Hat Size Calculator
US hat size from head circumference.
What it does
Hat sizes are head circumference divided by pi: the head is a circle and the size numbers its diameter in inches. Round to eighths for fitted caps or group into letters for stretch. This page runs both units to the same size.
The default is a 22.5 in head. Division gives 7.16, which rounds to 7 1/8 in the Large band. Metric defaults convert a 57 cm head to 22.44 in and land on the same 7 1/8, since the tape not the math differs.
Tape spot decides size. Measure where the hat will sit, keep one finger of ease, and take the largest of three pulls since heads compress. Thick hair and glasses arms add real circumference.
Bands are groups, not guarantees. Wool, stretch, and crown depth shift wear within a size, so try both neighbors when between eighths.
How to use
- Pick imperial or metric first.
- Measure the head three times.
- Enter the largest measure.
- Read US size first, then band.
- Try on between bands.
Formula
- head
- Head circumference where the hat sits.
- pi
- Circle constant 3.1416.
Heads measure where the hat sits, usually mid forehead above brow and ears. Fitted caps use eighths; S to XL bands group them for stretch and wool. Metric divides cm by 2.54 before pi so both modes agree.
Worked example
Example: 22.5 in head
22.5 in head (metric 57 cm).
- Size: 22.5 / 3.1416 = 7.16.
- Eighths: 0.16 x 8 = 1.3, round to 1.
- Size reads 7 1/8.
- Band: 7.16 lands Large.
- Metric: 57 cm gives the same 7 1/8.
- Tool shows 7 1/8 imperial and metric.
Reference table
| Head | US | Band |
|---|---|---|
| 21 in | 6 5/8 | Small |
| 22 in | 7 | Medium |
| 22.5 in | 7 1/8 | Large |
| 23 in | 7 1/4 | Large |
| 24 in | 7 5/8 | XL |
Limits
- Fitted math. Stretch varies by brand.
- One head per pass. Hair changes fit.
- Crowns and brims excluded.
- Between sizes need trying on.
Common mistakes
- Measuring over glasses arms wrong.
- Pulling tape tight like a tourniquet.
- Averaging three pulls instead of largest.
- Mixing cm with inches.
- Reading band as fitted size.
Try it yourself
Quick checks. Answers sit tucked under each question.
21.5 in head?
6 7/8 (21.5 / 3.1416 = 6.84, eighths 7).
23.25 in head?
7 3/8 (23.25 / 3.1416 = 7.40, eighths 3).
Questions
US hat size?
Circumference over pi, to eighths. Here 7 1/8.
S to XL?
Under 6.75 Small, 7.13 Medium, 7.5 Large, else XL. Here Large.
What is that in metric?
Same 7 1/8, since 57 cm is 22.44 in.
Where to measure?
Above the brow and ears, largest of three tapes.
Tall crowns differ?
Yes for depth, but size still follows circumference.
Exact fit?
No. Stretch and style shift fit within a size.