Why “US Men 9” is not one number
The US and UK scales are both built on the barleycorn — one third of an inch, the length of a grain of barley, fixed by Edward II in 1324 as the standard for three barleycorns to an inch. Every whole size step is 8.47 mm. The UK scale starts at 3 × foot length in inches − 23, and the US men’s scale is the same line shifted up.
Shifted by how much is where it falls apart. Wikipedia and Nike put US Men at UK + 1. adidas publishes charts where US 10 is UK 9.5 — an offset of 0.5. Both are real, both are current, and neither is wrong. This tool uses +1 as the default because it matches the more widely published convention, and shows the foot length in centimetres beside it so you can check against any brand chart yourself.
The women’s toggle, and why it exists
US women’s sizing has two competing conversions, and this is the single most common source of a shoe that arrives too small.
FIA (men + 1). The Footwear Industries of America standard. A women’s 8 is a men’s 7. adidas measures that as a 24.6 cm foot, which is exactly what the formula gives.
Common scale (men + 1.5). Also widespread. Nike lists a women’s 8 as a men’s 6.5.
That is a full half-size gap between two mainstream readings of the same label. The toggle defaults to FIA. If a brand chart disagrees with what you see here, the brand chart wins — but now you know which convention it is using.
No brand is consistent on all three axes
This is worth stating plainly, because size charts rarely admit it.
At a 27.0 cm foot the formula gives US M 10 / UK 9 / EU 42.5. Nike’s own chart says US M 9 / UK 8 / EU 42.5 — the EU axis matches, the US and UK axes are a full size off. At 26.3 cm adidas says US M 9 / EU 42⅔ where the formula gives US M 8.5 / EU 41.5 — now the US axis is close and the EU axis is over a size out. adidas also uses thirds (42⅔, 43⅓), which do not sit on the half-size grid at all.
So label-to-label conversion carries a built-in error of half a size to a full size, no matter who does the arithmetic. That is why this tool asks for a measurement rather than a size.
Measure it properly, once
Measure in the evening — feet swell up to 5% through the day. Wear your usual socks. Tape paper to a wall, put your heel against the wall, and stand with your weight on the foot; measuring seated reads short. Hold the pencil upright and mark the longest toe, which for many people is not the big toe. Measure both feet, use the larger, and don’t worry about a 5 mm difference between them.
Then check that number against the cm column of the brand’s own chart. Every reliable chart has one.