EU shoe size to US: why no conversion chart can be exact

Why there is no exact EU to US conversion

Every conversion chart you have used is an approximation, and the reason is not sloppiness. It is that the two systems are not measuring the same thing, and no body has ever reconciled them.

There is an international standard for footwear sizing. ISO 9407:2019 specifies Mondopoint, a system of designating and marking footwear size based on defined measurements of the foot the shoe is intended to fit. It was last reviewed and confirmed in 2026, so it is current.

What it does not do is standardise anything else. The standard's own text says that other sizing systems are not given in the document. EU sizing and US sizing sit outside it.

So the question "what is the correct conversion" has no authority to appeal to. There is a standard, and the systems people actually use are not it.

The part that makes charts unreliable

Here is the detail that explains most sizing frustration.

Some manufacturers mark the foot. Some mark the last, which is the mould the shoe was built around. ISO addresses this directly, noting that some manufacturers express size in centimetres rather than millimetres, and that others give size markings relating to last length rather than foot length, and that neither of these expressions of size conforms to ISO 9407.

A last is longer than the foot that goes in it, and by how much is a design decision. A roomy running shoe and a close-fitting dress shoe built for the same foot have different lasts. If one brand marks the foot and another marks the last, the same number on the box means two different measurements, and no conversion table can reconcile them because the discrepancy is not a constant.

Why regions differ in the first place

ISO gives the reason plainly: the dimension values for each size vary by market, because the statistical distribution of the dimensions of the feet to be fitted differs between regions.

That is a polite way of saying sizing systems were built around local populations. They were not designed to map onto each other, and the grading, which is how much length each step adds, differs as well. Mondopoint's own interval for manufacturers aligning to it is normally 5mm. Other systems step differently, which is why conversions drift at the ends of a range even when they look right in the middle.

What each system is counting

It helps to know that the numbers on a box are not three versions of one measurement. They are different quantities built on different units.

Mondopoint is a foot length in millimetres, of the foot the shoe is meant to fit. It is the only one of the three that states plainly what it measures, which is why ISO standardised it and not the others.

Continental European sizing is conventionally built on the Paris point, a unit of two thirds of a centimetre, and is applied to the length of the last rather than the foot.

UK and US sizing are conventionally built on the barleycorn, an old English unit of a third of an inch, with the US count offset from the UK one and men's and women's scales offset again from each other.

A caveat on those last two, and it is the article's point in miniature. The Paris point and the barleycorn are long-standing trade conventions rather than published standards. There is no ISO document fixing them, no regulator policing them, and no requirement that any brand follow them. They are what the industry has broadly done, which is a weaker thing than a standard and explains why two brands nominally using the same system still disagree.

Different units, most of them applied to the shoe rather than the foot, none of them standardised, and two separate US scales. A single conversion number cannot hold that. It is why charts drift at the extremes of a range, and why one person can be three sizes in three brands without anyone having made an error.

What actually works

Measure your foot, in millimetres, and compare it to the brand's own figures.

That is the whole method, and it works because it sidesteps the problem. Mondopoint is foot-length-based by design, and a brand that publishes the foot length each size fits is giving you the one number that converts cleanly. Many do, usually buried in a size guide rather than on the product page.

Two practical notes. Measure at the end of the day, when feet are at their largest, and measure both, because they commonly differ. Fit the larger one.

Why online makes it worse, and what ISO thought about that

The standard's stated purpose includes reducing confusion for customers, particularly those buying footwear online without trying it on.

That is the right diagnosis. In a shop, the number on the box is a starting point and the fit decides. Online, the number is the entire decision, which puts weight on a figure that was never built to carry it.

It is also why return rates on footwear are what they are, and why brands that publish real foot-length measurements tend to get fewer returns than brands that publish a conversion table.

The honest summary

A conversion chart is a reasonable first guess. It is not a fact, because:

• ISO standardises one system and explicitly leaves the others alone
• size values vary by market by design
• some brands mark the foot and some mark the shoe, and ISO says the second does not conform to the standard

If you want the right size, the number to find is the foot length in millimetres that the brand says each size fits. If a brand will not tell you, that is information too.

For the broader pattern of garment sizing drifting from any fixed measurement, vanity sizing is the same problem in clothing.

Only Mondopoint is standardised. European sizing (the Paris point, two thirds of a centimetre) and UK/US sizing (the barleycorn, a third of an inch) are trade conventions.