Here's how it works:

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Shoe size in the United Kingdom, Ireland, India, Pakistan and South Africa is based on the length of the last [a last is a foot-shaped template] used to make the shoes, measured in barleycorns (1⁄3 inch) starting from the smallest size deemed practical, which is called size zero. It is not formally standardised.

Note that the last is typically longer than the foot heel to toe length by 1⁄2 to 2⁄3 in or 1+1⁄2 to 2 barleycorns, so to determine the shoe size based on actual foot length one must add 2 barleycorns.

A child's size zero is equivalent to 4 inches (a hand = 12 barleycorns = 10.16 cm), and the sizes go up to size 13+1⁄2 (measuring 25+1⁄2 barleycorns, or 8+1⁄2 inches (21.59 cm)). Thus, the calculation for a children's shoe size in the UK is:child shoe size (barleycorns) = 3 × last length (in) − 12equivalent to:child shoe size (barleycorns) ≈ 3 × foot length (in) − 10.

An adult size one is then the next size up (26 barleycorns, or 8+2⁄3 in (22.01 cm)) and each size up continues the progression in barleycorns.[5] The calculation for an adult shoe size in the UK is thus:adult shoe size (barleycorns) = 3 × last length (in) − 25equivalent to:adult shoe size (barleycorns) ≈ 3 × foot length (in) − 23.

Although this sizing standard is nominally for both men and women, some manufacturers use different numbering for women's UK sizing.In Australia and New Zealand, the UK system is followed for men and children's footwear [Essentially the UK system, but they count the UK size zeros as the US size 1 and go from there]. Women's footwear follows the US sizings.

In Mexico, shoes are sized either according to the foot length they are intended to fit, in cm, or alternatively to another variation of the barleycorn system, with sizes calculated approximately as:adult shoe size (barleycorns) = 3 × last length (in) − 25+1⁄2equivalent to:adult shoe size (barleycorns) ≈ 3 × foot length (in) − 23+1⁄2.