Shoe Size Conversion Calculator

Why the scales disagree

All but one shoe scale measures the last — the form a shoe is built around — rather than the foot inside it. That is the root of every conversion problem. The last is longer than the foot by a wearing allowance that is not standardised, so two makers can use the same nominal size for lasts that differ by several millimetres.

The UK and US scales both descend from the barleycorn, a third of an inch, which is where the odd half-size steps come from: one size is only 8.5 mm of length. They differ because they start counting from different zeros. Men's US runs half a size above UK; women's US runs two above. Europe uses the Paris point instead, two thirds of a centimetre, applied to the last, which gives roughly one and a half European sizes per centimetre.

The exception is Mondopoint, ISO 9407, which is simply the foot length in millimetres rounded to the nearest five. It describes the wearer rather than the shoe, which is why it is used for military boots and ski boots, where getting it wrong has consequences beyond discomfort.

A worked example

A foot measures 270 mm from heel to longest toe. That is Mondopoint 270 and 27.0 cm, which is also the Japanese size. Adding the wearing allowance and dividing by the barleycorn gives roughly 33.5 barleycorns of last; subtracting the adult zero of 25 leaves a UK 8.5, and adding the half-size offset gives a US 9. In Paris points the same last is about a European 43.

Note how much rounding sits in that chain. The allowance is a convention, the zero is a convention, and the Paris point conversion produces fractional sizes that get rounded to whatever the maker stocks. The 270 mm is the only figure in the paragraph that was measured.

Where the answer misleads

Brands vary by up to a full size, and often by more between categories from the same maker. Running shoes are routinely bought half a size above dress shoes for the same foot, because feet swell during exercise and toes need clearance at the front. A conversion table cannot know any of this.

The practical approach is to convert once to centimetres and then shop in centimetres wherever the maker publishes them, which most athletic brands and most Japanese brands do. That skips every convention in the chain and compares your foot directly against the last.

Feet are also not a matched pair. A difference of several millimetres between left and right is ordinary, and the larger foot decides the size. Measuring one foot and assuming the other matches is how people end up with a shoe that fits on one side only.