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Hat Size Calculator

Use this free hat size calculator to find the hat that fits your head. Measure your head circumference in centimetres or inches and it gives your size in the US and UK systems, the EU centimetre size, and the letter size (S, M, L, XL) that most brands use. A hat that fits should sit comfortably without squeezing, and this calculator takes the guesswork out of ordering one online.

How it works

Hat sizes come from one measurement: the circumference of your head, taken where the hat band sits, about 1 cm above your eyebrows and ears. That number is the whole story. In the EU system it is the size itself: a head of 58 cm takes an EU 58 hat, a head of 56 cm takes an EU 56, and so on. There is no formula to learn for EU sizes, which is why so many brands print the centimetre size on the label even when they sell in other markets.

The US and UK systems take one extra step. A hat is roughly a circle, and the size is its diameter in inches: the width of the circle across the middle. Since circumference equals pi times diameter, the diameter is the circumference divided by pi (3.1416). A 58 cm head is 22.83 inches around, and 22.83 divided by 3.1416 is 7.27 inches across, which rounds to US 7 1/4. The UK uses exactly the same numbers, so a US 7 1/4 is a UK 7 1/4. Hat sizes step in eighths of an inch, so you will see 6 7/8, 7, 7 1/8, 7 1/4, 7 3/8, 7 1/2, and 7 5/8 on a full size chart, and this calculator rounds to the nearest eighth by default, with a nearest-quarter option for brands that only stock quarter sizes.

Letter sizes are a coarser version of the same scale. Roughly, a head under 55 cm is a small, 55 to 57 cm is a medium, 58 to 59 cm is a large, 60 to 61 cm is an extra large, and 62 cm and above is XXL, though the exact cut-offs vary a little by brand. Letter sizes are convenient, but they hide a full centimetre or more of difference, so two medium hats from different brands can fit differently. If you are buying a fitted hat, a baseball cap with a structured crown, or a hat you plan to wear all day, use the numbered size rather than the letter.

Measuring correctly matters more than the arithmetic. Use a soft tailor’s tape, not a metal one, and hold it level all the way around. Hair makes a difference: measure over the hair as you normally wear it, and if you wear your hair up under a hat, measure with it up. Measure twice, and if the two readings differ, use the larger one. A hat that is slightly loose can be fixed with a thin foam sizing strip tucked inside the sweatband, but a hat that is too tight will leave a red line on your forehead and give you a headache within an hour.

Heads are not perfect circles, and neither are hats. Most people’s heads are slightly oval, longer front to back than side to side, and hat makers shape their hats that way too. If a hat fits around the circumference but pinches at the front and back while gaping at the sides, you may need a long-oval fit rather than a larger size. Some brands sell long-oval versions of their popular hats, and a hat shop can also stretch a hat slightly on a wooden block. For kids, the same measurement works, but children’s heads grow fast, so check the size every few months and leave a little room to grow.

Hat size chart

Worked example

A shopper measures her head at 58 cm and wants to order a fitted hat from a store that lists US sizes.

Step 1: Convert to inches — 58 ÷ 2.54 = 22.83 inches.
Step 2: Divide by pi — 22.83 ÷ 3.1416 = 7.27.
Step 3: Round to the nearest eighth — 7.27 rounds to 7 1/4.
Step 4: Read the other sizes — EU 58, letter size L.

She should order US 7 1/4, or EU 58 if the store lists centimetres. Enter 58 cm in the calculator and it gives exactly that: US 7 1/4, UK 7 1/4, EU 58, and L. If her measurement had been 57 cm instead, the same steps give 22.44 ÷ 3.1416 = 7.14, which rounds to US 7 1/8.

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