Inches to centimetres

The commonest failure on this page is not arithmetic. It is that the inch figure printed on the thing is not the size of the thing. A "two by four" leaves the mill at two inches by four and reaches the shop planed down to 1.5 by 3.5, which is 3.81 by 8.89 cm. Nominal pipe size stopped describing any dimension of the pipe more than a century ago: half-inch pipe has an outside diameter of 0.84 in, or 2.13 cm. Screw gauges, wire gauges and sheet gauges are index numbers and not measurements at all. Convert one of those labels and you get a very precise translation of a name.

A tyre marking mixes three unit systems inside one string, and it repays being able to read. In 205/55 R16 the 205 is the section width in millimetres, the 55 is the sidewall height as a percentage of that width, and the 16 is the rim diameter in inches. So the sidewall is 112.75 mm tall and the rim is 40.64 cm across, and nothing in the code converts to anything else in it.

Paper is the other place the two systems meet badly. A4 is 210 by 297 mm; US Letter is 215.9 by 279.4. Letter is about 6 mm wider and about 18 mm shorter, so printing one on the other clips something at every setting except fit-to-page, which shrinks it instead. The A series is built on a ratio of the square root of two, so halving the long side gives exactly the next size down. Letter comes from a different tradition and has no such property, which is why an A3 poster halves cleanly and a Tabloid one does not.