How to Size Running Shoes
Two starting-point rules: leave about a thumb's width (roughly half an inch) between your longest toe and the shoe's end, and consider sizing up about a half size from your everyday shoe since feet swell on longer runs. Both are starting points — sizing still varies significantly by brand and last shape.
The one thing to know: Two common sizing rules of thumb: the thumb-width rule (roughly a thumb's width, about half an inch, of space between the longest toe and the shoe's end when standing) and sizing up about a half size from a runner's typical everyday-shoe size to accommodate feet swelling on longer runs, since foot volume increases measurably over the course of an hour-plus effort. Sizing still varies significantly by brand and last shape, so these are starting points for an in-store or return-eligible online fitting, not a substitute for trying a specific model.
Why feet swell on runs
Foot volume measurably increases over the course of an hour-plus effort as blood flow and impact cause tissue to expand slightly — which is why a shoe that fits perfectly standing in a store can feel tight at mile 10. The half-size-up convention exists to leave room for that change rather than to accommodate a permanently larger foot.
Why the rules of thumb aren't the whole answer
Last shape — the foot-shaped mold a shoe is built around — varies by brand and even by model line within a brand, so the same labeled size can feel narrow in one shoe and roomy in another. A specialty running store fitting, or ordering from a retailer with an easy return policy so you can test the actual fit on a short run, covers gaps that a spec-sheet rule can't.
This page describes general sizing conventions, not a fit guarantee for any individual foot or shoe model.
Frequently Asked Questions
A common rule of thumb is about a thumb's width — roughly half an inch — of space between your longest toe and the end of the shoe when standing. This leaves room for the foot to move forward on impact and swell without jamming the toes.
Many runners size up about a half size from their everyday-shoe size, since feet swell measurably over the course of longer runs. This is a starting-point convention, not a universal rule — some runners find their everyday size fits fine.
Yes, significantly. The same labeled size (e.g., US 10) can fit differently across brands and even across models from the same brand, because last shapes (the foot-shaped mold a shoe is built on) differ. Always check a specific brand's size chart rather than assuming your usual number applies.
Feet are generally at their largest later in the day after walking and standing, closer to how they'll feel mid-run, so trying shoes on in the afternoon or evening is commonly recommended over first thing in the morning.
Yes — trying shoes on with the actual socks you plan to run in (thickness affects fit) gives a more accurate sense of the true in-shoe volume than trying them on barefoot or with regular socks.