Why every shoe conversion table disagrees with the next one
Shoe sizes are not measurements; they are numbers on 3 different scales, each invented for a different job. A Chinese listing, a European listing and a British listing can describe the same foot, and the tables that convert between them disagree because they round at different points.
The useful model has 2 layers. The first is your foot in millimetres, which you can establish on a sheet of paper in about 2 minutes. The second is whatever number a brand prints on a box, which is a convention with an allowance built into it, and no table removes that allowance, which is the same point the [sizing topic](/table/sizing/) makes about garment charts.
Everything below is structure rather than conversion data, and there are 4 scales to keep apart. Where a figure appears it is arithmetic from the definition of the unit, and the chart published for the specific shoe still governs, because brands shape their lasts differently even when they share a numbering system.
One more thing belongs in the frame. A conversion table is a claim about a population of shoes rather than about the pair in front of you, so it can be right on average and still wrong about the 1 order you are placing.
One foot length in mm, taken standing and late in the day, decides more than any grid. Do that before reading any further.
China and Japan: the number is a foot length, not a shoe
Chinese and Japanese listings usually print 2 numbers, and the useful one is the second. The first is a European-style 码 that matches what you would see in a shop; the second is 脚长, the foot length in millimetres, and that figure is the one that survives translation.
Mondopoint, standardised as ISO 9407, formalises exactly that idea by expressing a size as a foot length in millimetres. Where a listing says 250, it is describing a 25 cm foot rather than a 25 cm shoe, and the gap between those 2 readings is the source of most of the confusion.
- What the number counts: foot length in millimetres, taken while standing.
- Where you meet it: Chinese and Japanese listings, and running shoes that print a mm figure beside the EU size.
- What to check: whether the listing gives 脚长 (foot length) or 鞋内长 (insole length), because a 250 mm insole is not a 250 mm foot.
Then measure your own foot properly on a sheet of paper against a wall, with your weight on it. Most feet measure slightly longer late in the day than early in the morning, and on a scale where a step is 5 mm, that difference is worth knowing about before you choose a size.
Record the number once and keep 1 copy of it. A foot length in mm is stable for years in an adult, while the size printed on a box is stable only for as long as you keep buying the same brand, which makes the mm figure the better thing to write down.
Europe: the number describes a last, not a foot
A European size counts Paris points, and 1 Paris point is 2/3 of a centimetre. What the number describes is the last, the foot-shaped form the shoe is built on, and the last is longer than the foot by an allowance the brand chooses.
The arithmetic is short. A size 42 is 42 Paris points, which is 28 cm of last length, and the foot that fits it is naturally shorter than that. Because the allowance varies between brands, 2 makers can both be correct about their 42 while the shoes fit differently on the same foot.
This is where most tables quietly break. A table that converts a 25 cm foot straight to a 40 is applying 1 brand allowance to every brand in the world, and rounding to the nearest half step hides that assumption inside a tidy grid.
- What the number counts: last length, in units of 2/3 cm.
- Where you meet it: European listings, and the first number on most Chinese listings.
- What to check: whether the brand publishes an insole length, since that is a figure you can verify against a shoe you own.
The UK and the US: one unit, four separate scales
British and American sizes count barleycorns, and a barleycorn is 1/3 of an inch. The unit is identical on both sides of the Atlantic, and the scales are not, because they begin counting from different zero points.
That is why a UK 8 and a US 9 are often printed as equivalents in the mens range while the same 2 labels describe different lengths in the womens range. Adults, children, men and women are 4 separate scales built on the same unit, and a table that ignores the split is a table to put down.
Width adds a 2nd variable that no number on a box carries. American listings often run from a narrow fitting through to a wide one, British listings use their own fitting letters, and a table that handles length only cannot tell you which of them you are ordering.
- What the number counts: barleycorns of 1/3 inch each, from a zero point that differs by scale.
- Where you meet it: UK and US listings, and the size printed inside shoes you already own.
- What to check: whether the label says mens, womens or youth, because one figure means 3 different lengths.
Children add a 4th scale on the same unit, which is why youth sizes are converted wrongly more often than any others. A youth figure and an adult figure drawn from the same range of numbers describe feet that sit 2 scales apart.
Four break points, and what to reach for instead
Four breakages cover most of the failures people run into, and all 4 of them are structural problems rather than badly made tables.
| Break point | Why it happens | What to reach for |
|---|---|---|
| Foot length to shoe size | the table applies 1 brand allowance to every brand | an insole length measured inside a shoe you own |
| Half steps and rounding | each scale rounds at a different point | treat a half step as a range rather than a fixed length |
| Mens, womens, youth labels | the same unit is counted from different zero points | the label on the shoe rather than the number alone |
| Width | most tables carry length and nothing else | the width in cm across the ball of the foot |
| Thick insoles and boots | internal volume changes the fit at the same length | the insole figure, not the figure printed on the box |
The second row is the subtler one. A half step is roughly 3 mm on the European scale and roughly 4 mm on the barleycorn scales, which is smaller than the difference between 2 brands sharing a nominal size, so a conversion that lands on a half step is really telling you that you are between 2 answers.
Returns mostly come from the last row. A boot with a thick insole can be 1 cm shorter inside than another boot of the same outside length, and the number on the box says nothing about it, which is why the [size comparison tool](/rig/size-compare/) asks for an insole figure before it asks for a size.
What to ask a seller for, and what to do with the answer
Ask for 3 things and the conversion problem largely disappears. None of them is a size, and all 3 are measurements a seller can take in about a minute with the shoe in hand.
- The insole length in cm, measured inside a shoe of the size you are considering.
- The width across the ball of the foot, measured inside that same shoe.
- The size printed on the box, so you can compare it against the size you already wear in that brand.
Then compare with a shoe you own. Take the insole out of a pair that fits, measure it, and treat that figure as your anchor; 2 or 3 anchors from different brands cover most of what you buy, and recording them takes about 10 minutes in total.
Where a seller cannot measure anything, buy 1 pair before buying 3. A single pair establishes your anchor for that seller, and the freight on 1 pair costs less than the freight on 3 pairs that all turn out to be the wrong length, which is the same order of thinking as [handing a link over](/steps/paste-link/) before committing to a full basket.
Keep those 3 answers with the order, as you would keep any other measurement. A seller who has supplied an insole figure once will usually supply it again, and the second order then takes 1 message instead of 3.
Two insole measurements from shoes you own are worth more than any conversion grid, because they carry the brand allowance inside them.
Worked example: converting in 2 steps — foot length in millimetres, then the brand allowance for the last — removes the rounding error that appears when a table converts a metre-based Chinese figure straight into a barleycorn UK size, which is where most half-size disputes start.
This note explains the mechanics; the list is where the items are.