OneTaskBy Tim King · Updated
Hat Size Calculator
Head Circumference to US, UK, EU and Letter Sizes
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How the Hat Size Calculator works
This hat size calculator converts your head circumference into a hat size in every common system. Wrap a soft tape around your head just above the eyebrows and ears, then enter the measurement in centimetres or inches. You get the US fitted size, the UK size, the EU size and the S, M, L letter size used for beanies and flex-fit caps.
Hat sizes come from three traditions. Europe simply uses the head circumference in centimetres. US fitted sizes, used for baseball caps, fedoras and many formal hats, are the circumference in inches divided by pi, as if the head were round, and go up in eighths. UK sizes follow the same idea but sit one eighth smaller. Stretch hats skip all of that and use letters.
How the hat size calculator works
The formula for the US size is circumference in inches ÷ π, rounded to the nearest 1/8. The UK size is 1/8 below the US size, and the EU size is the circumference in centimetres. The letter size follows common maker charts: XS up to 53 cm, S to 55 cm, M to 57 cm, L to 59 cm, XL to 61 cm and XXL to 63 cm.
First example. A head of 57 cm is 22.44 in. Divided by π that is 7.14, which rounds to US 7 1/8. The UK size is 7, the EU size 57 and the letter size M.
Second example. A head of 23 in, about 58.4 cm, gives 23 ÷ π = 7.32, which rounds to US 7 3/8, UK 7 1/4, EU 58 and letter size L.
How many centimetres per hat size? Each 1/8 US size is about 1 cm of circumference, so the step from 7 1/8 to 7 1/4 needs roughly 1 cm more head.
Head circumference with US, UK, EU and letter hat sizes
| Head (cm) | Head (in) | US | UK | EU | Letter |
|---|---|---|---|---|---|
| 52.0 cm | 20.5 in | 6 1/2 | 6 3/8 | 52 | XS |
| 52.5 cm | 20.7 in | 6 5/8 | 6 1/2 | 53 | XS |
| 53.0 cm | 20.9 in | 6 5/8 | 6 1/2 | 53 | XS |
| 53.5 cm | 21.1 in | 6 3/4 | 6 5/8 | 54 | S |
| 54.0 cm | 21.3 in | 6 3/4 | 6 5/8 | 54 | S |
| 54.5 cm | 21.5 in | 6 7/8 | 6 3/4 | 55 | S |
| 55.0 cm | 21.7 in | 6 7/8 | 6 3/4 | 55 | S |
| 55.5 cm | 21.9 in | 7 | 6 7/8 | 56 | M |
| 56.0 cm | 22.0 in | 7 | 6 7/8 | 56 | M |
| 56.5 cm | 22.2 in | 7 1/8 | 7 | 57 | M |
| 57.0 cm | 22.4 in | 7 1/8 | 7 | 57 | M |
| 57.5 cm | 22.6 in | 7 1/4 | 7 1/8 | 58 | L |
| 58.0 cm | 22.8 in | 7 1/4 | 7 1/8 | 58 | L |
| 58.5 cm | 23.0 in | 7 3/8 | 7 1/4 | 59 | L |
| 59.0 cm | 23.2 in | 7 3/8 | 7 1/4 | 59 | L |
| 59.5 cm | 23.4 in | 7 1/2 | 7 3/8 | 60 | XL |
| 60.0 cm | 23.6 in | 7 1/2 | 7 3/8 | 60 | XL |
| 60.5 cm | 23.8 in | 7 5/8 | 7 1/2 | 61 | XL |
| 61.0 cm | 24.0 in | 7 5/8 | 7 1/2 | 61 | XL |
| 61.5 cm | 24.2 in | 7 3/4 | 7 5/8 | 62 | XXL |
| 62.0 cm | 24.4 in | 7 3/4 | 7 5/8 | 62 | XXL |
| 62.5 cm | 24.6 in | 7 7/8 | 7 3/4 | 63 | XXL |
| 63.0 cm | 24.8 in | 7 7/8 | 7 3/4 | 63 | XXL |
| 63.5 cm | 25.0 in | 8 | 7 7/8 | 64 | XXXL |
Tips and pitfalls
- Measure where the hat sits. Run the tape about 1 cm above the eyebrows and over the tops of the ears, around the widest part at the back.
- Measure twice. Take two readings and use the larger; a hat that is slightly big can be padded, one that is too small cannot be fixed.
- Hair counts. Thick or long hair worn under the hat adds to the size.
- Between sizes, go up. Fitted caps and felt hats have little give; most have a sweatband that can take a sizing strip.
FAQ
How do I measure my hat size?
Wrap a soft tape measure around your head about 1 cm above the eyebrows and ears, where a hat brim would sit. Keep it level and snug but not tight. Use the number in centimetres or inches above.
What hat size is 57 cm?
A 57 cm head is a US 7 1/8, a UK 7, an EU 57 and a medium in letter sizes. It is close to the average adult head, so many one-size hats are made to fit around 57 to 58 cm.
What is the difference between US and UK hat sizes?
Both are the circumference divided by pi, but the UK size runs one eighth smaller for the same head. A US 7 1/4 is a UK 7 1/8. EU sizes avoid the confusion by using centimetres directly.
What size is a fitted baseball cap?
Fitted caps use US sizes such as 7 1/8 or 7 3/8. Adjustable and flex-fit caps use letters, often S/M around 55 to 57 cm and L/XL around 58 to 60 cm, so check the brand chart.
Do children's hats use the same sizes?
Children's hats are usually sold by age or by centimetres. A one-year-old's head is about 46 cm and a ten-year-old's about 53 cm, so measure rather than relying on the age label.
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