Should Running Shoes Be a Size Bigger? The Real Deal on Sizing Up

Does your big toe ever bang against the front of your running shoes? Or maybe you finish a run with numb toes—or worse, black toenails? It’s a classic runner problem, and it’s got a simple fix: make sure your shoes actually fit, which sometimes means going up half a size or even a whole size.

Here’s the thing—your feet swell when you run. Add in thick socks, hot weather, or just standing for longer than a few minutes, and suddenly your shoes feel way tighter than they did in the store. That’s why a lot of experienced runners buy their shoes a size bigger. It's not a weird trend; it’s about comfort and keeping your feet healthy.

Nobody wants to regret a new pair of shoes because of lost toenails or blisters. If you’re new to running, or even if you’re just scoring your first upgrade in years, it’s smart to get real about sizing. Forget what’s printed on the box—you need to know how that shoe fits when you’re actually running. Let’s break down what matters and how to choose the right fit for your next pair.

• Why Everyone Talks About Sizing Up
• How Feet Change During a Run
• Socks, Insoles, and the Sizing Equation
• When Going Bigger Makes Sense
• Fit Checks: Simple Tests at the Store
• Common Fit Problems and How to Fix Them

Why Everyone Talks About Sizing Up

Ask a group of regular runners about running shoes, and almost every one of them will tell you sizing up is a hot topic. It’s not just about comfort—it’s about what actually happens to your feet while you run.

Your feet expand. Seriously, it’s normal for them to flatten out and swell up to 5% by the end of a long run. If you buy shoes that fit snugly when you’re sitting in the store, you might be in for a rough ride once you’re three miles into a run.

Ever wondered how common this advice is? Check out these real-life numbers:

So if you keep hearing about this, you’re not alone. Even companies like Nike and Brooks say it’s smart to leave a thumb’s width between your longest toe and the end of your shoe. That buffer helps with swelling, keeps your toenails safe, and lowers your chances of black toenails or blisters. Think of it as a little extra insurance for your feet.

Besides, feet come in all shapes and sizes. Some folks have wider forefeet, while others have slightly different lengths between their left and right foot. Sizing up gives you wiggle room—literally.

Long story short, people talk about sizing up because it solves real problems for most runners, keeps you from getting hurt, and plain feels better when you’re putting in the miles.

How Feet Change During a Run

Your feet aren’t the same when you finish a run as when you start. That’s not just you being picky—it’s basic biology. When you run, blood flow to your legs and feet increases, especially as your workout drags on. This makes your feet swell up, usually by a half size, or even more if you’re running long distances or in the heat.

Ever notice your shoes feel tighter after the first mile? It’s not your imagination. Here’s a quick look at what actually happens inside your shoes on the move:

• Swelling: On average, feet get about 5-10% bigger during exercise. For someone with a size 9 foot, that’s up to half a size more in length and width.
• Spreading Out: Each footstrike flattens the arch a little, so your feet literally spread out wider and longer compared to when you’re sitting or standing.
• Heat: The hotter it gets, the more you sweat and the more your blood vessels expand. That just adds to the swelling.

Check out some actual numbers from a small study tracking foot size during runs:

So the next time you’re at the store trying on running shoes, remember: if they fit just right when you’re sitting, they might feel like foot prisons after a real run. Always factor in how your feet expand and demand more space as you go.

Socks, Insoles, and the Sizing Equation

Ever tried on shoes barefoot, then realized your thick running socks make the fit way tighter? Happens all the time. Socks are a bigger deal than most runners think. A plush, cushioned sock can eat up space—sometimes pushing your toes right up to the end. Always try shoes on with the socks you’ll run in, not some thin pair from your sock drawer.

Insoles can switch things up too. Companies toss in basic factory insoles, but if you use custom or supportive insoles, they’re usually thicker. Slide those into your shoe and, suddenly, what fit fine in the shop feels cramped out on the road. Even swapping out for a new, more padded insole midway through a shoe’s life can change the fit.

Curious how much space this stuff really takes? Here’s a quick look at a typical difference in foot volume caused by common sock and insole swaps:

Some runners ignore these small changes, but stack up thick socks and a beefy insole, and you’re talking up to half a size worth of extra volume. No wonder your toes feel squished.

Here’s the move if you want a comfortable fit every time:

• Running shoes should always be fitted with your typical running socks and any insoles you use regularly.
• Don’t assume your size stays the same if you change your gear—even a new brand of socks can make a difference.
• If you use orthotics or custom insoles, bring them with you to the store. Don’t just “eyeball it.”

It’s not just about length—fit is really about volume and what’s actually inside your shoe. Pay attention to these tweaks and you’ll save yourself a ton of discomfort down the road.

When Going Bigger Makes Sense

So, when should you actually buy running shoes a size up? It’s not about blindly picking a bigger number—it’s about real signs your regular size isn’t cutting it. Several legit reasons make sizing up the smart move.

First off, lots of runners find their feet swell up during runs, especially when it’s hot or they go long distances. Studies have shown that feet can lengthen by up to half a size after just an hour of running. If your shoes are already snug, that swelling squeezes your toes against the front, which causes pain, black toenails, and blisters.

Another reason: the type of socks you love. If you wear thick, cushioned running socks, they take up more room inside your shoes. Special insoles for arch support can also hog space. These add-ons can turn a perfect fit into a cramped one, so that extra half or full size can make all the difference.

Then there’s the shape of your feet. Have a wide forefoot or bunions? Standard sizes often come up short—literally. Your toes need to splay naturally to avoid hot spots or numbness. If you’re always feeling pinched at the end of your runs, it’s probably time to size up or even look for wide models.

Here are some sure signs sizing up is worth it:

• Your toes touch the end of the shoes when you’re standing up.
• You get blisters or black toenails even on moderate runs.
• Your toes curl up or feel squished after a run.
• You feel tingling or numbness during runs.
• Your current shoes feel perfect with thin socks, but tight with your usual running socks.

Remember, it’s all about the right fit, not the number on the box. If you’re in doubt, bring your favorite running socks to the store and try on shoes later in the day, when your feet are slightly swollen. Your running shoes should have about a thumb’s width between your longest toe and the tip of the shoe—this little trick helps keep your feet happy, run after run.

Fit Checks: Simple Tests at the Store

Shopping for running shoes in-store means you’ve got a real chance to nail the perfect fit. Before you fall for the look or hype, there are a few super simple checks you can do right there on the shop floor.

First up: always try shoes on in the afternoon or after a workout, because your feet will be a bit swollen—just like when you actually run. Wear the socks you plan to run in, not thin dress socks or whatever you can grab from your sock drawer.

• The Finger Test: Once your shoes are laced up, check the space in front of your longest toe (sometimes this is your second toe, not the big one). You should be able to fit about a thumb’s width—roughly half an inch—between your toe and the end of the shoe. Less than that and you risk bruised nails; too much and you’ll slip around.
• The Heel Pinch: With the shoes on and laced, stand up and walk a bit. Your heel should stay locked in and not slip up and down when you walk or jog in place. If your heel’s popping out, those shoes are too big or laced wrong.
• The Side Squeeze: Make sure there’s no pinching on the sides of your foot. Your toes should be able to wiggle—not swim, but move easily. A cramped toebox leads to blisters and that burning pain on longer runs.
• On-the-Go Test: Walk or jog around the store if you can. Don’t just stand there. Stores expect you to move around. If the shoes feel iffy now, they’ll feel worse five miles into a run.

Don’t trust just the number printed on the box. Brands run big or small, and your size in one brand often won’t match another. Bring a running sock, test both shoes (because your feet might not be exactly the same size), and remember these quick checks to dodge all the usual foot problems down the road.

Common Fit Problems and How to Fix Them

If you’ve ever finished a run wincing in pain or found mysterious marks on your feet, you’re not alone. Most runners battle sizing problems at some point. But before you swear off the sport, check out these typical fit issues and simple fixes.

Here are some of the most common problems people hit with running shoes:

• Toes hitting the front: Black toenails and sore tips usually mean your shoes are too small or not shaped right for your foot. A thumb’s width (about half an inch) between your longest toe and the shoe front is the golden rule. Try sizing up or picking a style with a roomier toe box.
• Heel slip: If your heel pops up when you walk or run, your shoes are probably too big, or the lacing isn’t right. Use the “runner’s loop” lacing trick and double check the fit standing up before buying.
• Blisters or hot spots: Tight shoes or bunched-up socks cause rubbing. Sometimes it’s as simple as picking different socks. You might also need shoes with a wider width or softer materials inside.
• Numbness or tingling: Too-tight laces or a shoe that pinches across the top can cut off circulation. Try loosening the laces or swapping for a different lacing pattern. Don’t settle for any shoe that cramps your foot.
• Shoes feel great at first but uncomfortable after running a few miles: Remember, feet swell when you exercise. Shoes that fit perfectly in the store might not work out on a long run. Always try on shoes in the afternoon (when your feet are slightly bigger) and bring your running socks.

Spotting a problem early can save you tons of pain. If you see these signs, it’s time for a change:

One more pro tip: keep an eye on the shoe’s age. Most running shoes last about 300 to 500 miles before they lose their shape and support. If you’re logging serious miles, track your shoe usage and swap them out before problems crop up again.

Solving fit issues isn’t rocket science—but ignoring them can wreck your run. Next time you shop, focus on comfort more than the number on the box. Your feet will thank you.