Helmet Size Calculator
Head circumference to helmet size with fit and strap checks.
What it does
A helmet protects only when it stays put. Too large and it slides over the eyes on impact; too small and it rides high, leaving the forehead bare. Tape time takes a minute and beats guessing by hat size.
The tool above converts head circumference to a size band. A 22.4 in head measures 56.9 cm, which lands in Medium. The metric match is 57 cm toward 22.44 in.
Bands run Child 48 to 52 cm, Small 52 to 56 cm, Medium 56 to 59 cm, and Large 59 to 63 cm. Shell shapes vary, so two Mediums can feel different on long versus round heads.
Straps finish the job. Keep one to two finger widths above the brow, V buckles at the ears, and one to two fingers under the chin. A snug helmet should not rock side to side when you shake your head.
How to use
- Pick imperial (in) or metric (cm).
- Wrap the tape above brows and ears.
- Enter the number: 22.4 in or 57 cm.
- Read the size first.
- Read circumference in both units.
- Check brow gap, V straps, and chin space.
Formula
- head
- Head circumference in cm or in.
- bands
- Size bands in cm from helmet guides.
- / 2.54
- Divide cm by 2.54 for inches.
Bands follow common bike helmet sizing: Child 48 to 52 cm, Small 52 to 56 cm, Medium 56 to 59 cm, and Large 59 to 63 cm. Measure above the brows with the tape level. Fit checks add the finger rules for brow gap and chin strap, plus a no rock test.
Worked example
Example: 22.4 in head
Imperial defaults: 22.4 in head. These are also the default values, so the page opens on this answer.
- Head: 22.4 in.
- Convert: 22.4 x 2.54 = 56.9 cm.
- Band check: 56.9 sits in 56 to 59, so Medium.
- Size: Medium.
- Tool shows Medium, 22.4 in, with 56.9 cm and a strap check.
- Metric match: 57 cm gives Medium at 22.44 in.
Reference table
| Head cm | Head in | Size |
|---|---|---|
| 50 | 19.69 | Child |
| 54 | 21.26 | Small |
| 57 | 22.44 | Medium |
| 61 | 24.02 | Large |
Limits
- Bike style sizing; moto and equestrian differ.
- Ignores head shape differences by brand.
- Kids between sizes need try on, not math.
- Post crash helmets need replacement, not resizing.
Common mistakes
- Measuring over thick hair or caps.
- Tilting the tape up at the back.
- Wearing the shell too far back.
- Leaving straps loose under the chin.
- Keeping a crashed helmet in use.
Try it yourself
Quick checks. Answers sit tucked under each question.
20.5 in head?
20.5 in is 52.1 cm, which is Small.
60 cm head in metric?
60 cm is Large at 23.62 in.
Questions
How do I measure my head?
Wrap a tape just above eyebrows and ears, level front to back, and read centimeters or inches.
What range is Medium?
Medium covers 56 to 59 cm, about 22.05 to 23.23 in.
How should it sit?
One to two finger widths above the brow with a level shell that does not rock side to side.
How tight is the strap?
Straps form a V at each ear with one to two fingers under the chin strap.
Same size across brands?
Shapes differ by brand, so the size is a starting point and the try on decides.
When do I replace a helmet?
Replace after a crash and check the CPSC label inside when buying in the US.