Hat size converter
Measure around your head, type the number, and see the matching fitted hat sizes. The conversion uses the common diameter rule, which is an approximation, so check the maker's own chart before buying.
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How to use it
- Measure your head and choose cm or inches, or start from a US or UK size.
- Type the number. Sizes can be written as 7 1/4 or 7.25.
- Read the circumference, the nearest US and UK fitted sizes, the EU number and an approximate S/M/L.
How the conversion works
| Value |
Rule |
| Inches |
cm ÷ 2.54 |
| US fitted size |
inches ÷ π, rounded to the nearest 1/8 |
| UK fitted size |
US size − 1/8 |
| EU size |
circumference in cm, rounded |
| S / M / L |
rule-of-thumb bands of the circumference: S 54–55.9, M 56–57.9, L 58–59.9 cm |
Worked example
A head of 57 cm is 57 ÷ 2.54 = 22.44 in. Divide by π: 22.44 ÷ 3.1416 = 7.14, so the nearest US size is 7 1/8, the UK size is 7 and the EU size is 57. It falls in the approximate M band.
Tips
- Between two sizes, take the larger for a stiff hat and the smaller for a soft cap you can adjust.
- Wool and felt can stretch slightly; stiff hats and helmets do not.
- Measure twice, and keep the tape in the same place.
Frequently asked questions
How do I measure my head for a hat?
Wrap a soft tape measure around your head about 1 cm (half an inch) above the eyebrows and ears, where a hat sits. Keep the tape level and snug, not tight, and read the value.
How is the US hat size calculated?
By the common diameter rule, the US fitted size is the head circumference in inches divided by π, the diameter of a round head. A 22.5 in (57.2 cm) head gives 22.5 ÷ π ≈ 7.16, so 7 1/8. It is a convention, not a standard, and makers may differ.
What is the difference between US and UK hat sizes?
In the common convention the UK size is the US size minus 1/8. A US 7 1/4 is a UK 7 1/8. The UK size is the same circumference, just labelled one eighth lower.
What is an EU hat size?
Usually the head circumference in whole centimetres, so a 57 cm head is EU 57. Some brands use it as a size label, with S/M/L covering a few of them.
Which S, M and L does 57 cm mean?
In this tool 56 to 57.9 cm is labelled M, but the bands are a rule of thumb. One brand's M can be another's L, so use the circumference when the maker lists it.