Hat size from head circumference
Measure around your head above the eyebrows and ears, then see your size in all three systems.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter head circumference and unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
US size = head circumference in inches ÷ π, to the nearest 1/8; UK = US − 1/8; EU = circumference in cm.
Example
Inputs: Head circumference = 57, Unit = centimeters. Result: US size: 7 1/8; UK size: 7; EU size (ISO): 57 cm.
Limits and caveats
- Letter sizes such as S, M and L vary by brand and are not calculated here.
- If you fall between sizes, go up; a hat can be tightened with sizing tape.
Questions about this tool
How do I measure my head?
Wrap a soft tape about 1 cm above the eyebrows and over the ears, where the hat sits.
My head is 57 cm. What size am I?
US 7 1/8, UK 7 and EU 57.
Why do US and UK sizes differ?
For the same head, the UK number is one eighth smaller than the US number.