Downhill Ski Boots: How to Choose

Ski boots are the key link between your body and your skis, so finding boots well-matched to the size and shape of your feet is a high priority.
Ski boots consist of a hard outer shell for support and soft liner for cushioning and warmth. Most adult boots are "front-entry overlap" designs, meaning that they open in the front like hiking boots and are secured by 3 or 4 buckles. Some kids' boots are "rear-entry" style. These models open in the back, which makes them more comfortable and user-friendly for beginners.
You should narrow your search for the best ski boots by focusing on fit and performance—style and color are secondary considerations. If at all possible, have yourself professionally fit by an REI ski boot specialist or other qualified person.

Ski Boot Flex Index

Ski boots are best shopped by the amount of flex the boots offer, which roughly correlates to your ability level: novice, intermediate or advanced.
Ski boot manufacturers rate boots on a flex index. The higher the number, the stiffer the boot. The more rigid your boot, the more power transfers to your ski's inside edge. REI.com product pages list the flex index under the "specs" tab. Unfortunately, flex index ratings are comparable only within a manufacturer's line, so the best way to compare when shopping is to put a boot on each foot, lean forward and flex them. (Boots will have a softer flex at a warm indoor temperature than they will out on the ski hill.)
When choosing your boots, be honest in your self-evaluation of your skill level. Below is a general guideline for matching flex index to your ability level, but know that within each level you can still refine your range.

Soft flex

Best for beginners. Flex index is typically below 80.
Soft-flexing boots are usually comfortable and warm. They’re typically best if you're new to the sport or you've been at it for a while but enjoy skiing leisurely on groomed green and blue runs. A soft flex does make it a bit harder to control the ski; go a step up if you expect to progress quickly.

Medium flex

Best for intermediate. Flex index ranges from about 80-100.
Medium-flex boots are geared to intermediate skiers and deliver increased responsiveness for improved turn-carving skills and higher speeds. They're ideal for those who can comfortably ski blues and easier black runs and are ready to tackle steeper terrain, moguls and ungroomed snow.

Stiff flex

Best for advanced. Flex index is typically above 100.
Stiff-flex boots are highly responsive and designed for those who ski with confidence, speed and aggressiveness on the steepest and most challenging terrain. Some boots offer features such as shock absorption for landing jumps or slamming bumps. Most advanced-level boots combine multi-density materials to make boots stiff in critical areas of energy transfer, but soft in other areas. Very stiff racing boots can be uncomfortably rigid for general use.

Ski Boot Sizing

Mondo Sizing: Downhill ski boots use mondo sizing, which refers to the boot’s inner sole length in centimeters. Sizes are not always uniform by brand or even from one model to another, but here is a general guide to get you started:

Mondo Ski Boot Sizes to US Sizes

Mondo	26.5	27	27.5	28	28.5	29	29.5	30	30.5
Women's US	10	10.5	11	11.5	12	NA	NA	NA	NA
Men's US	8.5	9	9.5	10	10.5	11	11.5	12	13

Mondo	21.5	22	22.5	23	23.5	24	24.5	25	25.5	26
Women's US	5	5.5	6	6.5	7	7.5	8	8.5	9	9.5
Men's US	NA	NA	NA	5	5.5	6	6.5	7	7.5	8

Last size: Last is the width of the ball of your foot and ranges from 97mm to 106mm. A tighter fit is more responsive for higher performance.

Ski Boot Liners

Most boots come with some amount of heat-moldable material in the liners. Typically, more expensive boots will feature more heat-moldable material. Some liners have down-filled toe boxes for added warmth.
There are several liner types to choose from:
Thermal-formable foam liners use your foot’s heat to achieve a custom fit. They break in after a day or so of skiing.
Custom moldable liners use an artificial heat source to achieve a custom fit. Though best done at REI or other ski shops with boot-fitting expertise, it is possible to do this at home with your oven or a hot-air blower.
Molded foam liners are made by molding liner tongues and foot sections individually, then sewing them together for a precise fit. These are not heat-moldable.

Additional Ski Boot Features

Ski/Walk Mode: Do your ski days include hikes up sidecountry ridges to find untracked powder? Many medium- to high-end boots let you separate the upper shell from the lower boot for more comfortable walking. When you're ready to ski the descent, you can lock the upper and lower shell together to maximize power transmission.
Flex Adjustment: Some ski boots have a switch so you can adjust the boots' stiffness to match a particular type of skiing, like powder, groomers or bumps. It is usually located on the back of the boot, around ankle height.