Hat Size Converter (head cm ⇄ US/UK fitted)

Understanding the hat size converter — concept and principle

Hat sizes are written in several systems: US fitted sizes such as 7 1/4, UK sizes that run one eighth smaller, European sizes in centimetres and loose letter sizes from S to XXL. This converter takes your head circumference in centimetres and shows each of them, which helps when buying fitted caps, fedoras or caps from overseas shops.

How it is calculated

• US fitted size: circumference in inches ÷ π, that is (cm ÷ 2.54) ÷ π, rounded to the nearest 1/8.
• UK size: US size − 1/8, rounded to the nearest 1/8.
• EU size: head circumference in centimetres, rounded to a whole number.
• Letter size (approx.): under 54 cm = S, 54 to under 56 = M, 56 to under 58 = L, 58 to under 60 = XL, 60 cm and above = XXL.

The principle

Traditional hatters described a hat by the average diameter of the head opening, in inches, not by its circumference. Dividing the circumference by π (about 3.1416) gives that diameter, which is why US sizes cluster around 7: a 57 cm head is about 22.4 inches around and 7.1 inches across. Sizes step in eighths of an inch of diameter, and one eighth of diameter equals about 1 cm of circumference (π × 0.125 in × 2.54 cm/in ≈ 1.0 cm). The UK labels the same head one eighth smaller by convention, while European sizing simply uses the circumference in cm, so each EU size step is also about 1 cm.

Worked example

1. Input: head circumference 57.5 cm.
2. Inches: 57.5 ÷ 2.54 = 22.64 in.
3. Divide by π: 22.64 ÷ 3.1416 = 7.206.
4. Nearest eighth: 7.206 × 8 = 57.65, rounded to 58, so 58/8 = 7 1/4 (US).
5. UK: 7.206 − 0.125 = 7.081, nearest eighth 7 1/8.
6. EU: 57.5 rounds to 58; letter size: 57.5 cm is under 58, so L.

How to read the result

A US 7 1/4 corresponds to a head of about 57.9 cm, so your 57.5 cm head sits between 7 1/8 (about 56.8 cm) and 7 1/4. If you prefer a snug fit, or the hat has a soft sweatband that stretches, try the smaller size; for a firm felt or straw hat, or when between sizes, the larger one with a sizing strip is usually more comfortable. Adult heads commonly fall between about 54 and 61 cm.

Common mistakes

• Measuring too high on the crown or over thick hair; measure just above the ears and eyebrows, around the widest part.
• Pulling the tape too tight; it should sit snug but relaxed.
• Treating US and UK sizes as the same, which puts you an eighth off.
• Assuming letter sizes are standard across brands.

Frequently asked questions

What is a size 7 1/4 hat in cm?

About 57.9 cm of head circumference (7.25 × π × 2.54).

How should I measure my head?

Wrap a soft tape around the head about 1 cm above the ears and eyebrows, across the widest part at the back.

Why are kids' hat sizes much smaller numbers?

The number is a diameter, so a child's head of about 50 cm is roughly size 6 1/4.

Sources: NIST Special Publication 811 (2008), Appendix B (1 inch = 2.54 cm exactly)