Should Running Shoes Be a Size Bigger? The Real Answer for Comfort and Performance
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Your Recommended Running Shoe Size
This gives you the ideal 10-12mm space at the front of your shoe for optimal comfort and performance. For long-distance runners, this prevents black toenails and blisters.
Most runners have been told at least once: get a size bigger for your running shoes. But is that actually true? Or is it just a myth passed down from one runner to another like a worn-out pair of socks? The truth isn’t simple. It depends on your feet, your pace, your distance, and even the time of day you run.
Why People Think Bigger Is Better
The idea that you need a bigger shoe comes from a simple observation: your feet swell during a run. That’s normal. After 30 minutes of pounding pavement, your feet can expand by up to half a size. That’s not a guess-it’s backed by podiatry studies from the American College of Sports Medicine. If your shoe is too tight when you start, your toes will be crushed by mile three. You’ll get black toenails, blisters, or worse: nerve damage from constant pressure. Add in downhill sections, long races, or hot weather, and that swelling gets worse. So the advice to go up a size makes sense on paper. But here’s the catch: going up a size doesn’t fix everything. It can create new problems.The Problem With Too Big
A shoe that’s too big doesn’t hold your foot in place. That means your foot slides forward with every step. Your toes bang against the front. Your heel slips. Your arch isn’t supported the way it should be. You’re not running in a shoe-you’re running in a shoe-shaped boat that’s drifting around your foot. That sliding causes blisters, especially under the ball of your foot and around the heel. It also messes with your stride. Your muscles have to work harder to stabilize your foot. That leads to fatigue faster. And if you’re training for a race, that inefficiency adds seconds-maybe minutes-over 10K or a marathon. I’ve seen runners who went up a full size because they were told to, only to end up with chronic heel pain and plantar fasciitis. Their shoes were too loose. Their arch collapsed. Their body compensated. It wasn’t the shoe size that broke them-it was the fit.The Right Fit: Not Size, But Space
Forget the idea of a fixed size increase. What you need is space-not extra length, but the right kind of space. Here’s how to check it yourself:- Put on your running socks-the same ones you’ll wear when you run.
- Stand up and push your foot forward until your big toe touches the end of the shoe.
- Slide two fingers sideways between your heel and the back of the shoe. If they fit snugly, it’s good. If they’re loose or you can’t fit them, it’s wrong.
- Walk or jog in place. Your heel shouldn’t lift more than a millimeter.
- Look down. You should see about a thumb’s width of space (roughly 10-12mm) between your longest toe and the front of the shoe.
Foot Swelling Isn’t the Same for Everyone
Not all runners swell the same. A 5K runner might only need 5mm of extra space. A marathoner running in 25°C heat? They might need 12mm. Your foot shape matters too. If you have wide feet, you might not need a bigger length at all-just a wider width. Many brands (like Brooks, Altra, and Hoka) offer wide versions of the same shoe. Going up a size in a narrow shoe just makes the shoe longer and floppier. It doesn’t solve the width problem. And if you have high arches or flat feet, your support needs change. A bigger shoe doesn’t add cushioning or stability. Only the right midsole design and heel counter will do that.
How to Choose the Right Size
Here’s a simple process that works every time:- Shop in the afternoon or evening. That’s when your feet are at their largest.
- Wear the socks you run in. No cotton socks. No thick hiking socks. Running socks are thin, moisture-wicking, and seamless.
- Try on both shoes. Your feet aren’t identical. One might be half a size bigger.
- Don’t assume your street shoe size is your running shoe size. Most runners need a half size up from their casual shoes. Some need a full size. Others? Same size.
- Test the shoe on a treadmill or walk around the store. If your toes feel squished, walk out. If your heel slips, walk out.
Brand Differences Matter
Not all running shoes are made the same. Nike runs narrow. Altra runs wide and zero-drop. Saucony tends to run true to size. Hoka has a roomy toe box but a snug heel. You can’t use the same rule for every brand. Check the brand’s sizing chart. Most have them online. Look for reviews that say things like “runs small” or “true to size.” Don’t rely on generic advice. A shoe that fits someone in Boston might not fit you in Dublin.When You Definitely Need a Bigger Size
There are times when going up a size is the right move:- You’re training for a marathon or ultra-long distances mean more swelling.
- You run in hot climates or summer months.
- You’ve had black toenails before.
- Your toes feel numb or tingly after a run.
When You Should Stick to Your Normal Size
And here’s when you should ignore the advice:- You’re doing short runs under 8K.
- You have narrow feet and the shoe already fits well in width.
- Your heel doesn’t slip and your toes aren’t touching the front.
- You’ve had no issues with blisters or toenail damage.
What Happens If You Get It Wrong?
Too tight? You risk:- Black toenails (from blood pooling under the nail)
- Morton’s neuroma (nerve pain between the toes)
- Plantar fasciitis (from overcompensating with your arch)
- Heel blisters
- Metatarsalgia (pain in the ball of the foot)
- Slower times from wasted energy
Final Rule: Fit Over Size
The best running shoe isn’t the one that’s biggest. It’s the one that fits your foot perfectly while allowing room to move. That means:- Thumb’s width at the front
- No heel slippage
- No side-to-side movement
- No pressure on your toes or arch
Next time you buy running shoes, skip the advice. Skip the myth. Test them. Move in them. Run in them-even if it’s just around the store. Your feet will thank you.
Should I always buy running shoes a half size bigger?
No, not always. Most runners need about half a size bigger than their regular shoes to allow for foot swelling, but this depends on your foot shape, the brand, and how far you run. If your current shoes fit well with no pain or slippage, stick with them. The goal isn’t size-it’s space. Aim for a thumb’s width between your longest toe and the front of the shoe.
Why do my toenails turn black after running?
Black toenails usually happen when your toes repeatedly hit the front of the shoe-often because the shoe is too tight or too short. This causes blood to pool under the nail. Going up half a size and making sure there’s enough toe room can prevent this. It’s not normal and shouldn’t be ignored.
Do I need a bigger size if I have wide feet?
Not necessarily. If your feet are wide, you likely need a wider width, not a longer size. Many brands offer wide versions of their shoes (marked with a “W” or “2E”). Going up a size in a narrow shoe just makes the shoe longer and floppier, which can cause heel slippage and blisters. Look for width options first.
Is it better to buy running shoes online or in-store?
In-store is better-especially if you’re unsure about fit. You can test the shoe with your running socks, walk or jog on a treadmill, and feel how your foot moves inside. Online shopping works if you know your exact size from a previous pair from the same brand and model. Never buy running shoes online without knowing the return policy.
How often should I replace my running shoes?
Most running shoes last between 500 and 800 kilometers. After that, the cushioning breaks down, and your feet take more impact. Even if the tread looks fine, the midsole loses its bounce. Replace them before they cause injury. Track your mileage with a running app or journal.
