Direct answer
Sneakers should fit with toe room, a held heel and no pinching
A well fitting sneaker leaves about a thumb width of space between your longest toe and the front of the shoe, holds your heel firmly without slipping, and lets the widest part of your foot sit comfortably without pinching at the sides. Your toes should be able to wiggle, your arch should feel supported rather than pressed, and the shoe should bend near the ball of your foot when you walk. If you run in the pair, start half a size up from your everyday sneaker size, because feet swell and slide forward with each stride.
Toe box room is the first fit check
Start every sneaker fit check at the toes. Stand up, because feet spread under body weight, and press the front of the shoe to feel where your longest toe ends. There should be about a thumb width of space between that toe and the inside front of the shoe. Less space than that means your toes will hit the front on downhills and at the end of a run, which leads to bruised toenails and sore toes.
Room in front is not the same as room at the sides. Your toes should also be able to spread and wiggle without pressing against the upper. A toe box that is too narrow squeezes the toes together even when the length is right, so check both directions. If the front feels roomy but the sides pinch, the problem is width, not length, and a wide fit or a different last will help more than a longer size.
Your heel should stay put when you walk
Walk a few steps and pay attention to your heel. It should feel held by the heel counter, the firm cup at the back of the shoe, with only a tiny lift as you push off. A heel that slips up and down with every step will rub blisters, and it usually means the shoe is too long, too wide at the heel, or laced too loosely. Try a heel lock lace pattern before you decide the size is wrong, because lacing can fix a surprising amount of slip.
The collar around the ankle should sit comfortably without digging into the Achilles area. Some sneakers have a soft padded collar and some have a firmer one, and the right feel depends on your ankle shape. If the collar presses hard on the back of your ankle when you stand, the heel height or the collar shape does not suit you, and a different model will fit better than a different size of the same model.
Width across the forefoot matters as much as length
Sneaker width is measured across the ball of the foot, the widest part, and it decides whether the shoe feels comfortable after an hour on your feet. The upper should lie flat across the forefoot without bulging out over the midsole at the sides. If you can see your foot pushing the upper outward in a clear bulge, the shoe is too narrow for you, even if the length is correct.
Width needs change through the day, because feet swell in warmth and after activity, so try sneakers on in the afternoon or evening when your feet are at their largest. Wear the socks you plan to use with the pair, since a thick running sock takes up more room than a thin everyday sock. If you need a wide fit, look for a wide version of the model rather than sizing up in length, because extra length will leave your heel loose while the width stays tight.
Arch support should feel present, not pressing
The arch area of a sneaker should meet your arch without pushing hard into it. For a medium arch, the midfoot should feel gently held, with no gap under the arch and no hard ridge pressing upward. For a low arch, a flatter insole with light support usually feels better, while a high arch often wants a contoured insole or a little extra cushioning under the midfoot to fill the space.
Arch feel changes once the shoe is laced and you are standing, so do not judge it while sitting with the shoe unlaced. Stand, lace the shoe evenly, and walk around the room. If the arch support feels like a lump under your foot, the insole shape does not match your arch, and a different insole or a different model will help. A good arch fit should fade from your attention within a few minutes of walking.
Running fit is roomier than everyday fit
Everyday sneakers can fit close, because you walk at a steady pace and your feet stay fairly settled. Running sneakers need more room, because feet swell during a run and slide forward with each landing. That is why the common running rule is to go half a size up from your everyday sneaker size, keeping about a thumb width of space in front of your longest toe even after your feet have warmed up.
Test a running fit by jogging in place or up and down a hallway. Your toes should never touch the front of the shoe, your heel should stay held, and the midfoot should feel locked down by the laces without pressure on the top of your foot. If your toes touch the front when you jog in place, the pair is too short for running, even if it feels fine for walking. For more detail on the general principles behind a good shoe fit, see how shoes should fit.
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Frequently Asked Questions
How much space should there be in front of my toes in sneakers?
About a thumb width between your longest toe and the front of the shoe when you stand, with enough room for your toes to wiggle and spread.
Should my heel slip at all in new sneakers?
Only a tiny lift as you push off is normal. Repeated slipping that rubs your heel means the shoe is too long, too wide at the heel, or laced too loosely.
Do sneakers need breaking in before they fit properly?
A new sneaker should feel comfortable on the first wear, with the upper flexing easily. Stiff uppers soften a little, but length and width problems do not improve with wear.
How do I know if a sneaker is too narrow?
The upper bulges outward over the midsole at the sides, or your toes feel squeezed together even when the length is right. In that case try a wide fit, not a longer size.
Should running sneakers fit differently from everyday sneakers?
Yes. Running sneakers usually need half a size more room than everyday sneakers, because feet swell and slide forward during a run.