maratoolHat Size Converter — US, UK & CM
Enter head circumference or a US/UK hat size to read every equivalent, including S/M/L letter sizing.
—Circumference (cm)
—Circumference (in)
—US size
—UK size
—Letter size
US sizes step in eighths (6 3/4 – 8). Enter e.g. 7.25 for 7 1/4.
How this conversion works. Uses traditional millinery sizing: US size = circumference in inches ÷ π (UK size = US − 1/8), with S–XL bands by circumference. Formula shown inline.
How to use
- Measure around your head just above the ears and brow.
- Enter the circumference in cm or inches (or a US/UK size).
- Read the US, UK, metric, and letter-size equivalents.
Frequently asked questions
How do I measure my head for a hat?
Wrap a soft tape around the widest part — about 1 cm above the ears and across the mid-forehead. Keep it snug but not tight; measure twice.
How are US numeric hat sizes defined?
US size = head circumference in inches ÷ π, rounded to the nearest eighth — a 23″ head is size 7 3/8. UK sizes run 1/8 smaller for the same head.
What size is a 58 cm head?
58 cm is 22.8″ — US 7 1/4, UK 7 1/8, or a Large in most S/M/L brands.
Is my data sent to a server?
No. All sizing math runs in your browser.
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