Barleycorn

A barleycorn is an old English unit of length: a third of an inch, or 8.47 mm (25.4 / 3). UK and US shoe sizes count in barleycorns, so one whole size is 8.47 mm of length and a half size about 4.2 mm.

The scales count from a fixed starting point, and they measure the last, the form a shoe is built on, which is longer than the foot. On the last, a UK adult size is 3 × length in inches - 25.

On a size chart

Starting from the foot instead, Ruban Size's engine adds two barleycorns (16.9 mm), the allowance of the Brannock device. That gives:

• UK = 3 × foot length in inches - 23
• US men's = UK + 1
• US women's = UK + 2
• Children: UK = 3 × foot length in inches - 10 and US = 3 × foot length in inches - 9.75, rolling over into adult sizes past UK 13.5

Each result is rounded to the nearest half size. A 26.5 cm foot is 10.43 in: 3 × 10.43 - 23 = 8.3, so UK 8.5, US men's 9.5 and US women's 10.5.

Brands set their own offsets. Ruban Size's Shoes template, written like a typical brand chart, puts US women's one and a half sizes above US men's (US men's 7 next to US women's 8.5), where the formulas put them one apart. Shoe size conversion chart from foot length has the full table.

In Ruban Size

Find my size for shoes asks for the foot length and ends its answer with "Usual equivalents (approximate)". For a 26.5 cm foot: EU 42 · UK 8.5 · US M 9.5 · US W 10.5 · JP 26.5. The shoe size converter runs the same formulas, for adults and children: