Your foot is one size. Yet a shoebox might call it a UK 8, an EU 42, and a US 9 (or 8.5, depending). Three numbers, one foot. It's confusing, but each system makes sense once you know what it's actually counting.
Three Systems, Three Starting Points
UK Sizing
The UK system is based on the length of the shoe's last (the mould it's built on), traditionally measured in barleycorns — an old unit of one third of an inch. UK sizes increase in steps of one barleycorn. It starts counting from a child's smallest size and runs up, so adult sizes are a continuation of the same scale.
EU Sizing
The European system (the Paris point) measures in units of two thirds of a centimetre. The number roughly reflects the length of the last in those units. Because the unit is smaller, EU sizes climb in finer-looking steps and the numbers are larger.
US Sizing
The US system is closely related to the UK one (it also uses the barleycorn step) but starts counting from a different point, which is why US numbers run a little ahead of UK numbers for the same foot. US sizing also differs between men's and women's scales, adding another layer.
How They Compare
Because the systems start from different zero points and, in the EU's case, use a different unit entirely, there's no single tidy formula you can do in your head. Here's the rough relationship for adults:
| UK | EU (approx) | US Men (approx) |
|---|---|---|
| 6 | 39–40 | 7 |
| 7 | 40–41 | 8 |
| 8 | 42 | 9 |
| 9 | 43 | 10 |
| 10 | 44–45 | 11 |
These are approximate because, as you'll see in the next point, brands vary. Treat any conversion chart as a starting point and check against your actual foot length where you can.
Men's vs Women's Numbers
To add one more wrinkle: the US and UK systems handle men's and women's sizing differently, so a women's US size and a men's US size with the same number are not the same length. When converting, make sure you're reading the right column for the right scale.
The Bottom Line
UK, EU and US sizes are three different rulers laid against the same foot, each starting in a different place. You don't need to memorise the maths — measure your foot length, convert from that, and you'll cut through all three systems at once.