Ski Boot Flex Explained

Published on 05/12/2023 · 13 min readSki boot flex is important to take note of when shopping for a new pair of ski boots! Check out this guide for what you'll need to know about the flex of your new boots.
Hunter R., Ski Expert
By Ski Expert Hunter R.

Photo by Wilfredor

There are a lot of options when shopping for ski boots. And, given the bad reputation that ski boots get—historically being one of the most uncomfortable parts of skiing for most people—choosing the right ski boot can feel like a high-stakes game that you almost can’t win! But it doesn't have to be that way! This guide will break down some of the things you will see when shopping for ski boots and the most important things to consider. We will lightly touch on fit and shape and dive into flex rating!

Why Is It So Important to Have the Correct Ski Boot?

First, it's important to understand why any of this matters. Ski boots are responsible for the energy transmission between your body and your skis. So having a ski boot that is an awesome fit while you’re on the slopes will make you feel more in control, help you get better quicker, and leave you less tired and having more fun on any given day. A ski boot that's a bad fit will leave you feeling out of control on the mountain, wondering why you can’t progress, and taking rest every other day because your legs are so sore.

Fit and flex index are the two main components you need to focus on when looking for downhill boots. There is a bit more to know when you get into alpine touring boots and freestyle boots, but in this article, we will stick to downhill boots.

The most obvious part of finding a good ski boot is finding something that fits well. The right-fitting boot will have a snug fit but not be tight to the point where it causes loss of blood circulation or any pressure points. When in ski stance (bending your knees and ankles), your toes should be touching the front of your boot but not feel crunched, and your heel should be firmly in the heel pocket in the back of the boot. A ski boot that's too large can cause things like shin-bang (where your shins hit the front of your boot and get deep bone bruises) and blisters. A ski boot that's too small can cause lost toenails and blisters and result in a higher risk for frostbite due to the loss of circulation in your feet. If you are struggling to find a ski boot with a comfortable fit, check out this guide for five rules of wearing ski boots which offers some suggestions on fit modification options such as using an aftermarket footbed, heat molding your ski boot liners, heat molding your ski boot shells, and more!

The second part of finding a good ski boot is the flex index. We will get into the details of this later on but understand that it's the main component responsible for power transmission from the body to the ski. The incorrect flex will severely diminish your control and energy transfer over your skis. Meaning that you will need to put more muscle into every turn, making your legs much more tired and leaving you much more frustrated.

Size

Ski boots are sized by what's called mondopoint or mondo size, which is a fancy way of saying the length of the foot in centimeters. If you take a tape measure and measure the length of your foot, that would be your mondo point size. Check the size chart below of the conversions from regular shoe size to ski boot size, so you’ll know what to look for!

Ski boots generally only come in half sizes, but if you are in-between sizes, the right size for you is generally the smaller one. For example, if you are a women's 7.5/8, normally you'd want a size 24.5 instead of a 25.5. Boots pack out as they are worn, so something that is a bit tight the first time can be packed out to be a perfect fit, whereas if you start with extra room in the first place, you will cause yourself a world of hurt later when the boot packs out.

Width

Aside from the length of the ski boot, another factor in fit is the width. Boots come in medium volume, high volume, or low volume. This is often noted in their name as MV, LV, or HV (for example, the Salomon S/pro HV 90 Women's Ski Boots). These indicate if the boot is high, medium, or low-volume, meaning either a wide, medium, or narrow fit, respectively. Those who have a wider foot or a narrower foot and have had to buy wide or narrow specific shoes in the past will want to look for something that is wide or narrow so that there isn't too little or too much room on the sides of your feet when in your ski boot.

Though it is often designated in the product name, as in the example above, many brands just have slightly different model names for their high or low-volume boots. For example, the Nordica Sportmachine and the Nordica Speedmachine are the exact same boot, except that the Sportmachine is 102 mm in width across the forefoot (high volume/wide), whereas the Speedmachine is 100 mm in width across the forefoot (medium volume/average). If the width is not obvious in the name, you can check for this by looking at the tech specs on a product under “last width.” Below is an example and a table of the last widths that would designate each volume style.

If you have a unique foot shape, such as a narrow heel and wide toe box, there are some options for you, too, such as narrow-fit boots that have the ability to be widened in the toe box!

Flex Index

What is a flex index?

Aside from fit, the other important thing in ski boots is the flex index. Flex index refers to the stiffness of a boot and will appear as a number between 60 and 130 in the product's name. A high flex, such as a 130, is considered a very stiff boot, good for expert skiers and racers, and a low flex, such as a 60, is considered a soft boot that is more beginner-oriented. You can tell if a boot has a stiffer or softer flex by trying to bend your knees and lean forward while in your boots. If you can bend quite a bit, it's a softer flex; if you can’t bend very much, it's a stiffer flex.

Stiffer boots are a less comfortable fit but have more efficient energy transfer from your legs to your skis. This, in turn, makes your skiing more efficient and requires less energy to get your skis to go where you want them to go. You can ski steeper and more difficult terrain without your legs feeling as tired!

What determines the flex?

A few things are responsible for determining the flex of the boot.

  • Material of the outer shell of the boot (the hard, plastic part)
  • Thickness of that material
  • Number of buckles and material of those buckles
  • Strength of the velcro strap at the top of the boot (called the power strap)
  • Thickness/material of the liner (inside, the soft part of the boot)

Generally, the stiffer the boot, the higher grade of materials, resulting in higher flex boots having a higher price point. For more on ski boot materials, check out What Are Ski Boots Made of and Does It Affect Your Performance?

Standardization

One important thing to note is that there is actually no standardization of this number. Though flex refers to the general stiffness, there is no governing body that ensures all ski boot manufacturers have the same amount of bend in an 85 flex boot, for example.

This means that an 85 flex boot from Atomic might feel slightly stiffer than an 85 flex boot from Salomon. They are generally pretty close to the same from brand to brand, but just worth mentioning because if you think an 85 is too stiff for you in one brand, it is worth trying an 85 in another brand to see if it's a better fit!

There are a few reasons that there is no standardization. First of all, the flex rating is determined by the type of foot that the boot is made for. A 130 flex in a high-volume boot will feel different than the exact same boot in the low-volume version. Similarly, standardization would require most boots to be of a similar design/material. The great part about searching for boots is that if you find one brand that doesn't work, you have several others to choose from that have slightly different materials/construction. And lastly, standards such as this require pricey testing, and boot molds are expensive to make, so if one brand had a boot that was just slightly out of the standardization, it could be financially detrimental.

Even without this standard, most companies still have some process, such as the one listed below, that makes them all within the same relative range across flex ratings.

How is it measured then?

Brand to brand is different, but here's one boot manufacturer's process, Atomic, just to give you an idea. 1. Purchase all boots in a 130 flex from competitors. 2. Test the flex on all of them using in-house measuring, field testing, and a robot. 3. Create a 130 flex boot that scores in the middle on all these measurements 4. After successfully doing this, blend softer plastics in to create the same version of that boot in softer flexes such as 120, 110, 100, etc.

What's the best flex for me?

Your height, weight, skill level, and preference determine the best flex for you. It would be best to check in with a Ski Expert on Curated since it varies so much from person to person. But below, we will look at all the flex categories so you can understand the pros and cons of each.

These specific numbers are general recommendations that depend greatly on your specific height and weight, so take the men's and women’s recommendations with a grain of salt.

Soft Flex

New-Beginner/Intermediate

  • Women: 55-70
  • Men: 65-80

Good for a beginner skier and anyone who is a bit lighter weight. If you are on the bunny hill, greens, and maybe the occasional blue—this is likely the best choice for you. Soft flex boots generally have the thinnest plastic liner, and for anyone who is just getting into skiing and is not yet familiar with the woes of wearing ski boots, it will be the least shocking entry to the sport since they are the most comfortable. They allow you to flex forward quite a bit, which offers the most control for beginners. They are also a great option for intermediate skiers who weigh a bit less.

The downfall to a soft boot is that it's usually an entry-level boot that offers less customization than a medium or higher flex boot would. The upside of that is that they are going to be the cheapest since they cater to those just getting into the sport and don't have all the bells and whistles.

Medium Flex

Beginner-Intermediate/Advanced

  • Women: 70-90
  • Men: 80-100

A medium flexing boot is great for most people who know what they are doing, maybe explore a bit off-trail, but want to do mostly leisure days on the groomers. They are also great for more aggressive, lightweight folks. For reference, as a 5’3, 105 lb expert skier, this is the range of flex I ski, mostly due to my weight. I also commonly recommend an 85 flex boot to larger men who are still in the learning phases of their skiing journey. Medium flex is the most common type of boot I find myself recommending as a Ski Expert on Curated because it's the best of both worlds in terms of comfort/warmth and enough responsiveness that it doesn't kill your legs to ski all day (as would a soft boot for those who aren't beginners).

These generally also have the most customizable options in terms of either having walk mode, an adjustable upper cuff, and more micro-adjustment buckles.

Stiff Flex

Intermediate-Advanced/Expert

  • Women: 90-100
  • Men: 110-125

This flex will be great for those who are aggressive or advanced skiers. Maybe you want to do a bit of on-trail skiing but mostly want to go off-piste. They offer great control, particularly at higher speeds than soft/med flex boots. They are usually a different, higher-grade plastic than medium or soft flex boots, and they will have a slightly thinner liner.

Very Stiff

Advanced/Expert/Professional/Competitive

  • Women: 105+
  • Men: 130+

A very stiff boot would be a good option for heavier, taller skiers as well as expert/aggressive skiers and competitive ski racers. These stiff boots will be very responsive and efficient, requiring less energy transfer from the skier to the ski. Great for those who need this performance, but not so great for the occasional skier who is just looking to get out and do some groomer runs! The added stiffness comes with an often thinner liner and thicker shell, which means not quite as warm or comfortable on the inside. This is especially true in race boots which will pretty much always be a very stiff flex. Unless you're a pro, you probably do not want a boot with these flex ratings. Get some softer flex boots. You'll thank me later!

Final Thoughts

As I mentioned earlier, it's a real case-by-case basis figuring out what the best flex is for you specifically. This answer would need to take into consideration your height, weight, where you ski, as well as other specifics. Some people with knee or ankle issues prefer a lower flex than someone of the same height/weight/skill level without a knee or ankle issue would need.

There are also some boots that have special features such as an adjustable flex, which would be great for those who are looking to advance more in the next season or two but, as for now, are still getting their feet under them in the sport.

No matter how many or few days of skiing you get per year, it's important to have a ski boot that's perfect for you, and the correct flex can be a very hard thing to figure out on your own. The best option, if you are still feeling unsure about the correct flex by the end of this article, would be for you to check in with a Ski Expert here on Curated. Before chatting with an expert, you will fill out a questionnaire asking many specifics that are relevant to determining the best flex for you. You will then be connected to a real person who can take any other info into account (an old ankle injury, for example) and make a recommendation based on the whole picture. We look forward to helping you find your perfect boot!

Curated experts can help

Have a question about the article you just read or want personal recommendations? Connect with a Curated expert and get free recommendations for whatever you’re looking for!

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