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Everyday Life Calculators
Hat Size Calculator
Convert a head circumference measurement into a numeric hat size and letter size.
Hat size (US)
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Letter size
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About the Hat Size Calculator
US hat sizes are based on the inside diameter of the hat, derived from the circumference of your head measured just above the ears and eyebrows.
Formula: Hat Size = Head Circumference (in) ÷ π, rounded to the nearest eighth. This is also mapped to common letter sizes (S, M, L, XL) based on the circumference in centimeters.
Measure with a soft tape wrapped snugly around the widest part of your head for the most accurate result.
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