How to Choose Your Running Shoe Size (and Why It Isn't Your Street Size)

If you work behind the counter of a running store, there's one mistake you see every single day: most people get the size wrong. They ask for the same size they wear in everyday shoes, because that's what they've always worn. And for running, that's often too small.

Why your street size doesn't work for running

When you run, your foot doesn't stay still inside the shoe: it slides forward with every stride, and that movement gets bigger on downhills. Your feet also swell with exercise and heat, and gain volume over the course of a long run.

If you don't leave room at the front, your big toe hits the toe box again and again, mile after mile. The result is predictable: black toenails, toenails that fall off, blisters on the tips of your toes, and a ruined end to your race.

How to find the size you need, in four steps

1. Measure your foot. Don't guess. Measure standing up, with your weight evenly spread, and measure both feet: almost everyone has one that's longer. Go by the bigger one.
2. Add roughly half an inch (about 1 cm) to that measurement. That's the room you need at the front.
3. Check with your thumb. With your heel snug at the back, there should be about a thumb's width between your longest toe and the end of the shoe.
4. Keep the heel locked in. Room at the front, hold at the back. If your heel slips, the fix isn't a different size: try another model or another fit.

If that comes out half a size or a full size bigger than your everyday shoes, that's normal.

The myth: "they'll stretch"

Don't count on a running shoe getting longer with use. If there isn't enough room at the front on day one, there probably still won't be later.

How to try on running shoes in the store

• Go in the afternoon. Your feet swell over the day, just as they swell when you run. What feels just right at 9 a.m. can be too small at mile eight.
• Bring your running socks. Thickness changes the fit more than you'd think.
• If there's a treadmill, use it. Run for a few real minutes: that's when you notice whether your heel slips, whether something rubs, or whether the toe box pinches.
• Try several models back to back. With only one, you have nothing to compare: comfort is relative.

Size is only one of the mistakes

Getting the size right is the quickest fix, but not the only one. In What the Running Store Won't Tell You I go through the criteria for choosing a running shoe (cushioning, drop, foot strike, and why pronation shouldn't be the first thing you look at), when to really replace them, and the mistakes that come up most in the store, with sources cited at the end.

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