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Shoe Size Conversion Basics: Why Sizes Never Quite Line Up

Shopping for shoes online often runs into mismatched "US size" and "EU size" labels β€” understanding the logic behind a few sizing systems can help you avoid a bad fit.

Some countries size directly by foot length in millimeters

Some regions size shoes directly by foot length in millimeters (or centimeters) β€” a 245mm foot gets a size 245, for example. This approach is fairly intuitive: a bigger number simply means a longer foot.

EU sizing uses a unit called the Paris point

EU sizing is based on a unit called the "Paris point," where each size step corresponds to roughly two-thirds of a centimeter of foot length. Because that ratio is not a round number of centimeters, EU sizes do not map onto exact foot-length millimeters in a particularly intuitive way.

US and UK sizing are not the same, even though both use similar units

US and UK sizing both commonly use fractional inch-based units, but they start counting from different baseline points β€” so the US and UK size for the same actual shoe usually differ by roughly one whole number, and the two cannot be used interchangeably.

US sizing itself splits into men's and women's scales

Within the US system, the same actual shoe length is labeled with a different number depending on whether it is a men's or women's size β€” women's numbers typically run higher than the equivalent men's number, so it is worth confirming which scale a listing is using before buying.

The same size number can still fit differently between brands

Even under the exact same sizing system and number, the actual internal length can differ between brands and shoe lasts β€” which is exactly why a size conversion chart can only ever be a rough reference, not a guarantee of a perfect fit.

The most reliable method: measure your foot, then check the brand's own chart

Rather than relying on a generic conversion formula, it is more reliable to measure your foot length (in millimeters) in the afternoon or evening, when feet are slightly more swollen, then compare that against the specific brand's own official size chart β€” especially useful for cross-border or overseas shoe shopping, where it can meaningfully cut down on size-related returns.

Why a shoe size chart is only ever a rough guide

A shoe size conversion chart is typically built from the commonly observed correspondence between systems, but because of the EU "Paris point" ratio, the men's/women's split within US sizing, and differences in brand lasts and shoe width, the same converted number can still fit noticeably differently on an actual product β€” especially near the edges of a size range.

Foot length and foot width matter equally

Most sizing systems only reflect one dimension β€” length β€” but foot width and arch height also affect how a shoe actually feels. Two shoes marked as the "correct" size can feel completely different if their lasts are cut narrower or wider, which is why it is worth paying attention to width and last shape when shopping online.

Frequently Asked Questions

I bought shoes online using a size conversion chart, but they still run large or small β€” why?

A conversion chart shows the commonly observed correspondence between systems, not a specific brand's exact measurements. Differences in last shape between brands, and even between product lines from the same brand, are common β€” it is better to check the brand's own official size chart or a precise foot-length-in-centimeters comparison rather than relying only on a generic conversion chart.

Is there a single international shoe size standard everyone uses?

Not yet. The International Organization for Standardization has proposed a foot-length-based system called Mondopoint, which is used by some athletic and military footwear, but US, EU, and UK sizing all remain in common everyday use and have not been unified into a single global standard.