Stack height is the total thickness of material between the bottom of your foot and the ground. It includes the midsole foam, the insole, and the outsole rubber, all measured in millimeters. A typical running shoe has a stack height somewhere between 20mm and 40mm, though some maximalist designs push past 50mm. This single number tells you more about how a shoe will feel underfoot than almost any other spec on the box.
Stack Height vs. Heel-to-Toe Drop
These two numbers get confused constantly, but they measure different things. Stack height is the total amount of material under your foot. Drop (also called offset) is the difference in height between the heel and the forefoot. A shoe with 30mm of material under the heel and 22mm under the forefoot has a stack height of 30mm at the heel and a drop of 8mm.
Here’s what trips people up: a low stack height doesn’t necessarily mean a low drop, and a high stack height doesn’t mean a high drop. You could have a 35mm shoe with only a 4mm drop if the foam is distributed almost evenly from heel to toe. Or a 22mm shoe with a 10mm drop if most of the cushioning is concentrated in the heel. When you’re comparing shoes, you need both numbers to understand what you’re getting.
How Stack Height Categories Break Down
Shoes generally fall into three tiers. Low stack height (roughly under 20mm) puts you close to the ground with minimal cushioning. These minimalist designs promote a more natural foot strike and tend to shift impact toward the forefoot and ankle rather than the knee. Mid-range stack heights (20mm to 30mm) cover the majority of traditional running shoes and offer a balance between ground feel and cushioning. High stack height shoes, sometimes called maximalist, run from 30mm up to 50mm or more and prioritize shock absorption above everything else.
The Adidas Adizero Prime X, at 50mm of heel stack, represents the extreme end of the spectrum. Most major racing shoes from Nike, Asics, and New Balance sit in the 35mm to 40mm range, which has become the sweet spot for competitive road running.
What More Cushioning Actually Does to Your Body
Thicker midsoles absorb more shock on impact. Research published in the Journal of Sports Science and Medicine found that thicker midsoles provide better cushioning and reduce the rate at which force loads into your legs during landing. Thinner midsoles, by contrast, increase vertical loading rates, meaning the impact hits harder and faster with each footstrike.
But more foam isn’t a simple upgrade. That same body of research found that thick midsoles decrease plantar sensation, your foot’s ability to feel the ground. When your foot can’t sense what’s happening beneath it, your body may compensate in ways that change your gait. Softer midsoles were linked to increased ankle stiffness and, surprisingly, higher peak impact forces in some runners. The body appears to adjust its landing mechanics based on how much cushioning it perceives, sometimes stomping harder on softer surfaces.
Maximalist shoes do reduce peak pressure on the forefoot compared to traditional shoes, which matters for runners dealing with metatarsal pain or stress reactions. A 2025 study on recreational runners found significantly lower forefoot plantar pressure in maximal shoes, along with a shorter center-of-pressure path, meaning the foot rolled through a more compact motion during each stride.
On the other end, minimalist shoes with low stack heights encourage midfoot or forefoot striking and shift load away from the knee. That comes at a cost: greater stress on the ankle joint, the Achilles tendon, and the small joints of the forefoot. Runners who transition to low-stack shoes too quickly often develop Achilles tendon issues or metatarsal stress injuries for exactly this reason.
Why Modern Shoes Keep Getting Taller
The explosion in stack heights over the past decade comes down to a single material innovation. Traditional midsole foams like EVA are relatively dense and heavy. Stacking more of it meant a heavier, clunkier shoe. The game changed when brands started using foams based on PEBA (polyether block amide), a thermoplastic material that is simultaneously soft, bouncy, and light. Nike’s ZoomX foam, made from a version of PEBA, was the first major application in running shoes.
PEBA-based foams have alternating hard and soft molecular segments that give them high energy return without the weight penalty of older materials. This meant designers could build shoes with 35mm or 40mm of foam that still weighed less than traditional 25mm trainers. Pair that foam with a rigid carbon fiber plate embedded in the midsole, and the combination creates a propulsive feel that has reshaped competitive running. Every major shoe brand now offers its own version of this formula: tall stack, premium foam, stiff plate.
Racing foams do sacrifice durability for performance. The lighter and bouncier the foam, the faster it breaks down. A high-stack racing shoe might hold its cushioning properties for 200 to 300 miles, while a denser training shoe lasts 400 to 500.
Competition Rules and Stack Limits
World Athletics, the governing body for track and field, caps how tall racing shoes can be. For road races and race walking events, the maximum allowable stack height is 40mm. For track and field events, the limit drops to 20mm for both spike and non-spike shoes. These regulations, updated for January 2026, exist specifically because of the performance advantages that high-stack, plate-equipped shoes provide.
The 40mm road limit is why most elite marathon shoes top out right at that number. Shoes like the Adizero Prime X, which exceeds 40mm, are legal for training and recreational racing but banned from World Athletics-sanctioned competitions. If you’re racing at a level where results are officially recorded, the stack height printed on the spec sheet matters.
Choosing the Right Stack Height
Your ideal stack height depends on what you’re asking the shoe to do. For long runs and recovery days, higher stack heights (30mm and above) reduce cumulative impact on joints and are generally easier on the body over many miles. For speed work and shorter races, a moderate stack (20mm to 30mm) offers better ground feel and responsiveness without excessive bulk. For trail running on technical terrain, lower stack heights improve stability and proprioception, your ability to feel and react to uneven ground.
Heavier runners often benefit from more stack height simply because impact forces scale with body weight. Lighter runners with efficient mechanics may prefer less material underfoot for a more connected feel. If you’ve had knee issues, higher stacks tend to reduce knee loading. If you’ve dealt with Achilles or forefoot problems, be cautious with very low stacks that shift stress to those areas.
The number on the spec sheet is the starting point, not the full picture. Two shoes with identical 32mm stack heights can feel completely different depending on the foam density, the outsole geometry, and whether there’s a plate inside. Stack height tells you how much material is there. The foam type tells you what that material actually does.