How this calculator works
Traditional hat sizes are based on the diameter of the head, not the circumference. The circumference divided by pi gives the diameter in inches, and that number is the US hat size, written to the nearest eighth of an inch. A 57 cm head is 22.44 inches around, a diameter of 7.14 inches, so the size is 7 1/8. UK sizes run about one eighth smaller, and the continental size is simply the head circumference in centimetres. The letter sizes are broad bands that vary by maker, so treat them as a starting point.
Formula
head in inches = circumference in cm / 2.54. US hat size = head in inches / pi. UK hat size = US hat size - 1/8. Continental size = circumference in cm, rounded to a whole number.
Worked example
A head circumference of 57 cm is 22.44 inches. Dividing by pi gives 7.14 inches, which rounds to US 7 1/8. UK is 7, and the continental size is 57. A 55 cm head is US 6 7/8, UK 6 3/4, continental 55.
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Frequently asked questions
Where exactly should I measure?
Around the widest part of the head, about one centimetre above the ears and across the middle of the forehead. Keep the tape level and snug; a loose tape reads the hat too large.
Why is a hat size smaller than the circumference?
Hat sizes are diameters, not circumferences, because a hat is a circle. Dividing the circumference by pi converts the measurement around the head into the width across it.
Are letter sizes reliable?
Less than the numbered sizes. A small from one maker can be a medium from another, and the ranges here are generic. Use the numbered size or the circumference when you can, and check the maker chart.
How much ease should a hat have?
It depends on the style. A knitted beanie should measure slightly less than the head so it grips; a structured felt hat usually matches the head or adds half a centimetre. The chart gives body measurements, not the finished hat size.