Use the free Hat Size Converter below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
What the hat size converter does
Hat sizes are confusing because they are sold in at least four ways at once: centimetres, inches, a US number like 7 1/8, and a letter like M. This free hat size converter takes one measurement, the circumference of your head, and gives you all of them. Type your head measurement in centimetres or inches and it shows the nearest published hat size, the matching US size, the letter size, and the full table so you can check nearby sizes. It works entirely in your browser, with nothing uploaded.
The table inside the tool runs from 52 centimetres up to 63 centimetres, which covers the great majority of adult heads. Each row links a head circumference to a US hat size and a letter size. When your measurement falls between two rows, the tool picks the nearest one and tells you to size up, because a hat that is slightly loose can be adjusted with a sizing strip, while a hat that is too tight will give you a headache.
How to use it
Measure first, then convert. Wrap a soft tape measure around your head just above your eyebrows and ears, where a hat actually sits, and read the number in centimetres. Type that number into the tool and press Convert. If your tape only reads in inches, type the inches instead and the tool converts them to centimetres first. The big result shows your size in centimetres, your US size and your letter size, and the table below shows the whole range.
Do not use a hat size you remember from years ago without measuring again. Head size changes very little in adulthood, but hair changes a lot: a thick hairstyle, braids or a lot of hair under a hat can easily add a centimetre. Measure with your hair as you will wear it under the hat.
How to measure your head correctly
Use a soft tailor tape, not a metal builder tape, and keep it level all the way around. It should sit about a finger width above your eyebrows and just above the tops of your ears, then around the widest part at the back of your head. Pull it snug but not tight: you should be able to slip one finger under the tape. Write down the number, then measure a second time. If the two numbers differ, use the larger one.
Measure in the middle of the day if you can, and avoid measuring straight after a haircut if you normally wear your hair longer, because that can shift the result by a centimetre. If you are buying a hat as a gift and cannot measure the person, a medium fits many adults, but it is a guess, and a gift receipt matters more than a clever estimate.
A worked example
Suppose your head measures 57 centimetres. Type 57 and convert. The result is US size 7, letter M, which is 22.4 inches around. If the shop lists hats by US size, you order 7. If it lists by centimetres, you order 57. If it only sells S, M and L, you order M. One measurement, three answers, no second chart needed.
Common questions
What is the difference between US and UK hat sizes? For most modern hats they are the same number: a US 7 is generally sold as a UK 7. Older British sizing sometimes used different labels, so when a shop gives both, trust the centimetre measurement, which is unambiguous.
What if I am between sizes? Size up. A hat that is a little loose can be tightened with foam sizing tape inside the band, which costs almost nothing. A hat that is too tight cannot be stretched much, and it will be uncomfortable within an hour.
Do all hats in my size fit the same? No. Head shape matters as much as circumference. Some people have rounder heads and some have longer, oval heads, and a hat shaped for one will feel tight on the other even in the right size. If you buy a structured hat like a fedora, check whether the maker describes the fit as round or oval.
Should I measure over my hair? Yes, exactly as you will wear the hat. Thick hair, a bun or braids all add bulk, and a measurement taken on a shaved head will give a hat that will not fit over your normal hairstyle.
Limits of this tool
This converter translates a head measurement into published hat sizes. It cannot measure your head shape, your hairstyle bulk, or how a particular maker cuts its crowns. For an expensive hat, try it on or buy from a shop with easy returns. For everyday hats, a careful measurement plus this conversion is usually enough.
Pingback: Ring Size Converter - US, UK and EU Ring Sizes