The basics

Why

People say a chain "stretches", but its plates don't stretch. The pins and the inside of the rollers wear: each link gains a little play, and the whole chain gets longer. It no longer sits right at the bottom of the teeth. Shifting gets worse, then the chain wears the sprockets and chainrings to its own shape.

Replaced in time, a chain is cheap. Replaced too late, the new chain skips on the worn sprockets, and the cassette has to go too, sometimes the chainrings as well.

When to do it

• About every 1,000 km, more often for mountain biking, in the rain or on an e-bike. KMC, a major chain maker, even recommends a check every 500 km on dry roads, and every 150 km off-road or in bad weather.
• As soon as shifting gets poor although the derailleur is well adjusted, or the chain skips when you push hard on the pedals.
• Before buying a cassette or chainrings: if the chain is worn, replace it at the same time.

The threshold

It depends on the number of rear gears (count the sprockets on the cassette):

These are Park Tool's thresholds. Each drivetrain maker has its own: when in doubt, your chain's manual takes precedence. Waiting for the limit gains nothing: the chain costs less than what it wears out.

Steps

1. Wipe the chain

Hold it in a rag and backpedal a few turns. Dirt between the rollers skews the reading.

2. Measure with a checker

Measure on the lower run, with the chain taut. Set the hook (1) between two rollers, on a link with inner plates: you then measure against the rollers, not the plates. Keep slight tension on the chain and press the tip (2) down (3), without forcing.

• With a two-sided gauge (such as Park Tool's CC-3.2), start with the 0.5 side: if the tip doesn't go in, the chain hasn't reached 0.5% yet. If it drops all the way in, flip the gauge and try the 0.75 side.
• With a dial or window checker, read the value directly, without pushing past the point where it stops.

Measure in two or three places along the chain and keep the highest reading.

3. No tool: use a ruler

A bicycle chain has a half-inch pitch: 24 new pins measure exactly 12 inches (30.5 cm). Line a pin up with the ruler's zero (1), count 24 pins (2) and see where the last one falls against the 12-inch mark (3). If it's past it by more than 1/16 inch (1.6 mm), the chain needs replacing. Use a ruler in inches: in centimetres the gap is too small to read reliably.

4. The chainring test, for a first opinion

Put the chain on the big chainring and the smallest sprocket, then pull the chain forward, off the middle of the chainring teeth. If it lifts by more than half a link and you can see daylight under it, it's probably worn. This test isn't accurate: it only tells you to measure with a proper tool.

Reading the result

• Below the threshold: nothing to do. Log the reading and the distance in your logbook.
• Close to it: get a new chain ready. Measure again in a few hundred kilometres.
• At the threshold: replace the chain: see Replace your chain.
• Well past it (beyond 1% on a multi-speed drivetrain): the cassette has probably worn to the chain's shape. If the new chain skips on some sprockets, the cassette will need replacing too.

Common mistakes

• Measuring a dirty chain: mud between the rollers skews the reading.
• Forcing the tip: push hard enough and any gauge drops in.
• Mistaking noise for wear: a squeaky chain often just needs oil: see Clean and lube your chain.

Sources

This guide was checked against the following documents:

• Park Tool, When to Replace a Chain on a Bicycle (thresholds, ruler method, chainring test)
• Park Tool, CC-3.2 Chain Checker
• KMC, Technical maintenance (check intervals, chainring test)

Values vary between models: your part's manual always takes precedence.