Heel Drop and Stack Height Explained: What the Numbers on the Box Actually Mean

Stack height is the amount of material between your foot and the road, in millimeters, measured separately at the heel and the forefoot. Heel-to-toe drop is the difference between those two figures. A shoe listed as “39 / 31 mm, 8 mm drop” has 39 mm under the heel, 31 mm under the forefoot, and the 8 mm gap is the drop. Both numbers describe geometry, not quality. Neither one tells you whether a shoe is good, and for most runners at most levels the right answer is the geometry you are already used to.

Both numbers at a glance

Those category boundaries are RunRepeat’s, from the lab where they cut shoes in half and measure them, and the drop range and the 10 mm convention come from the same lab’s heel-drop guide. We do not measure shoes ourselves; every figure on this page is attributed to whoever published it.

What is stack height?

Stack height is how thick the sole is: the foam, the insole and the outsole rubber stacked between your foot and the ground, quoted in millimeters at the heel and at the forefoot. A shoe with a 41 mm heel has about four centimeters of material under your heel.

There is a standard for where to measure it. RunRepeat follows the World Athletics method and marks the sole at 12% and 75% of the shoe’s inside length, which gives the forefoot and heel figures, and it includes the insole because runners run with the insole in. That detail matters when you compare numbers across sources: a measurement taken with the insole counted will be a few millimeters taller than one without.

What more stack buys you is protection over distance and, usually, a softer first impression underfoot. What it costs you is weight, ground feel and sometimes stability. The trade is easiest to see in two shoes from our registry: the Saucony Kinvara 16 sits on a 29 mm / 24 mm stack and weighs 7.3 oz in a men’s 9 per Running Warehouse, while the ASICS Superblast 3 sits on 46.5 mm / 38.5 mm per Doctors of Running and is described by RunRepeat as offering virtually no ground feedback. Neither is the better shoe. They are built for different jobs, which is the argument of our running shoe types explained guide.

What is heel-to-toe drop?

Heel-to-toe drop, also called heel drop, offset or pitch, is the heel stack minus the forefoot stack. It describes how much your heel is raised relative to your toes, and it is the single number most often confused with cushioning.

It is worth separating the two ideas properly, because a shoe can be soft and flat, or firm and steeply pitched. RunRepeat’s lab sorts drop into four bands: zero drop (0 mm, with tolerance up to 0.9 mm), low drop (1-4.9 mm), mid drop (5-7.9 mm) and high drop (8 mm and up). Most mainstream daily trainers land in the mid or high band. The Brooks Ghost 18 is a classic high-drop shoe: 10 mm listed, 10.8 mm when RTINGS measured it.

Mechanically, drop changes where the load goes rather than how much of it there is. A flatter shoe asks more of the calf, Achilles and ankle because the forefoot is relatively higher; a steeper shoe tips a little more of the work forward to the knee and hip. That is why the practical advice is about your history, not about an ideal: RunRepeat’s guide suggests a higher drop for runners managing a stiff Achilles or calf issues, and a lower drop for forefoot and midfoot strikers. If something hurts now, that is a conversation with a clinician, not a shoe-shopping decision.

The two numbers are independent

This is the part the spec sheet hides. Stack height and drop vary separately, so all four combinations exist on shelves right now, and a shoe’s drop tells you nothing about how much foam it has.

The More v6 is the clearest example of why the two must be read together. RunRepeat notes its 38.5 mm lab-measured forefoot is taller than the heel of most shoes it has tested, yet its drop is one of the lowest in our registry. “Low drop” and “minimal” are not synonyms.

Why the box and the lab disagree

Brand-stated drop and lab-measured drop frequently differ, sometimes by several millimeters, so treat the number on the box as a design intention rather than a measurement.

RunRepeat put numbers on the problem. In an analysis of its own lab results, 122 shoes were tested and 115 used for the deviation breakdown after the most extreme outliers were set aside. Of those 115: 40 came within 0-9% of the stated drop, 32 within 10-19%, 15 within 20-29%, 9 within 30-39%, 4 within 40-49%, and 15 were off by more than 50%. By brand, the average deviation ran from Nike at 10% and adidas at 12% up to HOKA at 27% and New Balance at 34%. The extremes were startling: a Skechers Max Cushioning Elite listed at 6 mm measured 16.1 mm, and a New Balance Fresh Foam X 1080 v12 listed at 8 mm measured 3.3 mm.

Shoes in our own registry show the same spread, which is why we print both figures wherever they disagree:

• ASICS Gel-Cumulus 28: 8 mm listed, 11.7 mm lab-measured by RTINGS — a mid-drop shoe on paper that measures like a high-drop one.
• HOKA Bondi 9: 5 mm listed, 9.1 mm in RunRepeat’s lab.
• HOKA Clifton 11: 8 mm listed, 10.2 mm per RTINGS.
• ASICS Gel-Nimbus 28: 8 mm listed, 5.2 mm measured by RunRepeat — the deviation runs both ways. The same review measured the stack at 39.5 / 34.3 mm against the 43.5 / 35.5 mm ASICS lists.

Does heel drop change injury risk?

The best available trial says no, not on average. Drop did change injury risk in opposite directions for two different kinds of runner, which is a more useful finding than a single recommendation.

Malisoux and colleagues (2016) gave 553 leisure-time runners one of three versions of the same standard cushioned shoe — 10 mm, 6 mm or 0 mm drop — and followed them for six months, with runners logging their training and any injury that cost them at least a day of running. Overall, injury risk was no different in the 6 mm group (hazard ratio 1.30, 95% CI 0.86-1.98) or the 0 mm group (HR 1.17, 95% CI 0.76-1.80) than in the 10 mm group.

The split by training history is the interesting part. Among occasional runners — fewer than six months of weekly running in the previous year — the low-drop shoes were associated with a lower injury risk (HR 0.48, 95% CI 0.23-0.98). Among regular runners, the same shoes were associated with a higher risk (HR 1.67, 95% CI 1.07-2.62). The authors’ reading: low-drop shoes may suit newer runners and could be more hazardous for established ones. One plausible explanation is adaptation. A runner with years of 10 mm shoes has tissue tuned to that geometry; a near-beginner has no such habit to break.

Lab work fills in the mechanism. Besson and colleagues (2019) had 15 female runners run a 15-meter runway at their preferred pace in 0 mm, 6 mm and 10 mm shoes. The zero-drop condition produced significantly higher loading rates and impact transient peaks than either the 6 mm or the 10 mm shoe, plus higher ankle dorsiflexion moments during braking. Between 6 mm and 10 mm there was no significant difference at all — worth remembering the next time a 2 mm change is marketed as a redesign.

Does stack height change anything?

Measurably, yes, but mostly in how you run rather than in how fast or how safely. The clearest recent study found the tallest stack changed stride mechanics and two stability measures.

Kettner, Stetter and Stein (2025) put 17 experienced runners on a treadmill at 10 and 15 km/h — about 9:39 and 6:26 per mile, our conversion — in three shoes that differed mainly in stack height: 50 mm, 35 mm and 27 mm. The 50 mm shoe produced longer ground contact relative to stride time than the 27 mm shoe, and vertical movement of the center of mass rose as stack height rose. It also kept the foot in an everted position longer than the 35 mm shoe, and local dynamic stability at the hip was lower than in the 27 mm shoe. The authors flag those last two as “changes related with instabilities” that may matter for injury and performance.

Read that carefully before drawing a conclusion. It is 17 runners on a treadmill, it measured no joint loads and no oxygen cost, and the 50 mm test shoe is taller than almost anything sold as a road trainer. What it does not support is the idea that more foam is automatically more protective. If you want a tall stack, choose one with a wide, stable platform and spend the first few runs on flat ground. Our best max-cushion running shoes list ranks the tall shoes with stability and fit range in the rubric.

The 40 mm racing limit

A road racing shoe’s sole may not be thicker than 40 mm, and it may contain no more than one rigid embedded plate or blade. Those are World Athletics’ competition shoe rules, which state that “the sole must be no thicker than 40mm” and that a plate may be made of several parts only if they sit sequentially in a single plane, without stacking or overlapping.

Two things follow for ordinary runners. First, the rule covers shoes worn in competition under World Athletics rules, not your training shoes — which is exactly why a 46.5 mm trainer like the Superblast 3 can exist and sell well. Second, race shoes now cluster right under the ceiling: the ASICS Metaspeed Ray measured 39.8 mm at the heel in RunRepeat’s lab, 0.2 mm inside the cap. If you are running a local 10K, none of this constrains you; if you are chasing a qualifying time, check that the shoe is on the approved list before race day. More on the trade-offs in are carbon-plated shoes worth it?

Five real shoes, 29 mm to 46.5 mm

Here is the geometry argument in five shoes you can buy: one for each corner of the stack-and-drop grid, plus the conventional default. We have not run in any of them. The numbers are lab measurements and published specs, attributed on each product page, and our scoring method is set out at how we pick.

What to pick at your level

If you are not sure which tier you are in, the Level Finder sorts it by weekly mileage and goal race, and the rotation guide shows how many pairs each tier actually needs. Beginners choosing a first pair should start with how to choose running shoes, where fit and width matter far more than either number on this page.

How to switch drop safely

If you are moving more than about 4 mm in either direction, treat it as a change in training, not a purchase. RunRepeat’s guide puts the threshold in the same place: at 4 mm and up, plan an adaptation period, and treat it as mandatory when going to a low or zero drop.

• Keep the old pair in rotation. Run the first week in the new geometry on short, easy runs only, and do the long run in the shoes your legs already know.
• Expect the calf to notice first. Going lower loads the calf and Achilles more. Mild stiffness that settles within a day or two is ordinary adaptation; pain that builds run to run is not.
• Do not stack changes. A new drop, a new stack height and a new mileage block at the same time makes it impossible to tell what caused what.
• Keep race week boring. Whatever the spec sheet says, race day is for the geometry you have already run long in.

One last reason not to assume: drop and stack can change between versions of the same model, so the shoe you loved two years ago may not have the same geometry today. Check the current numbers rather than your memory of them.