Stand on a flat surface and follow the measurement method in the product’s size guide. Measure both feet rather than assuming they are identical.
Compare the correct measurement
Foot length, insole length and recommended foot length are not interchangeable. Check which measurement the chart provides, and whether it gives additional guidance about width.
Think about the whole fit
The socks you wear and the shape of the toe box can affect comfort. A conversion between US, UK and EU sizes may not match every brand exactly.
If your measurement falls between sizes, consult the guidance for that style. Avoid relying on a generic rule to always size up or down.