What the convention comes down to
A ski boot’s flex index is a number printed on the cuff — commonly 60 to 130, occasionally higher on race boots — meant to describe how hard the boot resists being bent forward.
There is no published method behind it. No body defines the test, the temperature, the load or the point of measurement. Each maker sets its own figure, on its own scale, in a way it does not publish.
What that produces is a number that is meaningful within one maker’s range and unreliable between makers. A 100 from one brand and a 100 from another can differ by what feels like ten points, and everybody in the trade knows it.
Snowboard boots use the same idea more loosely still, usually as soft / medium / stiff or as a number out of ten, with the same absence of a method.
What it does not promise
It does not describe the skier. It is a property of a plastic shell, and reading it as a grade — beginner 60, expert 130 — is how people end up in boots they cannot bend at the end of a cold day.
It does not hold at temperature. Plastic stiffens in the cold, and the figure was not measured in the cold.
It does not survive the liner. A packed-out liner changes how the boot feels long before the shell changes.
It does not describe lateral stiffness, which is what actually transmits an edge, and which no maker publishes at all.
What it is read alongside
The boot sole standards, which do have methods and do define dimensions, and which say nothing about stiffness. The binding assembly procedure, which uses the boot’s sole length and ignores its flex. Between them, the flex index is the only number on a boot that no document supports.
Where it turns up on this site
Ski boots and snowboard boots.
This site handles it in one way, consistently. A boot page prints the flex index as the maker published it, with the maker named, and states in the same breath that the scale is not comparable between brands. It never orders a family by flex, never converts a flex into an ability level, and never describes a stiffer boot as a better one.
The questions asked most often
What flex should I buy?
This site will not answer that, because the answer depends on weight, speed and how a particular boot is built. What the pages do is print the figure, name the maker, and say that it is not comparable across brands.
Does flex change with temperature?
Plastic stiffens as it gets colder, and every boot on this shelf is made of plastic. A boot bought in a warm shop is stiffer on a −15 °C morning, and no maker publishes a figure for that.
Do women’s boots use the same scale?
They use the same numbers, and the ranges offered are generally lower. That is a product decision, not a different scale.
Last reviewed 28 September 2026