Hat Size Calculator
Convert head circumference to hat size.
The calculator converts a head measurement to US and UK hat sizes and shows an international size band.
Result
--
Run the calculation to see the hat size.
| Metric | Value |
|---|---|
| US size | -- |
| UK size | -- |
| International size | -- |
| Measured circumference | -- |
Quick Answer
Hat size is based on head circumference. In the adult chart, US sizes are usually one eighth larger than UK sizes.
How to Calculate
- Measure the circumference of the head.
- Choose the length unit and mode.
- Enter the age only if you are using child mode.
- Click Calculate.
Formula
US size = round(circumference in inches x 8 / pi) / 8
UK size = US size - 1/8
International size = size band based on the rounded US size, or an age band in child mode
Worked Examples
Example 1: 22 inches gives about US size 7 and UK size 6 7/8.
Example 2: 24 inches gives about US size 7 5/8 and UK size 7 1/2.
Example 3: 50 cm is in the child hat-size range and usually maps to the 12-24 months chart band.
How to Interpret Your Results
| Output | Meaning | Action |
|---|---|---|
| US size | American hat size based on circumference. | Use it for most hat brands in the US. |
| UK size | British hat size, usually one eighth smaller. | Use it for UK labels and charts. |
| International size | XS through XXXL for adults or an age band for children. | Match it to the product chart when shopping. |
Frequently Asked Questions
Measure around the widest part of the head, just above the ears and eyebrows.
The UK chart is offset by one eighth of a size compared with the US chart.
The calculator still converts the circumference, but child mode also shows an age band.
Related Calculators
References
- OmniCalculator reference page
- American hat sizes are usually slightly larger than British sizes.
- Last reviewed: March 2026.