Shoe Sizes Don't Actually Convert Cleanly Across Countries
Why a shoe size conversion chart is a reference, not a formula, and why brands routinely disagree with it.
Published May 13, 2026
Unlike converting meters to feet, which has one mathematically correct answer, converting a US shoe size to UK or EU sizing doesn't have a single correct answer, because shoe sizing was never standardized the way length units were. What exists instead are widely-used conventions that most brands roughly follow and regularly deviate from.
Why there's no universal formula
Different countries developed their own shoe-sizing systems independently, based on different measurement conventions (some based on foot length in specific increments, others on different base units entirely), long before there was any coordination between them. Conversion charts are an attempt to map between systems after the fact, not a shared underlying formula both systems were built from.
Why brands disagree with the chart, and with each other
Shoe sizing also depends on the actual last (the physical mold a shoe is built around), which varies by brand, by shoe style, and sometimes by specific model. A size 9 running shoe and a size 9 dress shoe from different brands can fit meaningfully differently even though both claim the same size, because the size number was never tied to one universal, physically fixed measurement.
How to actually use a size conversion chart
As a starting point for what size to try, not a guarantee. When a brand publishes its own size chart, that specific chart is more reliable than a general conversion for that brand's products, since it reflects the actual lasts that brand uses rather than an industry-wide average.