How do I know when a chain is worn out?

Three signs, and any one of them is enough: the slack varies noticeably as the wheel is rolled, the adjusters have reached the end of their travel, or the sprocket teeth have taken on a hooked profile. Your machine’s manual also publishes a wear measurement.

Uneven slack, which is the clearest sign

Roll the wheel and check the slack at several points. A new chain reads much the same all the way round. A worn one has stretched unevenly, so there is a tight spot and a loose spot — and adjustment then sets the tight spot correctly while leaving the rest too slack, which is hiding the problem rather than fixing it.

The end of the adjustment

The axle adjusters have a finite range. A chain that needs the axle near the end of its travel to reach the specified slack has stretched about as far as it usefully can, and the geometry of the swingarm and the wheelbase have moved with it.

The sprockets, which tell you before the chain does

Look at the teeth. New teeth are symmetrical; worn ones lean, with a hooked profile on the driving face. If the sprockets are hooked, fitting a new chain onto them destroys it quickly — which is why chains and sprockets are sold as kits and replaced together.

The measurement your manual publishes

Manufacturers publish a wear limit, usually as a length measured over a defined number of links with the chain under tension. It is the only numeric answer, it is specific to your machine and chain, and it lives in that manual rather than on a comparison page.

The noise, which is a prompt rather than a measurement

A chain that has started to sound different — a rhythmic slapping, or a dry rasp that returns quickly after lubrication — is asking to be looked at. It does not tell you the chain is finished; it tells you to go and check the three signs above.