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 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.

Is my measurement stored?

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

Hat Size Converter

Turn head circumference into US, UK, EU and S–XL hat sizes.

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.

The US hat size is the circumference in inches divided by π; UK sizes sit one eighth below that, and the metric size is simply the circumference in whole centimetres. Everything is computed in your browser.