How to measure your foot properly
Everything else depends on this one measurement, so it is worth getting right. Put a sheet of paper on the floor against the wall, rest your heel against it standing up — sitting down the foot measures less, because it carries no weight — and mark where the longest toe reaches, which is not always the big one. Measure from the edge of the paper to the mark.
Two details change the answer: do it in the afternoon, when the foot is at its most swollen, and wear the sock you intend to use. And measure both feet: almost everyone has one larger than the other, and the larger one sets the size.
Where each scale comes from
The three big scales measure the same thing in different units, and none of them measures the foot: they measure the last, the mould the shoe is built on, which runs one to two centimetres ahead of the foot so the toes do not hit the end when you walk.
- Europe uses the Paris point, two thirds of a centimetre. A size 39 is 26 points, that is 26 centimetres of last. The scale dates from 1885 and has no official half sizes, though plenty of makers produce them anyway.
- The United Kingdom uses the barleycorn, a third of an inch, which is the average length of a grain and the oldest unit in footwear: it goes back to a decree of Edward II in 1324. Children's sizes run to 13 and adult sizes start again at 1.
- The United States uses the British scale shifted: one size up for men and two for women. There is no deep reason for it, it is a commercial convention.
- Japan and the ISO standard use mondopoint: the foot in millimetres, nothing else. It is the only system that says what it measures.
The formulas, to do it by hand
- Last = foot + allowance. With a 25 cm foot and 1.5 of allowance, 26.5 cm.
- European = last ÷ 0.667. With 26.5 cm that is 39.75, rounded to 40.
- British = last in inches × 3 − 25. 26.5 cm is 10.43 inches → 31.3 − 25 = 6.3, so a 6½.
- American = British + 1 for men and + 2 for women: 7½ and 8½.
What is worth knowing
The size measures length and nothing else. Width, instep and the shape of the toe box are each maker's decision, and that is what explains a 40 fitting in one brand and not in another. Some British and American brands also label the width with a letter — D is the men's standard, B the women's — and that letter says as much as the number does.
About published conversions: shop tables do not agree with each other, and not because some are wrong but because each starts from a different allowance. That is why the allowance is a field here. For closed shoes, or if you will wear an insole, raise it; for sandals, lower it.