Hat size calculator.

How the size is worked out

Diameter = circumference ÷ pi. US fitted size = that diameter in inches, rounded to the nearest 1/8. UK size = US size minus 1/8, the usual convention. EU size = circumference in whole centimetres.

Worked example. A 57 cm head is 22.44 in around. 22.44 ÷ 3.1416 = 7.14 in, which rounds to 7 1/8 US, 7 UK and 57 EU.

The formula treats the head as a circle. Real heads are oval and brands such as New Era and Stetson round and shape their hats differently, so the result can be one size off a given brand. If you are between sizes, check that brand's size guide. For knit or crochet hats, including baby and toddler sizes, compare your measured circumference with the finished circumference in the pattern and follow its sizing note, since stretchy fabric is normally made a little smaller than the head. This page does not publish a letter-size (S/M/L/XL) chart because no standard defines one.

FAQ

What size hat is a 22.5 inch head?

22.5 inches divided by pi is 7.16, which rounds to 7 1/8 US fitted. In centimetres that is 57 cm.

Is UK hat size the same as US?

Usually it is one eighth smaller, so a US 7 1/4 is commonly a UK 7 1/8. This page applies that convention. Brands differ, so check the maker's own size guide.

Why does my New Era or Stetson size differ from the calculator?

The formula is plain geometry (diameter = circumference / pi). Brands round, shape and size their hats slightly differently, and oval heads fit differently from the round head the formula assumes. Treat the result as a starting point and check the brand's guide.

What is the difference between hat sizes and S/M/L/XL?

Letter sizes are brand-defined ranges and no standard says which fitted sizes they cover, so this page does not guess. Use the circumference or fitted size against the brand's own chart.

How do I choose a size for a crochet or knit hat?

Measure the head circumference the same way, then compare it with the finished circumference in your pattern. Knit and crochet fabric stretches, so patterns usually finish a little smaller than the head. Follow the pattern's own sizing note, and for a baby measure the actual head rather than guessing from age.

Should I size up if I am between sizes?

If the measurement falls between two sizes, most people choose the larger for comfort, and a hat that is slightly big can be padded with a sweatband strip. Your preference for a snug or loose fit decides it.