figure, 2.4 m, which is not the number a rider would give you.
An oak tree. Twenty-two metres here, and individual trees vary enormously. It is a typical mature specimen rather than a standard, and anyone using it as a unit is using an average of a very wide distribution. Against General Sherman at 83.8 m the ratio is 3.81, and the sequoia is the far better-defined figure because it is a specific named tree.
A football pitch. The worst offender, and it has a guide of its own.
Body-scale units
The oldest informal units are body parts, and they persist because they are always to hand. The figure the catalogue uses for a person is 1.71 m, a global average for adult men, and national averages spread by more than fifteen centimetres around it.
That spread is the thing to keep in mind. As a unit for scaling a whale it is entirely adequate, because a fifteen-centimetre uncertainty against a twenty-seven-metre animal is noise. As a unit for scaling a door it is useless.
A hippopotamus at 3.5 m against a person gives 2.05, which is a good example of the unit doing exactly the job it is suited to: a ratio near two, at a scale where a human is genuinely comparable.
The unit that dates
An iPhone 16 is 14.76 cm tall, fixed to a tenth of a millimetre because cases are manufactured to it. It is the most precisely specified object in the catalogue and the one that ages fastest.
Phones make excellent references for about three years and then become historical objects. Pikachu at 0.4 m against the phone gives 2.71, and in a decade that comparison will need an explanatory footnote in a way that the credit card one never will.
Choosing a reference
Three criteria, in order.
- Does the reader know it physically? A container is precisely defined and most people have never stood beside one. A bus is imprecise and everyone has.
- Is it actually fixed? If the answer is no, say so, or choose something else.
- Is it the right order of magnitude? A credit card is a poor unit for a building and a superb one for an insect. Using a unit more than about five times away from the thing being measured reproduces the same problem you were trying to solve.
The catalogue holds 393 objects partly so that there is always something in the right band. That, more than any individual figure, is what makes a comparison land.