Only one system measures your foot
This is the fact that explains every frustration with shoe sizes, and almost no conversion chart mentions it.
Mondopoint — the system Japan and China use, and the one printed on ski boots and military issue — is a foot length in millimetres. Size 265 means a 265 mm foot. Nothing is hidden in it.
US, UK and EU sizes measure the last: the wooden or plastic mould the shoe is built around. A last is longer than the foot it is meant for, because a foot needs room in front of the toes to move as it rolls through a step. How much room is the maker’s decision. A running shoe is built with more; a dress shoe with a long pointed toe may have a great deal more in the shape and none of it usable.
So converting a US size to an EU size is converting between two descriptions of a shoe, not two descriptions of a foot, and the quantity separating shoe from foot is not standardised. That is the whole reason two pairs marked the same size fit differently, and why the difference between brands is routinely a full size — larger than most of the conversions on this page. It is not sloppiness in the charts. The thing being converted genuinely does not have one value.
Which is why this tool asks for a foot length. It is the only input that means the same thing in every country, and everything else is derived from it.
How to measure, in two minutes
Put a sheet of paper on a hard floor with one edge against a wall. Stand on it with your heel touching the wall, wearing the socks you would wear with the shoes. Stand up properly — sitting down changes the reading, because a foot spreads and lengthens under your weight.
Mark the paper at the tip of your longest toe. That is not always the big toe; in a good proportion of people the second toe is longer, and measuring to the big toe out of habit is a common way to end up half a size short. Measure from the wall edge to the mark. That number, in centimetres, is what this page wants.
Three details are worth the extra minute. Measure both feet and use the larger — most people have a noticeable difference and shoes are sold in matched pairs, so the smaller foot has to live in the larger foot’s shoe. Measure in the late afternoon or evening, when feet have swollen to their normal daily maximum; a morning measurement can be several millimetres short of how your foot will be for most of the time you are wearing the shoes. And measure again if it has been a few years — adult feet do lengthen and widen slowly with age, and after pregnancy.
US women’s has two different answers
For the same foot, there are two published US women’s sizes, and they differ by half a size. This is not a rounding disagreement; both conventions are in active use, and which one applies depends on who made the shoe.
The Brannock device— the hinged metal slide in every US shoe shop — has a women’s scale set exactly one size above its men’s scale. That is the retail standard, and it is what you get when someone measures your foot in a shop.
Athletic brands publish a one-and-a-half-size offset instead. A men’s 9 and a women’s 10.5 are the same shoe in most sportswear sizing, which is why unisex models are usually labelled with both numbers and why the gap between them is 1.5 rather than 1.
Half a size is about 4.2 mm of foot, so this is small in absolute terms and still enough to matter at the margin. The practical rule: use the brand’s own chart when there is one, take the athletic figure for trainers and the Brannock figure for dress and work shoes, and treat a unisex listing’s men’s number as the reliable one, since that is usually the size the shoe was actually built to.
Half a size is smaller than your own feet differ
A whole US or UK size is one barleycorn — a third of an inch, about 8.5 mm. Half a size is therefore about 4.2 mm. That is the finest resolution the system has, and it is finer than the foot being measured is stable.
Most people’s two feet differ in length by more than that. Feet swell through the day by a comparable amount, and more in heat, after exercise, or on a long flight. So the difference between a 9 and a 9.5 is smaller than the difference between your left foot and your right, and smaller than the difference between your own foot at breakfast and at dinner.
Two things follow. Agonising over a half size in the abstract is chasing a number below the noise floor of the measurement — the way to settle it is to put both on. And because it is below the noise floor, it is the wrong thing to blame when a shoe does not fit: the usual culprits are width, the shape of the toe box, and the volume through the instep, none of which any of these numbers describe.
When a foot does fall between two sizes, take the larger. A shoe that is slightly long can be managed with a thicker sock, a lacing change, or an insole. A shoe that is short cannot be fixed by anything, and the damage from wearing it is to the foot rather than the shoe.
Where each system is used
US and Canada use the barleycorn scale with separate men’s and women’s numbering. The UK and Ireland use the same barleycorn step from a different starting point, which puts UK sizes one below US men’s throughout — a UK 8 is a US men’s 9. India largely follows the UK scale, a legacy of the same shoemaking tradition.
Continental Europe uses Paris points of two thirds of a centimetre. Because a Paris point is smaller than a barleycorn, the two scales drift against each other: across 40 mm of foot the US scale gains about 4.7 sizes and the EU scale gains 6. That is why EU half sizes do not line up with US half sizes, why many European brands do not offer half sizes at all, and why a conversion that is right at one end of the range is off at the other.
Japan and China use mondopoint, in centimetres or millimetres. It is the least confusing of the four and the most useful number to know about yourself, because every other system can be derived from it while none of them can be reliably derived from each other.
If you buy across regions often, the thing worth memorising is not a conversion. It is your own foot length in centimetres.
Sources and methodology
The formulas here come from the two units the scales are actually built on — the barleycorn, one third of an inch, and the Paris point, two thirds of a centimetre — rather than from a transcribed chart, so every figure is derived and can be checked. What the sources cannot settle is the part that matters most: the toe allowance inside the last is a maker’s decision, not a standard, so the conversions below are the arithmetic relationship between the scales and not a promise about any particular shoe.