Black Toenails from Running? It's Probably Your Shoe Size | RunSized

Black toenails — medically called subungual hematoma, which is just bruising under the nail — are one of the most common running injuries. Ask any runner who's trained for a marathon and they've probably lost at least one nail. Many runners treat this as an inevitable cost of mileage. It isn't.

The vast majority of running black toenails are caused by shoe fit, not distance. Specifically: shoes that are too short, too narrow, or laced too loosely. The injury happens because your toe is repeatedly hitting the front or side of the shoe with each foot strike — small trauma, hundreds of times per kilometre, adding up to a bruised nail bed.

The fix isn't to run less or accept the damage. It's to find shoes that actually fit your feet. This guide explains the mechanics, how to diagnose your specific issue, and how to get the right fit permanently.

Why Running Shoes Cause Black Toenails

There are four main mechanisms, and it's worth understanding which one applies to you because the fix is slightly different for each:

Toe hitting the front of the shoe — especially on downhills

This is the classic cause. On downhills, your foot slides forward in the shoe and your toe impacts the front with each step. The longer and steeper the descent, the worse it gets. A shoe that fits perfectly on flat terrain can cause black toenails on a 5km downhill finish. You need enough space to absorb that forward slide.

Foot swelling during long runs

Feet expand during exercise — sometimes by up to half a shoe size. A shoe that fits at the start of a 5km might be too short by kilometre 25 of a marathon. This is why the standard advice is to go half a size up for training and a full size up for very long distances.

Shoe too narrow — lateral toe compression

When a shoe is too narrow, your toes are pressed sideways against each other. The outer toenail gets compressed against the adjacent toe, not the front of the shoe. This causes nail bruising on the side rather than the tip — a tell-tale sign that width, not length, is your problem.

Lacing too loose — foot sliding forward

Even a correctly-sized shoe will cause toe-strike issues if it's not laced tightly enough. A loose shoe lets your foot slide forward on every downhill step, putting your toes in exactly the same position as a shoe that's too short. Proper midfoot lockdown is as important as size.

Is It Definitely Your Shoe Size?

Not every black toenail is a shoe fit problem, though most are. Use this checklist to identify the likely cause:

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Which toe is affected? Big toe or second toe (if your second toe is longer than your big toe — this is called Morton's toe) suggests a length issue. Little toe or the side of any toe suggests a width issue.
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Where exactly is the nail bruised? Bruising at the tip of the nail = length/impact. Bruising at the side of the nail = width/compression. Bruising at the base = very unusual, possibly trauma or infection — see a doctor.
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When did it start? Did it start after buying new shoes? Strong signal it's fit-related. Did it start after an unusually hilly or long run in shoes that normally feel fine? That's swelling or terrain-related — consider sizing up slightly.
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Is it one foot or both? One foot only usually means that foot is slightly larger than the other (very common), and you've been fitting the smaller foot. Measure both and fit the larger one.

Other (non-fit) causes include fungal nail infections, a single traumatic injury, and running on extremely technical trail surfaces. But these are less common than fit issues. If the fit checklist lines up, start there.

How to Fix It — Getting the Right Shoe Size

Here's the step-by-step process for getting a fit that prevents black toenails:

1
Seat your heel properly
Put on the shoe and tap your heel against the floor two or three times to seat it fully in the heel cup. This is important — an unseated heel will make the shoe seem longer than it is.
2
Lace up at running tension
Lace to the same tension you'd use on a run — firm but not constricting. Too loose and your foot will slide forward; too tight and you'll constrict blood flow.
3
Check toe space with weight on foot
Stand with your full weight on the shoe. Press down on the toebox — you should feel your longest toe (not always the big toe) with about a thumb's width of space in front of it. If you can feel your toe right at the end, the shoe is too short.
4
Check from running position
Lean slightly forward as if starting a run. Your toes will slide toward the front of the shoe. Is there still enough room? This forward lean simulates what happens on a flat road; on a downhill, your toes slide even further forward.
5
Test on a short run first
New shoes should be tested on a 20-30 minute run before committing to long distances. If your toe is hitting the front within 20 minutes, the shoe is too short for training — regardless of how it felt in the store.

One practical note on sizing for different run types: some runners keep two pairs — a slightly snugger pair for short, flat runs and a half-size-up pair for long runs and races with significant elevation. This is especially relevant for ultramarathoners, whose feet can swell dramatically over many hours.

Other Fit Factors That Cause Black Toenails

Even with the right size, other fit variables can contribute to black toenails:

Lacing technique

The standard crisscross lace isn't optimal for downhills. Try the heel-lock technique: at the second-to-last eyelet, create a loop by threading the lace through the same eyelet twice. Then cross both loops and thread the opposite lace end through each loop before tying. This creates a much more secure midfoot lock that prevents your foot from sliding forward.

Sock thickness

Thick socks take up space inside the shoe. If you train in thick merino wool socks but try shoes on in thin cotton, you're testing the wrong configuration. Always try running shoes in the socks you'll actually run in. Conversely, switching to thinner race socks in a shoe that fitted with thick socks will give you slightly more room — useful if you're right on the boundary.

Toebox shape

Some shoes have a tapered, curved toebox that pushes the big toe medially (inward). Even if the shoe is long enough, a tapered toe can cause lateral nail compression on the second toe. Shoes with a straighter, more foot-shaped toebox (Altra, Topo Athletic, and some Hoka models) allow your toes to sit more naturally.

Custom insoles

Orthotics and custom insoles are thicker than the stock insole they replace, which effectively makes the shoe smaller inside. If you've added custom insoles and started getting black toenails, you may need to go up half a size to compensate for the space they occupy.

Finding the Right Shoe

When shopping for running shoes specifically to address black toenail issues, look for:

Adequate length — at least one thumb's width from your longest toe to the end of the shoe, measured in running position.
A roomy toebox — not just wide, but shaped to let toes sit naturally rather than being compressed together. Altra is the benchmark here, with a toebox shaped to match the widest part of the natural foot.
Proper width — if your issue is side bruising, you need more width, not just more length. See the separate guide on wide feet for brand recommendations.
Good heel lockdown — a shoe that holds your heel firmly prevents forward slide. Look for a snug heel counter and try heel-lock lacing regardless of the shoe you choose.

RunSized community reports often specifically mention whether a shoe runs narrow in the toebox or causes toe-related issues. These are practical fit notes from runners who have experienced the shoe across a range of distances — worth checking before buying.

Check how specific shoes fit before you buy — community reports from runners who've logged the mileage.

Check sizing reports from runners before you buy

When to See a Doctor

Most running black toenails heal without intervention. The nail may discolour, loosen, and eventually fall off, with a new nail growing in over several months. This is normal and doesn't require medical treatment.

See a doctor if you notice:

Severe pain that doesn't ease within 24-48 hours (a doctor can relieve this by draining the haematoma)
Redness, warmth, or swelling spreading beyond the nail bed (signs of infection)
Pus or discharge under the nail
Fever or red streaking up the toe or foot
The nail separating in a way that leaves raw tissue exposed and open to dirt

Preventing infection: While the nail is loose or partially detached, keep it clean and dry between runs. Consider a thin bandage or toe cap to protect the exposed nail bed. Don't forcibly remove a nail that's still attached — let it fall off naturally.

Once healed, address the root cause. The nail will grow back, and if you're still in the same shoes, it will blacken again.

Frequently Asked Questions

Do black toenails from running go away on their own?

Usually yes. A black toenail (subungual hematoma) from running is a bruise under the nail caused by repeated impact. In most cases it resolves over weeks to months as the nail grows out, sometimes falling off entirely before a new nail grows in underneath. If the nail is extremely painful, a doctor can relieve pressure by draining it. See a doctor if the area shows signs of infection (increasing pain, redness spreading beyond the nail, pus, fever).

How much space should I have in my running shoes to avoid black toenails?

The standard recommendation is approximately one thumb's width (about 1cm or half an inch) between your longest toe and the end of the shoe. This accounts for foot swelling during a run. Measure this with your heel pushed firmly to the back of the shoe and your foot bearing weight — not while sitting down with a loose shoe.

Can lacing technique fix black toenails?

Sometimes. The heel-lock (runner's loop) lacing technique can prevent your foot from sliding forward on downhills, which is a common cause of toe-strike blackening. Loop the lace through the top eyelet before crossing, creating a loop on each side, then thread the opposite lace through each loop before tying. This locks the heel in place and reduces forward slide.

Why do I only get a black toenail on one foot?

It's very common to have one foot slightly larger than the other. If your shoes fit your smaller foot correctly, the larger foot is being cramped — particularly at the longest toe. Measure both feet and fit the larger one. You may also have an anatomical difference (a longer second toe on one side) that causes asymmetric impact patterns.

Black Toenails from Running? Here's How to Fix It