How is hat size calculated from head circumference?

The US hat size equals the head circumference in inches divided by pi (about 3.1416), rounded to the nearest eighth. UK sizes run one eighth smaller, and EU/metric sizes equal the circumference in whole centimetres.

Where should I measure for a hat?

Measure around the widest part of your head, roughly one centimetre above your ears and eyebrows, keeping the tape level all the way around.

What head circumference is a medium hat?

In this tool the letter size is based on centimetres: under 54 cm is XS, 54–55 is S, 56–57 is M, 58–59 is L, 60–61 is XL, and 62 cm or more is XXL. So a 57 cm head is a Medium.

What is the average adult hat size?

For men the average head circumference is around 57–58 cm (US 7 1/8 to 7 1/4); for women it is around 55–56 cm (US 6 7/8 to 7). Children and teens vary widely, so always measure rather than assume.

Why does the same head fit different hat numbers in the US and UK?

Both systems divide circumference (in inches) by pi, but UK sizes are conventionally one-eighth smaller than US sizes for the same head, so a US 7 1/4 is a UK 7 1/8.

Is my measurement stored?

No. The calculation happens entirely in your browser and nothing is sent to a server.

Hat Size Converter — Gera Tools

Hat size converter

Hat sizes use four different systems, and they all start from one number: your head circumference. Enter that measurement in centimetres or inches and this tool gives you the matching US, UK, EU/metric and letter (S–XL) hat size at once — useful when buying from international sellers whose size labels do not match your own.

How it works

The tool first converts your measurement to inches and centimetres. The US hat size is the circumference in inches divided by π (≈ 3.1416), rounded to the nearest eighth — so a 22.5-inch head gives 22.5 / 3.1416 ≈ 7.16, rounded to 7 1/8. The UK size is one eighth smaller than the US size. The EU/metric size is simply the circumference in whole centimetres. The letter size is banded by centimetres (S = 54–55 cm, M = 56–57 cm, L = 58–59 cm, and so on).

Example

For a head circumference of 58 cm (≈ 22.83 in):

US: 22.83 / π ≈ 7.27 → 7 1/4
UK: 7 1/4 − 1/8 → 7 1/8
EU / metric: 58
Letter: L

Circumference	US	UK	EU	Letter
55 cm	6 7/8	6 3/4	55	S
57 cm	7 1/8	7	57	M
58 cm	7 1/4	7 1/8	58	L
60 cm	7 1/2	7 3/8	60	XL

Everything is computed in your browser; nothing is uploaded.