Ski boot size explained: mondopoint, width, flex and how ski boots should fit

Ski boots are the part of your kit you feel most on the slope: you steer your skis through them, and when they do not fit, your feet hurt and the skis respond less well. Salomon and Fischer both cite an estimate that up to three quarters of skiers ride in boots one or two sizes too big, and Atomic calls an oversized boot the most common mistake skiers make. Here is how boot size works, how a boot should fit and what to check when you rent.

Mondopoint: size by foot length

Ski boot sizes use the mondopoint system, based on the length of your foot in centimetres: a size 26.5 fits a foot roughly 26.5 cm long. Boots come in half sizes, and according to Atomic two neighbouring sizes, such as 27 and 27.5, often share the same shell, with the liner and insoles of different thickness making up the difference.

Why is your street shoe size not enough? The continental European size describes the last the shoe is built on, not your foot, and sizes are two thirds of a centimetre apart, so conversion charts differ from brand to brand. The Czech ski magazine SNOW.cz suggests starting about one size below your street shoes and then forgetting street sizes. Measuring your foot is more reliable.

Bear in mind that feet swell a little during the day and spread slightly wider and longer under load, as SNOW.cz points out. So measure standing up, measure both feet and note down both figures.

Boot width: narrow, medium or wide

Length is only half the story. The other key measurement is the width of the boot at the widest part of the forefoot, known as the last and given in millimetres. Salomon groups it like this:

• around 92 mm: very narrow, for racers only
• 96–98 mm: narrow, for narrow feet or for skiers who value precision over comfort
• 100 mm: medium, suits most skiers
• 102 mm: for wide feet
• 104 mm: for very wide feet

According to SNOW.cz, the quoted width applies to size 26.5 and grows by a few millimetres with each larger size, and the boundaries vary between sources: SNOW.cz puts purely comfort-oriented boots at up to about 106 mm. A boot that is too wide will not hold your foot and makes the skis harder to control; one that is too narrow squeezes from the sides. Instep height matters too: a high instep will suffer in a low-volume boot.

Flex: how stiff should a ski boot be?

The flex index describes how easily the boot bends forward at the ankle. The higher the number, the stiffer the boot and the more directly it passes your movements to the skis. Softer boots are more comfortable and forgiving, so they suit beginners and lighter skiers. Salomon gives these rough ranges:

• beginners: men 60–90, women 60–70
• intermediates: men 90–110, women 70–90
• advanced skiers: men 110 and up, women 90 and up
• racers: 140–150

Treat these figures as a rough guide. The flex index is not a standard and every brand measures it differently, so a flex 100 boot from one brand can feel stiffer than a 120 from another, SNOW.cz notes, and other sources give other ranges. Temperature matters too: shell plastic stiffens in the cold and softens in the warmth, so a boot feels stiffer on the slope than in the shop.

How a ski boot should fit

Atomic compares a well-fitting boot to a firm handshake: it should wrap your foot but must not cut off circulation or press anywhere. When trying boots on, look out for the following:

• Standing upright, your toes may lightly touch the front; when you bend your knees and lean forward into a skiing stance, they should pull back.
• Your heel stays down and does not lift when you flex forward. SNOW.cz sums it up: no heel movement, but a little freedom for the toes.
• Neither your foot nor your shin slides forward or sideways in the boot, and nothing presses at a single point.
• Close the buckles from the bottom up and without much force. Buckles closed too tightly create pressure points of their own, Atomic warns.

A boot that is too big feels pleasant in the shop, but on the slope your foot shifts around, which according to Atomic leads to foot cramps and pain in the shins, toes and ankles. If your foot measures between two sizes, Salomon advises going for the smaller one, as the liner compresses over time. If you are buying, SNOW.cz recommends wearing the boots for at least 20 minutes before you decide.

The shell fit test in a ski shop

In a good shop, the fitter takes the liner out and you step into the empty shell in a thin sock until your toes touch the front; then the gap behind your heel is measured. Atomic says around 2 cm suits most skiers, 1 cm is a tight performance fit and 3 cm means the boot is too big. SNOW.cz is stricter and considers up to about 1.5 cm a good basis.

Socks, and what does not belong in a ski boot

One thin ski sock is all you need. Atomic recommends tall socks reaching just below the knee, made of merino or a quick-drying synthetic that wicks moisture, unlike cotton. A thick sock is not warmer, says Atomic; it just takes up room, squeezes your foot and makes cold toes more likely. Do not tuck long underwear or ski trousers into the boot either: the snow gaiter of your trousers goes over it.

Rental boots: what to tell us and how to try them on

At our rental in Vrchlabí we rent ski boots in a set with skis and poles, or on their own. To help the first size we prepare fit well, put your shoe size in the booking and, if you know it, the length of both feet in centimetres. For children, age, height and shoe size are enough.

• Bring the ski socks you will ski in when you pick up.
• Put the boots on, buckle them, stand in them for a while and bend your knees a few times.
• If something pinches or your heel slips, tell us straight away: sorting it out here is quicker than driving back from Špindlerův Mlýn.
• Have your own ski boots? Bring them along, because the bindings on rented skis are adjusted partly according to their sole length.

Our rental is in Vrchlabí on the main road to Špindlerův Mlýn, with parking right in front. Book online no later than the day before and we will prepare your gear from your details, so on the spot we only fine-tune the boot size.

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