Ski boot sizing and Mondo sizes explained

Ski boots often use Mondo or Mondopoint sizing instead of the UK, US or EU sizes seen on everyday footwear. Mondo gives you a useful place to begin, but a number on the shell cannot tell you whether a particular used boot will suit the shape of your foot.

Flex numbers and sole markings explained

Flex describes how stiff a ski boot feels when you press forwards. Within a comparable range, a higher number is stiffer: a 120 usually takes more force to bend than an 80. Softer boots are generally easier to flex; stiffer boots need the strength, technique and fit to use them well. Brands do not share one flex standard, so use a boot fitter to match the stiffness to you.

GripWalk identifies a walking-friendly sole system that needs a compatible binding. The boot’s sole marking and the binding’s compatibility chart must agree. Mondo, boot sole length and flex each answer a different question: foot size, binding fit and stiffness.

What does Mondo size mean?

Mondo size is based on foot length in centimetres. A foot measuring about 26.0 cm therefore starts its search around Mondo 26.0. Measure both feet because they may differ, and use the longer measurement as your starting point.

Atomic's boot-fitting guidance notes that foot length is only the beginning. Forefoot and heel width, instep height, foot volume, ankle movement and tolerance for a close fit also affect the right choice. Some adjacent half sizes share the same shell, depending on the manufacturer, so do not assume that every 0.5 step means a different shell.

How to measure your feet at home

1. Put a sheet of paper on a hard floor with one edge against a wall.
2. Following Atomic’s measuring method, stand barefoot with your heel lightly against the wall. Try boots later with your usual thin ski socks.
3. Keep your weight on the foot and mark the end of the longest toe.
4. Measure from the wall edge to the mark in centimetres.
5. Repeat for the other foot and record the longer length.

This measurement narrows the search. It does not replace trying the boots or advice from a professional boot fitter.

Mondo size is not the same as boot sole length

Boot sole length, often marked as BSL, is the outside length of the plastic shell in millimetres. It may appear near the heel or along the side of the shell. BSL is used when setting up bindings and is not your foot length or Mondo size. Two boots with the same Mondo size can have different BSL measurements.

Width, volume and liner condition matter

The last is the width of the boot around the forefoot, usually stated for a reference size. A narrow, low-volume shell can feel very different from a wide, high-volume shell with the same Mondo number. Used liners may also have compressed around the previous owner's foot. Check the listing photos and notes for liner wear, heel wear, damaged tongues, buckles that do not close cleanly, cracks in the shell and excessive sole wear.

What should a ski boot feel like?

A ski boot normally fits more closely than a street shoe. Your foot should feel supported without sharp pressure, numbness or obvious heel lift when you flex forward. Thick socks are not a reliable fix for an oversized shell and can change how the boot feels. If you are unsure, a boot fitter can measure length, width and volume and assess the shell and liner.

Check boot and binding compatibility

Do not assume that a boot will work with a binding because both are alpine products. Sole standards and binding compatibility vary, including Alpine and GripWalk systems. Look for markings on the boot and binding and ask a qualified ski shop to check the combination when anything is unclear. Used ski bindings should also be inspected, adjusted and tested for the skier and boot before use.

For the actual combination, use the manufacturer’s compatibility information and a qualified technician. Salomon’s compatibility guidance explains why markings on both pieces matter; a matching Mondo size alone does not establish compatibility.

Using ReGear ski boot listings

ReGear product pages show the actual boots and record marked sizing evidence where it can be read. Compare the listing with your measurements, examine the shells, liners, buckles and soles, and read all condition notes. Browse current second-hand ski boots, pair them with used skis, or return to all snow gear.

Further reading: Atomic ski boot fit guidance and our guide to buying second-hand snow gear in New Zealand.

References for these explanations

Checked 26 September 2026. Salomon: ski-boot flex; Salomon: boot and binding compatibility.

Updated 26 September 2026: added plain-language explanations of ratings, terms and buying choices.