How to Use the Hat Size Converter
Buying a hat online gets confusing fast — some brands list a fitted number like 7 1/8, others use S/M/L labels, and a few just print centimeters. This calculator takes your head circumference in inches and converts it to all three at once: the US fitted size, a cm reference, and a letter label.
The US fitted size is calculated by dividing your head circumference (in inches) by pi (π ≈ 3.14), then rounding to the nearest eighth of an inch — the standard sizing method most American and European hatmakers use. For example, a 22.8 in circumference divided by π comes out to about 7.26, which rounds to the nearest labeled size of 7 1/4. This math reflects the fact that a hat wraps around your head in a roughly circular shape, so the size number represents the hat's inner diameter rather than its full circumference.
To get an accurate measurement, wrap a soft tape measure about 1 inch above your eyebrows, running across the back of your head at its widest point — usually just above the ears. Keep the tape snug but not tight; too loose and your result will read smaller than your actual size, too tight and it'll read larger. A finger's width of slack under the tape is a good rule of thumb.
Frequently Asked Questions
Wrap a tape measure about 1 inch above your eyebrows, across the widest part of your head just above your ears, to get an accurate circumference.
Head circumference in inches is divided by pi (Ï€) and rounded to the nearest eighth of an inch. For example, a 22.5 in circumference works out to roughly a 7 1/8 fitted size.