Hat Size Calculator
Turn your head circumference (cm or inches) into a US fitted hat size (6 7/8, 7 1/4 …), the UK size, the metric size and the S/M/L letter size, with a full hat size chart.
Last updated: October 2026
Hat size
7 1/4
Formula below · 3 sources (WHO, stetson.com, hatsandcaps.co.uk) · Updated Oct 2026
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About this calculator
Fitted hats are sized by the circumference of the head, and the three common systems are three ways of writing that one measurement:
• Metric (most of Europe and Asia): the circumference in centimetres, 58 for a 58 cm head. • US: the circumference in inches divided by π, which is the diameter of a circle with that circumference, rounded to the nearest eighth of an inch. 58 cm is 22.83 in; 22.83 ÷ 3.1416 = 7.27, so the US size is 7 1/4. • UK: historically the diameter measured on a slightly different hat block, so it runs one eighth below the US size; a US 7 1/4 is a UK 7 1/8. Sizes in both countries step by eighths (6 7/8, 7, 7 1/8, 7 1/4 …), and each eighth is about 1 cm of circumference.
Letter sizes (S, M, L, XL) are not standardised; the ranges used here (S 55-56 cm, M 57-58, L 59-60, XL 61-62) match most hat makers, but a brand's medium can span 56 to 58 or 57 to 59, so always check its chart when it offers fitted sizes.
Measuring: run a soft tape around the head where the hat will sit, about 1 cm (⅜ in) above the ears and across the middle of the forehead, level all the way round, snug but not tight. Take the largest reading from two or three tries, and measure over the hairstyle you will wear under the hat. Most adult heads are 54 to 62 cm; the average is about 57 cm for women and 58 cm for men.
If you fall between sizes, go up: a hat that is a touch loose can be padded with sizing tape behind the sweatband, but one that is too tight leaves a mark and cannot be stretched much. Wool felt and straw hats fit a little tighter than knit or soft cotton hats with the same label.
Hat size chart: head circumference to US, UK and letter sizes
Scroll sideways to see all columns →
| Circumference (cm) | Circumference (in) | US size | UK size | Letter |
|---|---|---|---|---|
| 52 | 20.5 | 6 1/2 | 6 3/8 | XS |
| 53 | 20.875 | 6 5/8 | 6 1/2 | XS |
| 54 | 21.25 | 6 3/4 | 6 5/8 | XS |
| 55 | 21.625 | 6 7/8 | 6 3/4 | S |
| 56 | 22 | 7 | 6 7/8 | S |
| 57 | 22.5 | 7 1/8 | 7 | M |
| 58 | 22.875 | 7 1/4 | 7 1/8 | M |
| 59 | 23.25 | 7 3/8 | 7 1/4 | L |
| 60 | 23.625 | 7 1/2 | 7 3/8 | L |
| 61 | 24 | 7 5/8 | 7 1/2 | XL |
| 62 | 24.375 | 7 3/4 | 7 5/8 | XL |
| 63 | 24.75 | 7 7/8 | 7 3/4 | XXL |
| 64 | 25.25 | 8 | 7 7/8 | XXL |
| 65 | 25.625 | 8 1/8 | 8 | XXL |
How to use
Measure around your head, choose cm or inches, and pick the size system you want. The tiles give the circumference in both units and the decimal US and UK sizes. Example 1 (defaults): 58 cm. 58 ÷ 2.54 = 22.83 in; 22.83 ÷ π = 7.27 → US 7 1/4, UK 7 1/8, letter size M. Example 2: 22 inches. 22 ÷ π = 7.00 → US 7, UK 6 7/8; 22 in = 55.9 cm, so letter size S (or a small M in some brands). Example 3: 61 cm. 24.02 in ÷ π = 7.65 → nearest eighth 7 5/8 → US 7 5/8, UK 7 1/2, letter XL. Example 4: a child's head of 50 cm. 19.69 in ÷ π = 6.27 → US 6 1/4; children's hats are usually sold by cm or age instead.
Frequently asked questions
What hat size is a 23-inch head?
23 ÷ π = 7.32, which rounds to the nearest eighth as 7 3/8 in US sizing (UK 7 1/4). 23 inches is 58.4 cm, so a metric 58 or 59 and a letter size M or L depending on the brand.
Why are US and UK hat sizes different by an eighth?
Both are the head circumference divided by π, but the British trade measured its hat blocks slightly differently, so the same hat carries a UK number one eighth smaller than its US number. A US 7 is a UK 6 7/8.
What is the average hat size?
About 7 1/4 for men (58 cm) and 7 1/8 for women (57 cm); most adults fall between 6 7/8 and 7 5/8 (55 to 61 cm). Sizes above 7 3/4 (62 cm) count as large and are stocked by fewer makers.
How do I measure my head without a soft tape?
Wrap a string or a strip of paper around the head where the hat will sit, mark where it overlaps, and measure the length against a ruler. Enter that length in cm or inches.
Do letter sizes mean the same thing in every brand?
No. S/M/L ranges vary by a centimetre or two between makers, and some use S/M and L/XL pairs that each cover 3 cm. Use the fitted size or the cm measurement when a brand lists it, and the letter only as a fallback.
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Sources & references
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