Converting Shoe Sizes Between US, UK, and EU Correctly
US, UK, and EU shoe sizes each use a different numbering scale based on different underlying measurement systems — a US men's size is typically about 1 size larger than the equivalent UK size, and EU sizes use a completely different numbering system based on foot length in a unit called Paris points, which is why the conversion isn't a simple, uniform offset across the whole range.
Shoe size conversion has real, documented general relationships, but it's worth understanding why it's not as simple as a single fixed number to add or subtract.
US and UK sizes
For men's shoes, a US size is commonly about 1 size larger than the equivalent UK size (a US size 9 is roughly a UK size 8) — women's sizing has its own separate relationship (commonly about 1.5-2 sizes different between US and UK women's scales), which is a real, distinct conversion from the men's relationship.
EU sizes
EU sizing uses an entirely different numbering system based on foot length measured in Paris points (each point equal to 2/3 of a centimeter) — this produces size numbers in a completely different range (a US 9 is roughly an EU 42), and the relationship to US or UK sizes isn't a simple fixed offset the way US-to-UK roughly is.
Why the conversion isn't perfectly uniform across the whole size range
Because different sizing systems were developed independently with somewhat different underlying measurement logic, the exact relationship between two scales can shift slightly at different points across the size range — which is why a proper conversion chart (or a tool using one) is more reliable than assuming a single fixed offset applies uniformly to every size.