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Hat Size Calculator

Convert your head circumference into a US fitted hat size, plus EU, UK and S/M/L alpha sizes. Works in inches or centimeters.

Hat sizes are approximate and vary by brand and style — use this as a guide and check the maker's chart when possible.

About this tool

This calculator turns a single head-circumference measurement into hat sizes across the common systems, entirely in your browser — nothing is uploaded. Wrap a soft tape around the widest part of your head, just above the ears and eyebrows, and enter the number in inches or centimeters.

The method mirrors how fitted hats are numbered: a US fitted size is the head diameter in inches, so it is the circumference divided by π (about 3.1416) and then rounded to the nearest eighth. EU sizes are simply the circumference in centimeters rounded to the nearest whole number, UK fitted sizes run about one-eighth smaller than US, and the S–XXL alpha size comes from centimeter brackets (roughly S 55, M 57, L 59, XL 61 cm).

Use the US fitted number when buying structured or fitted caps, and the alpha size for beanies and flex-fit styles. If your measurement lands between two sizes, size up for comfort.

Frequently asked questions

How do I measure my head for a hat?
Run a flexible tape measure around the largest part of your head — about 1 cm above the ears and across the middle of the forehead — keeping it level. That circumference is what you enter.
Why is the US size shown as a fraction like 7 1/8?
US fitted hats are numbered by the head's diameter in inches, which naturally lands on eighths. The tool rounds circumference ÷ π to the nearest eighth and shows it as a mixed fraction.
Are these sizes exact for every brand?
No. Hat sizing is approximate and varies by brand, material and style. Treat the result as a starting point and check the maker's own size chart when you can.
Is my measurement sent anywhere?
No. The calculation runs entirely in your browser with JavaScript; nothing you type is stored or transmitted.

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