You've probably heard the number: replace running shoes every 300 to 500 miles. It's a decent starting point and a terrible absolute rule. Two runners can hit 400 miles and one pair's fine while the other's been dead for a month. Mileage is a proxy for what really matters—the breakdown of the foam that cushions each stride.
What actually wears out
It's not the outsole, usually. It's the midsole foam. Every footstrike compresses those EVA or foam cells, and over thousands of strikes they stop springing back. The shoe still looks fine on the shelf but it's lost the cushioning and energy return it had new. Your legs notice before your eyes do.
The wear signs that beat the odometer
Flip the shoe over and look at the midsole from the side. Compression wrinkles that don't bounce back, or foam that's visibly flattened under the heel or forefoot, mean it's spent. Press your thumb into the midsole—new foam resists and rebounds; dead foam stays dented.
Then there's how you feel. New aches in your knees, shins, or hips that show up after runs and weren't there before are a classic sign the cushioning's gone, especially if your training hasn't changed. A lot of runners chase these with stretching and ice when the fix is a new pair.
Uneven outsole wear that exposes the white midsole through the black rubber, or a shoe that visibly lists to one side on a flat surface, is also game over.
Why the mileage range is so wide
Heavier runners compress foam faster. So do harder surfaces—concrete is brutal, trails and tracks are gentler. Your gait matters, midsole tech matters (some modern foams last longer, some race foams die fast), and rotating between two pairs lets foam decompress between runs and genuinely extends the life of both.
Track it, roughly
You don't need to be precise. Note the month you started a pair, or let a running app tag mileage to a shoe. When you cross 300 miles, start paying attention to how they feel. When you cross 500, assume they're on borrowed time unless they honestly still feel springy.
A cheap insurance policy
Buy your next pair before the current one's fully dead and rotate them in. You'll feel the difference side by side, which makes the "is it time?" question answer itself. Browse our running shoes and keep a fresh pair in the wings—your knees will thank you.