Kids' bike helmet size guide
Helmets are sized by head circumference in centimetres. Age only tells you where to start looking.
Checked against UK retailer charts on 18 October 2026.
Helmet size by age
| Halfords size | Recommended age | Head circumference |
|---|---|---|
| Toddlers | First years | 44–50 cm |
| Kids | 3+ years | 48–54 cm |
| Juniors | 7+ years | 52–59 cm |
BikeRadar gives similar general ranges, and points out that brands differ:
| Type | Typical circumference |
|---|---|
| Infant | 44–50 cm |
| Toddler | 46–52 cm |
| Kids' | 48–55 cm |
| Junior | 52–58 cm |
How to measure
- Use a soft tape measure, or a piece of string and a ruler.
- Wrap it level around the head, about an inch (two finger-widths) above the eyebrows and just above the ears.
- Read the measurement in cm and choose a helmet whose range includes it.
Fit checks
- Level: sits level across the forehead, about two finger-widths above the eyebrows, not tipped back.
- Snug: with the strap undone, it shouldn't fall off when they bend forward (BikeRadar's test).
- Straps: the Y of the side straps sits just below the ear; the chin strap is secure but lets them open their mouth.
- Too small: sits high on the head or leaves a red line (Halfords).
- Too big: moves around even with the dial tightened.
Don't buy a helmet to grow into. BikeRadar warns a helmet that's too big won't be secure in a fall. In the UK, look for a helmet that meets BS EN 1078.
Questions
What size helmet for a 5 year old?
Halfords’ Kids size (3+ years) covers 48–54 cm. Measure their head to check.
Can a child grow into a helmet?
No. A helmet that is too big won’t protect them properly. Buy for the head they have now; the rear dial gives a little adjustment.
Where these numbers come from
Checked 18 October 2026. Brands change their charts, so always check the size guide of the shop you're buying from.