Hat Size Calculator — US, EU, UK & Japan
Measure your head circumference once and instantly get your hat size in every major sizing system — European (cm), US/Canada (fractional), UK and Japan.
Unit
Head circumference rounded to the nearest centimetre
- 1
US hat size (circumference in inches ÷ π)
57 ÷ (π × 2.54) = 7.143Rounded to the nearest ⅛ inch to give the traditional US fractional hat size - 2
EU hat size = round(circumference in cm)
round(57) = 57
How does this calculator work?
Measure your head circumference in cm. Your EU hat size equals that number rounded. US/UK size = circumference in inches divided by π, rounded to the nearest ⅛ inch. For example, a 57 cm head → EU 57, US 7⅛. Japan uses the same cm figure as the EU.
Formula
How this is calculated
Hat size systems all derive from the same measurement: the circumference of your head at its widest point — typically 1–2 cm above your eyebrows. In the European (and most of the world) system the hat size simply equals that circumference in centimetres, rounded to the nearest whole number. A 57 cm circumference gives EU size 57.
The US and Canadian system divides the circumference in inches by π to get a "diameter" (in inches), then rounds to the nearest eighth of an inch. A 57 cm head is 22.44 inches in circumference; divided by π gives ≈ 7.14 inches, which rounds to 7⅛. The UK system uses the same numerical scale as the US, with very minor rounding differences that rarely appear in retail. Japanese hat sizes are essentially the same as EU sizes (circumference in cm), sometimes offset by ½ cm.
For the most accurate fit, measure twice with a flexible tape measure while looking straight ahead. Different hat styles (baseball caps, fedoras, beanies) also affect fit — stretchy materials accommodate a half-size range.
Frequently asked questions
Wrap a soft tape measure around your head about 2 cm above your eyebrows, passing above your ears. Keep the tape snug but not tight. Take the reading in centimetres — that is your head circumference and the basis for all hat sizes.
The US system expresses the nominal hat size as a diameter in inches, traditionally expressed in eighths: 7⅛ means a diameter of 7.125 inches. This equals a circumference of about 22.4 inches (56.9 cm), so EU 57. You will find these fractions stamped inside the brim of most US caps.
Go up to the next size for structured hats (fedoras, hard caps) and down for stretch-fit or knit hats. If you are exactly between sizes, hat tape inserted inside the brim can snug up a slightly large hat.
Also known as
TG we-Calculate Editorial Team. (2026). Hat Size Calculator — US, EU, UK & Japan [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/hat-size-calculator
TG we-Calculate Editorial Team. "Hat Size Calculator — US, EU, UK & Japan." TG we-Calculate. 2026. https://we-calculate.com/calculator/hat-size-calculator.
TG we-Calculate Editorial Team, "Hat Size Calculator — US, EU, UK & Japan," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/hat-size-calculator
@misc{wecalculate_hat_size_calculator, title = {Hat Size Calculator — US, EU, UK & Japan}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/hat-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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