What size ski helmet do I need, and how should it fit?

Measuring, in ninety seconds

1. Use a soft tailoring tape, or a piece of string you then measure against a ruler.
2. Run it around the head about 1 cm above the eyebrows at the front and over the widest point at the back, usually just above the bump at the base of the skull.
3. Keep it level all the way round. A tape that sags at the back reads short and gets you a helmet a size too small.
4. Measure over the hair you will actually ski with, and under nothing else. A modern ski helmet is lined; you do not wear a wool hat under it.
5. Read it in centimeters. Do it twice.

The adult size ranges, side by side

The point of putting these next to each other is the last two rows. Head circumference above about 63 cm is where the cheaper end of the market simply stops, and that is worth knowing before you spend an evening comparing helmets that do not come in your size.

The fit check, once it arrives

Measuring gets the right size in the box. This tells you whether it fits your head, which is a different question, because heads are different shapes.

1. Put it on without fastening the chin strap. The front edge should sit about two fingers’ width above your eyebrows, and the helmet should sit level rather than tipped back.
2. Shake your head side to side, then nod. The helmet should move the skin of your forehead with it. If it slides over your skin, it is too big.
3. Tighten the dial at the back until it is snug. Leave it a minute and take it off: a red line across your forehead means too tight, no line and no movement means correct.
4. Check for pressure points. Even, all-round snugness is right. Pressure at one spot means the shell shape does not match your head shape, and that is a different helmet, not a different size.
5. Fasten the chin strap. Two fingers between strap and chin, no more.
6. Put your goggles on and look for a gap between the goggle frame and the helmet brow. That fit has its own page, because it is where most people get caught out.

Head shape, which no chart covers

Helmet shells are molded to an assumed head shape, and heads vary from relatively round to relatively long front-to-back. A helmet that is the right circumference but the wrong shape presses at the front and back, or at the sides, and no amount of dial adjustment fixes it.

Some brands publish a round-fit variant of the same model for exactly this reason, and if a helmet is right on the tape and wrong on the head, that is what to look for. This is also one of the genuine advantages of buying in a shop, and we would rather say so than pretend a website solves it.

Long hair and ponytails

Measure with the hair where it will sit under the helmet. A ponytail worn high changes the measurement and the fit; worn low, through the gap at the back of most helmets, it changes neither. Many helmets have a shaped notch at the rear for this and it is worth checking before buying.

When a fitting helmet stops being one

• After any impact you noticed. The liner protects by crushing and does not crush twice, whatever the shell looks like.
• When the dial or strap stops holding. The retention system is what keeps the helmet on your head in the moment it matters.
• Never buy used. You cannot see whether a helmet has already done its job once.

Once you have your size, the picks on the adult roundup are ranked partly on how many sizes each maker publishes, because a helmet that does not exist in your size is not a pick for you. Children have their own chart.