maratoolHow to find your hat size from head circumference
Measure your head, type the number — read US, UK, cm, inch, and letter sizes.
Fitted hats use a numbering system most people meet exactly once — while buying a hat. The Hat Size Converter turns a tape measurement into US eighths, UK sizes, and S/M/L letters.
How it works
- Measure — around the head, 1 cm above the ears.
- Enter — cm, inches, or a known US/UK size.
- Read — every system, including letter sizes.
The formula
Traditional millinery sizing divides head circumference in inches by π and rounds to the nearest eighth: a 22.75″ (57.8 cm) head → 22.75/3.1416 = 7.24 → size 7 1/4 US. UK sizes sit exactly one eighth lower.
Letter sizes
Common bands: S = 55–56 cm, M = 57–58 cm, L = 59–60 cm, XL = 61–62 cm. Between sizes? Go up — most fitted hats shrink slightly and sweatbands can be padded.
Sources & methodology
The formula is arithmetic stated inline above — no external dataset is involved. Everything runs client-side in your browser; nothing is uploaded.
More conversion guides
- How to convert clothing sizes between US, EU, and UK
- How to read tire sizes and compare 205/55R16 to another size
- How to calculate TV and monitor dimensions from the diagonal
Or browse everything at once in the new unit & sizing converters roundup.