Stack height is the amount of material between your foot and the road. It’s usually quoted as two figures, heel and forefoot, and the gap between them is the drop. Once you can read both numbers, most of a shoe’s character is on the page.
Reading a stack figure
“36mm heel / 30mm forefoot” tells you three things at once: there’s a substantial amount of foam under the heel, a substantial amount under the forefoot, and the shoe has a 6mm drop.
More stack generally means more material to absorb impact and more foam to store and return energy. Less stack generally means a lower, more connected ride where you feel the ground and can react to it faster.
Stack and stability
Height and stability pull against each other. The higher you stand above the ground, the more leverage there is to tip sideways — which is why tall shoes are usually built with wider platforms, sidewalls that cup the foot, or a plate to stop the foam folding.
This is also why very tall shoes are a poor fit for technical trail. On tar, height is comfort. On uneven ground, height is a wobble.
Stack and race rules
Competitive road racing under World Athletics rules caps the permitted stack height for road shoes, and the limits differ for track events. If you race at a level where shoe compliance is checked, confirm the current rule for your event and distance before you buy — the governing-body list is the only authority worth trusting on this.
For everyone else, at parkrun, a club 10km or a weekend marathon, this simply isn’t a consideration.
Choosing a stack
Higher stack for long, easy, high-volume road running where comfort compounds. Lower stack for speed work, short racing and anything where you want ground feel and control. Most runners are best served by owning one of each rather than splitting the difference.
Stack is how much shoe. Drop is how it’s tilted. Read both together.
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