What this calculator does
Hat size works out hat size number from head circumference, plus the letter size. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaUS hat size = head circumference (in) / π; letter size from standard cm bands
The inputs explained
| Field | What to enter |
|---|---|
| Head circumference (cm) | A number, measured in cm. Starts at 57. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with head circumference
Every other input is held at the calculator’s starting values while head circumference varies. Select any row to load that scenario into the calculator.