Hat Size Calculator. Enter your head circumference in inches or centimeters and get your hat size in International, US (American), and UK (British) sizing systems. Choose your unit of measurement and the calculator returns your exact size across all three standards — plus your general fit category from XS to XXL. Also try the Circle Skirt Calculator.
Wrap a flexible tape measure around your head, positioning it mid-forehead just above your eyebrows and ears, and around the largest part of the back of your head. Keep the tape snug but not tight, and make sure it sits level all the way around. Note the measurement in centimeters or inches. See also our calculate Ring Diameter (mm), US / Canada Size & UK / Australia Size — Ring Size Converter.
Divide your head circumference in inches by π (3.14159) to get your US hat size, then round to the nearest 1/8. For example, a 22.5-inch circumference gives 22.5 ÷ 3.14159 ≈ 7.16, which rounds to 7 1/8. The EU/International size is simply your circumference in centimeters.
The most common men's hat size is 7 3/8 (US), which corresponds to a head circumference of about 23 inches (58–59 cm). In international sizing, this falls in the Large (L) range.
A head circumference of 23 inches (approximately 58.4 cm) corresponds to a US hat size of around 7 3/8, a UK size of 7 1/4, and an International/EU size of 58–59. This falls in the Large (L) fit category.
US hat sizes are typically 1/8 larger than UK hat sizes for the same head circumference. For example, a head that measures 57 cm (22.5 inches) is a US size 7 1/8 but a UK size 7. Always check the specific sizing system used by the hat brand before purchasing.
If you fall between sizes, it's generally recommended to size up for a more comfortable fit. Many hats also come with adjustable sizing bands inside the crown that can provide a little flexibility for borderline measurements.
Yes — the International (EU) hat size number is essentially the same as your head circumference in centimeters. A 57 cm circumference equals EU size 57. This makes the EU system straightforward to use compared to fractional US and UK sizes.
Children's hat sizes are often based on age groups rather than exact measurements, but a circumference-based chart is more accurate. Measure the child's head in the same way as an adult — mid-forehead and around the widest part — and use the circumference to look up their size. You might also find our Curtain Size Calculator useful.