Hat Size Calculator
Convert a head circumference into fitted US and UK hat sizes, centimeters and a practical letter-size range.
Calculate your hat size
This Hat Size Calculator converts an accurate head measurement into common fitted and letter-size estimates. Because manufacturers use slightly different blocks, materials and sizing charts, treat the result as a starting point and check the specific brand before purchasing.
How to measure your head for a hat
Use a soft measuring tape. Place it around the widest part of your head, usually about 1 centimeter or 3/8 inch above the eyebrows, just above the ears and around the small bump at the back of the head. Keep the tape level and comfortably snug without pulling it tight.
Measure two or three times and use the largest consistent value. If you are between sizes, choose the larger size or use the round-up option. Hair volume, liners, hat style and personal comfort can all change the best final fit.
Hat size calculation method
A traditional US fitted size is estimated by dividing adjusted head circumference in inches by pi. The result is rounded to the nearest one-eighth size, or upward when you select the safer round-up option. Traditional UK fitted sizing is approximately one-eighth below the corresponding US fitted size.
US fitted size = adjusted circumference in inches ÷ π
The calculator also returns the adjusted circumference in centimeters and a broad alpha size. Alpha labels are not universal: this calculator uses practical adult bands of XS below 54 cm, S from 54 to under 56 cm, M from 56 to under 58 cm, L from 58 to under 60 cm, XL from 60 to under 62 cm and XXL from 62 cm.
Hat size chart
| Head circumference | US fitted size | General size |
|---|---|---|
| 54 cm / 21.3 in | 6 3/4 | Small |
| 56 cm / 22.0 in | 7 | Medium |
| 57 cm / 22.4 in | 7 1/8 | Medium |
| 58 cm / 22.8 in | 7 1/4 | Large |
| 60 cm / 23.6 in | 7 1/2 | XL |
| 62 cm / 24.4 in | 7 3/4 | XXL |
Hat Size Calculator example
Suppose your head measures 58 centimeters. That equals about 22.83 inches. Dividing by pi gives approximately 7.27, which rounds to a US fitted size of 7 1/4. The matching traditional UK estimate is 7 1/8, while the general letter size is Large. A rigid felt hat may need more careful brand-specific fitting than an adjustable cap.
Tips for a comfortable hat fit
Use the Hat Size Calculator after measuring carefully, then compare its result with the retailer chart. The Hat Size Calculator is especially useful when a store lists fitted sizes instead of centimeters.
- Measure where the sweatband will actually sit.
- Keep the tape level and do not compress your hair or skin.
- Choose the larger size when your measurement falls between two fitted sizes.
- Consider a relaxed allowance for thick hair, liners or long wear.
- Check whether the seller uses US, UK, centimeter or alpha sizing.
For consistent metric measurement and length-unit terminology, consult the NIST guidance on length units.
Frequently asked questions
What if I am between two hat sizes?
Choose the larger size or select round up. A hat can often be reduced slightly with sizing tape, while a hat that is too small may be uncomfortable.
Are US and UK fitted hat sizes the same?
No. Traditional UK fitted sizes are generally about one-eighth lower than the corresponding US size.
Does hair affect hat size?
Yes. Thick hair, braids or a liner can increase the required circumference, so measure with the hairstyle you expect to wear.
Can children use this calculator?
The circumference conversion works, but children’s age labels vary widely. Always use the child’s actual measurement and the manufacturer chart.
Why does a brand chart show a different size?
Hat blocks, materials, shapes and intended fit differ. The brand chart should take priority over a general conversion.
Related everyday calculators
Hat Size Calculator example
A 58.0 cm head circumference converts approximately to US fitted size 7 1/4, UK size 7 1/8, and general letter size L. Keep the tape level and comfortably snug, then compare the Hat Size Calculator result with the manufacturer’s chart.