When to replace running shoes

The midsole is a foam that compresses on every footfall and springs back. Over thousands of repetitions it stops springing back completely, and the cushioning it provided is permanently reduced. This happens without any visible change, which is why a shoe can look almost new and be finished.

The outsole rubber, which is what people inspect, is chosen for durability and often survives long past the midsole. A shoe with plenty of tread left and a dead midsole is the normal end state, not an anomaly.

The signals, in rough order of usefulness:

Distance covered

The primary measureCommonly quoted as 500–800km. Wide because it depends on the shoe, the surface and the runner.

New aches after runs

The most reliable subjective signParticularly in shins, knees or feet, appearing gradually with no change in training.

Compression creases in the midsole

Visible on the side wallDeep horizontal wrinkling that does not relax means the foam has taken a set.

The shoe rocks when set on a flat surface

Uneven collapseUsually one side of the heel. The shoe is now steering you.

Tread wear

The least useful signalIt tells you about the rubber, which is not the part doing the cushioning.

Almost nobody logs shoe distance reliably, and the simplest workaround is to write the date of first use inside the tongue in permanent marker. Combined with a rough sense of your weekly distance, that turns "are these old" into arithmetic rather than guesswork.

Rotating two pairs is worth doing once weekly distance is high enough that one pair would wear out in a few months. The foam recovers between runs, so alternating pairs extends the total life of both, and it also means an injury caused by one shoe’s particular geometry shows up as a pattern rather than as a constant.

Midsole foams degrade slowly even unused, so a pair bought several years ago and left in a cupboard is not the bargain it appears to be. This is worth knowing before buying heavily discounted old stock of a discontinued model.

Never dry wet running shoes on a radiator or in front of a heater: the adhesives and foams do not tolerate it, and the shoe distorts. Loosely stuffed with newspaper at room temperature is slower and does not destroy them. Retired shoes are usually still fine for walking, gardening or the gym, which is where most of them should go rather than in a bin.