What this calculator does
Hat size is based on head circumference. This calculator estimates centimetre size, UK hat size and a general S to XL label.
Formula used
Head circumference is converted from centimetres to inches. UK hat size is circumference in inches divided by pi, rounded to the nearest eighth.
How to read the result
Hat fit also depends on shape, hair, material and how the hat is intended to sit.
Assumptions
- Measures around the widest part of the head.
- Returns an approximate UK hat size.
- General S to XL labels vary by brand.
Sources and checks
This calculator uses a standard public formula. Where rules or thresholds can change, source links are listed on the relevant page.
Frequently asked questions
Where should I measure my head?
Measure around the forehead and widest part of the back of the head, keeping the tape level and comfortable.