How to Measure Bike Helmet Size: A Step-by-Step Fit Guide

Helmet sizes are based on head circumference, not hat size. Run a soft tape measure around your head roughly 1–2 cm above your eyebrows, keeping it level all the way round and passing just above your ears. Read the number in centimetres, then match it to the size chart for that exact helmet — every brand draws its S/M/L boundaries in slightly different places, so a medium from one maker is not a medium from another.

Measure twice. If the two readings differ, use the larger one. That number, plus a proper check of how the helmet actually sits once it is on your head, is the whole job.

• A soft dressmaker's tape measure — the flexible fabric kind, not a rigid DIY tape.
• No tape? A piece of non-stretchy string, ribbon or a strip of paper, plus a ruler to measure it against.
• A mirror, or someone to help. Judging whether the tape is level at the back of your own head is surprisingly hard.
• Your hair as you normally wear it when riding.
• A pen and paper, or your phone, to note the number down.

Wrap a piece of string, ribbon or a strip of paper around your head on the same line, pinch or mark the point where the ends meet, then lay it flat alongside a ruler. Read the length in centimetres.

Avoid anything elastic — it stretches as you pull and will give you a flattering, smaller number. The same goes for pulling a fabric tape tight: a snug, flat tape is what you want, not a tourniquet.

Find the size chart for the specific model, not the brand in general. Sizing often varies between a maker's road, mountain bike and kids' ranges, and it can change from one generation of a helmet to the next.

Many charts give a range per size, and those ranges often overlap. If your measurement falls in the overlap, either size may work and the fit checks below decide it.

If you are between sizes, remember what the retention system can and cannot do. A dial at the back will usually take up a small amount of slack, but it cannot make a shell that is too narrow any wider, and it cannot make an oversized shell sit low enough on your head.

Two people with identical measurements can have quite different head shapes — one rounder, one longer and narrower front to back. A helmet that matches your circumference can still pinch at the temples or leave a gap at the front, because shells are shaped differently too.

That is why the measurement narrows the field but does not finish the job. Your number tells you which size to pick up; the fit checks tell you whether that size is right for your head.

Have the child sit still and look straight ahead rather than tilting their head up or down, and keep the tape level as you go round. Children often wriggle, so take two readings and use the larger — but only if both look honest.

Re-measure every few months. Children's heads grow steadily rather than in jumps, and a helmet that was right in the spring can be tight by the autumn. Check the fit each time rather than relying on the label.