About the Hat Size Calculator
This calculator converts a head circumference measurement into a standard hat size using the traditional hat-block sizing formula: hat size equals head circumference divided by pi (3.14159), rounded to the nearest 1/8 inch. That formula exists because a hat size number is meant to describe the approximate inside diameter of the hat — the wooden block a milliner shapes felt or straw around — and circumference divided by pi is exactly how you get a diameter from a circle's circumference. The calculator reports that number as a US/UK size, restates your circumference in metric centimeters, buckets it into a letter size (XS through XXL), and shows the circumference range that would still round to the same size.
How to measure your head
- Use a flexible cloth or sewing tape measure, not a rigid ruler
- Wrap it around your head about 1/8 inch above your eyebrows and ears — the widest part of most heads
- Keep the tape level and snug against your head without pulling it tight or leaving it loose
- Take two or three measurements and use the largest reading for the best fit
Reading your results
The US/UK size is expressed as a whole number plus an eighth (like 7 1/8), which is how most North American and British hat brands label sizes inside the hat. The metric size is simply your head circumference in centimeters, which is how European and many Asian brands label sizes — no pi conversion needed there. The letter size groups nearby numeric sizes together for brands that only stock S/M/L/XL, and the fit range shows how much your head circumference could vary and still land on the size you calculated, since hat sizes step in 1/8-inch increments rather than a continuous scale.
Practical notes
If your measurement falls exactly between two sizes, most hatters recommend sizing up slightly, since a hat that is a touch loose is usually more comfortable than one that pinches — many hats also include an adjustable sweatband that can take up a small amount of slack. Fitted hats, fedoras, and other structured styles rely on this sizing system closely; stretchy knit caps, snapbacks with adjustable straps, and "one size fits most" hats are more forgiving of small measurement differences.