Four systems, four different rulers
Shoe sizes look like simple numbers, but each system counts with a different unit and starts from a different point. That's why you can't add a fixed number and get a reliable answer.
US and UK sizes step in barleycorns, an old English unit of one third of an inch, about 8.5 mm. EU sizes step in Paris points of two thirds of a centimeter, about 6.7 mm. Mondopoint skips the step idea and just states foot length in millimeters.
- US and UK: 1 size = 1/3 inch (about 8.5 mm)
- EU: 1 size = 2/3 cm (about 6.7 mm)
- Mondopoint: foot length in mm
US and UK sizes
The two use the same step size but a different starting point, so the UK number is lower. The usual rule of thumb is that US men's sizes are one higher than UK sizes.
Brand charts don't always follow it. Nike's adult chart matches the rule: US men's 9 is UK 8. Adidas puts the gap at half a size, so its US men's 9 is UK 8.5. If you're buying from a UK site, look up the brand's own chart.
EU sizes and their odd fractions
Because a Paris point is smaller than a barleycorn, EU numbers climb faster. One US size covers about one and a third EU sizes. Some brands show that honestly with thirds: Adidas lists EU 42, 42 2/3, 43 1/3 and 44. Others, like Nike, round to halves.
That's why the same US 9 shows up as EU 42 2/3 on one site and EU 42.5 on another. Neither is wrong. They're two brands rounding the same foot differently.
Why women's and men's sizes don't line up
In the US, women's and men's sizes are separate scales. The common rule says women's sizes are men's plus one, and some charts use plus one and a half instead. Adidas uses one. Nike uses one and a half.
Length isn't the only difference. In US sizing the standard width is D for men and B for women. So a woman switching to a men's size may find the toe box roomy, and the reverse can pinch. EU sizes are meant to be the same for everyone, which makes them handy for comparing, though some brands still number women's UK sizes differently.
Mondopoint: the measurement-first system
Mondopoint, covered by ISO 9407, labels shoes by foot length and width in millimeters. A 263 mm foot is labeled 263. It's common on ski boots, skates and ballet pointe shoes, and NATO uses it for military footwear.
It's the cleanest system because it describes your foot, not a historic scale. If you know your foot length in mm, you have most of what any chart needs.
How to convert without guessing
Measure first. Stand on paper, mark your heel and longest toe, and measure both feet. Use the longer one.
Then use the chart for the brand you're buying, and read one row across. Our shoe size converter does this with the Adidas and Nike adult charts. It also shows the rows on either side, so you can see how close you are to the next size.
Frequently asked questions
Why is my shoe size different in every brand?
Each brand builds shoes on its own lasts and publishes its own chart. A 26.3 cm foot can be labeled differently by two brands even when both are measured correctly.
Are EU shoe sizes unisex?
In principle, yes. The Continental European system uses one scale for men, women and children, while the US uses separate men's and women's scales. UK sizes are nominally unisex too, but some brands number women's UK sizes differently.
What is a barleycorn in shoe sizing?
It's an old English unit equal to one third of an inch, about 8.5 mm. Each full US or UK shoe size is one barleycorn longer than the last.
What is the standard shoe width?
In North American sizing, D is the standard width for men and B is the standard for women. A shoe without a width letter is usually one of those, which is why a men's shoe feels roomier at the same length.
References
These sources support the method or guidance used for Shoe Size Conversion Calculator. Verify time-sensitive rules at the source.
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