Shoe scales are built from different units entirely, which is why the arithmetic never feels clean.
Different base units
The EU (Paris point) scale increases in steps of two-thirds of a centimetre. US and UK scales use the barleycorn, one-third of an inch. Because the increments differ, no single number converts one scale to the other perfectly across the full range.
Men's and women's scales differ
US women's sizes typically run about 1.5 numbers above US men's for the same foot length, while EU sizing uses one continuous scale for both. Converting a women's US size to EU without accounting for that offset is a common source of error.
Measure in centimetres instead
The most reliable approach is to measure foot length in centimetres and convert from there. Stand on paper, mark heel and longest toe, measure the distance, and add roughly 0.5-1.0 cm of wearing room depending on shoe type.
Fit factors beyond length
Width, instep height and last shape vary between brands and often matter more than a half size. Running shoes are usually bought slightly longer than dress shoes, and boots need extra allowance for thick socks.
Frequently Asked Questions
Why do two EU 42 shoes fit differently?
Each manufacturer builds shoes on its own last. The label reflects the last, not a standardised interior length.
How much room should I leave?
Around a thumb's width at the toe for everyday shoes, slightly more for running.