How this calculator works
Sock sizes are sold as letters, and each letter covers a band of shoe sizes rather than a single number. The calculator first converts the size you entered into a US shoe size. UK women's sizes run about two and a half sizes below US women's; UK men's sizes run about one size below US men's. European sizes need a different offset for women and men. The result is the sock letter whose band contains that shoe size. Children's socks are usually sold by age band, which is why the child version asks for an age instead of a shoe size.
Formula
US women from UK = UK + 2.5. US men from UK = UK + 1. US women from EU = EU - 30.5. US men from EU = EU - 33. Women's sock letters: S 4-6.5, M 7-9.5, L 10-12.5. Men's: S 6-8.5, M 9-11.5, L 12-14.5, XL 15-18.
Worked example
A women's UK shoe size 5.5 converts to US 8, which falls in the women's M band of US 7 to 9.5. A men's EU size 42 converts to about US 9, which is men's M. A child aged 4 falls in the 3 to 6 age band, which covers US child shoe sizes 7 to 10.
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Frequently asked questions
Are sock sizes the same in every brand?
No. Sock labels vary between letter sizes, numeric ranges and age bands, and a small in one brand can be a medium in another. Compare the shoe range on the label with the result rather than the letter alone.
Why do children's socks use age?
Children's feet grow at different rates, so a shoe size is more accurate if you have one. Age bands are a convenience for gifts and for buying ahead, but they are broad, and a child at the edge of a band may need the next size.
What about compression or sports socks?
Compression socks are sized by calf circumference and shoe size together, not by the sock letter. Sports socks often use the same shoe range but a firmer knit, so if you are between sizes choose the larger one for comfort.
Does the sock shrink?
Cotton and wool socks can shrink a little in a hot wash. If the size is at the top of the band, wash cool or choose the next size up. Merino and synthetic blends usually shrink less than pure cotton.