Hat size chart: cm, inches, UK and US sizes

How to use the chart

1. Place a tape measure around your head at the middle of the forehead and just above the ears.
2. Read the circumference in centimetres or inches.
3. Choose head circumference in the finder and enter the value. The finder highlights the first size that is large enough.
4. To convert a size, choose UK or US and enter it, for example 7 1/4.

Without JavaScript the finder is unavailable, but the chart is complete.

How the sizes are derived

• European size: the head circumference in centimetres.
• US size: the circumference in inches divided by 3.1416, rounded to the eighth of an inch. It is the diameter of a circle with that circumference.
• UK size: the US size minus one eighth.

The chart is calculated with these rules and equals the chart of a London hatter, Lock & Co., for all thirteen sizes from 53 to 65 cm. We found no standard that defines hat sizes, so the chart rests on this one source.

Worked example

Your head measures 57.5 cm.

1. The first size that is large enough is European 58.
2. 58 cm are 22.83 inches. Divided by 3.1416 that is 7.27, which rounds to 7 1/4: the US size.
3. The UK size is one eighth smaller: 7 1/8.

Letter sizes

Caps and knitted hats are often sold as S, M, L or as one size. Makers assign letters to different circumferences, so the chart leaves them out. Look for the circumference range in the product description.

Common mistakes

• Measuring too high or too low. The tape belongs at mid-forehead and just above the ears.
• Mixing up UK and US sizes. For the same hat the US number is one eighth larger.
• Reading a UK or US size as a circumference. Size 7 is not 7 inches around the head; it is about 22 inches.
• Rounding down. A hat that is too small cannot be worn; choose the next size up if you are between two.

Sources and checks

The values in the chart are calculated from the definitions in the sources below. They are not copied from another chart. Last checked against the sources on September 28, 2026.

The continental size is the head circumference in centimetres. The US size is that circumference in inches divided by pi, rounded to the eighth of an inch; the UK size is one eighth smaller.

The calculated sizes equal the chart of Lock & Co. Hatters for all thirteen sizes from 53 to 65 cm. No standard that defines hat sizes was found; the chart rests on this one source.

Letter sizes such as S, M and L are left out because makers assign them differently.

• Lock & Co. Hatters, hat sizing

Used for: What the three systems measure, that North American sizes are one eighth larger than UK sizes, where to measure the head, and a chart from 53 to 65 cm used to check the calculation.