Hat Size Calculator

How it is calculated

You enter head circumference in centimetres. The US size is the diameter of a circle with that circumference, in inches: US size = (circumference ÷ π) ÷ 2.54. The EU size is simply the circumference in centimetres rounded to a whole number, which is how most European hats are labelled. The tool also repeats your measurement to one decimal. The US value is shown to two decimals, for example 7.14, and is not rounded to the eighth-inch steps that shops print (7, 7 1/8, 7 1/4); read it as a point between two sizes and pick the nearest. Only circumferences from 30 to 100 cm are accepted; anything outside that range shows no result, which also catches a measurement typed in inches.

Common mistakes

• Measuring in inches and typing the number in the centimetre box. A 22-inch head typed as 22 is below the 30 cm minimum and shows no result; if you only have inches, multiply by 2.54 first (22 in is about 56 cm).
• Pulling the tape too tight or measuring over hair that is pressed down. Keep the tape level, just above the eyebrows and ears, and snug but not tight.
• Expecting a fraction. The US result is a decimal such as 7.14; match it to the nearest size on the brand's chart, usually in steps of 1/8.
• Assuming every brand follows the same scale. Some label by S, M, L or by centimetres with their own cut-offs, so compare with the brand's chart before buying.
• Choosing the size down when between two sizes. For a hat it is usually easier to fit with padding than to stretch a tight one, so many people go up.