How hat sizes are determined
Hat size is based directly on the circumference of your head, measured around the widest part — typically just above the ears and eyebrows. US hat sizes express this circumference as a diameter-like number rather than the raw circumference.
How this calculator works
The standard formula is: US Hat Size = Head Circumference (inches) ÷ π, rounded to the nearest 1/8th, since hats are traditionally sized in eighth-inch increments (e.g. 7, 7 1/8, 7 1/4). This calculator also maps that number to a common letter size (S-XXL) and shows your circumference in centimeters for brands that use metric sizing.
For the most accurate measurement, wrap a soft tape measure snugly (but not tight) around your head about 1/8 inch above your ears and eyebrows — the fullest part of your head.
Fit can still vary by hat style and brand, so use this as a reliable starting point and check the specific manufacturer’s size chart when possible.