Hat Size Calculator

Hat size is a label for the distance around your head where a hat sits. Each country writes that distance in its own way. This calculator takes one measurement and shows the matching size in every common system.

What Hat Size Measures

A hat size describes the inside of the hat's band, the ring that touches your head. It does not measure head shape, so two hats with the same size can still fit a little differently. EU sizes are the head circumference in centimeters. US and UK sizes are smaller numbers, such as 7 or 7 1/8, because they come from the diameter, not the circumference.

The Formula

Traditional US hat size is the inner band circumference divided by pi, which gives the band's diameter in inches¹. UK sizes use the same idea but are one eighth smaller, so a UK 7 is a US 7 1/8².

EU size = head circumference in cm

US size = circumference in inches / 3.1416, to the nearest 1/8

UK size = US size - 1/8

Because 2.54 x pi is almost exactly 8, you can also divide the circumference in cm by 8 to get the US size⁶. For example, 57 cm / 8 = 7 1/8.

How This Calculator Computes Your Result

Enter your head measurement in cm or inches. Inches can be typed as a fraction, such as 22 1/2. The tool turns inches into cm (1 inch = 2.54 cm) and rounds to a whole centimeter, which is your EU size. It works out the US size from that same number and takes one eighth off for the UK size, so all four results match one row of a hatmaker's chart. If you are between sizes, the "Round up" option gives you the next size.

What Your Result Means

These rows come from the formula and match the metric columns of the Lock & Co. and Stetson charts, and Christys' chart on every row except 53 cm² ³ ⁴.

Each one eighth step in a US or UK size is about 1 cm of head size. Lock & Co., for example, makes hats from small at 55 cm to XX-large at 64 cm². The letter sizes are a shop convention, not a standard, so treat them as a rough guide.

How to Measure Correctly

• Use a soft tape measure, or a piece of string you can lay against a ruler.

• Wrap it around your head where the hat will sit: across the middle of the forehead and just above the ears² ⁴. This is usually just above the eyebrows at the front.

• Keep the tape level all the way round and pull it firm, but not tight⁴.

• Do not measure over a thick ponytail, hair clip or headband.

• Measure two or three times. If the readings differ, use the larger one.

• If you land between two sizes, hatmakers suggest the larger size⁴ ⁵. A slightly loose hat can be fixed with padding or an insert in the band⁵. A tight one is much harder to fix.

Why US and UK Hat Sizes Differ

Both systems count in eighths of an inch, but they start one step apart. Lock & Co., a London hatter, says North American sizes are an eighth of an inch larger than British ones². So a hat marked 7 in London fits like a 7 1/8 in New York. Christys' chart shows the same one step gap on nearly every row from 49 to 65 cm³. If a label just says "7", check whether the brand is British or American.

The pi rule holds up on real hats. In a small classroom dataset, students measured 26 hats from Stetson, Borsalino, Dobbs and Beaver. Hat size against band circumference gave a slope of about 0.33, close to 1 divided by pi (0.318)¹.

Why Letter Sizes Vary by Brand

There is no standard for S, M or L hats. Brands agree well at odd numbers. Christys' calls 55 cm S, 57 cm M and 59 cm L³, and Stetson does the same⁴. At even numbers they split. Christys' labels 56 cm as S/M and 58 cm as M/L³, while Stetson puts 56 cm in Medium and 58 cm in Large⁴. That is why the calculator shows two letters for even sizes. Inch columns also differ: Stetson lists a size 7 as 22 1/4 inches, while Lock & Co. and Christys' list it as 22 inches² ³ ⁴. All three agree that a size 7 is 56 cm, so the calculator works from centimeters.