How to Measure Chain Wear With a Ruler or a Gauge

There are two simple ways to check a chain: a metal ruler, or a small gauge made for the job. Both measure the same thing, how far the links have drifted apart. If you're not sure why that happens, read what chain stretch means first.

The ruler method

Wear rates are highly variable, so measuring beats guessing.¹ A ruler or tape measure can check the chain without taking it off the bike.²

1. Use a metal ruler or tape measure. Wood, plastic and cloth can expand or shrink.²
2. Keep the chain under some tension, either on the bike or hanging.²
3. Put an inch mark at the side of one link pin. Then look at the matching link pin twelve complete links away.²

On a new, unworn chain, that second pin lines up exactly with an inch mark. On a worn chain, the pin lands past the mark.²

What the reading means

One repair reference gives these steps for a ruler marked in inches.²

At the 1/8 inch point, a new chain may run without skipping. But the worn sprockets will make it wear much faster than it should.²

With a metric ruler

In metric measurement, ten links of a new chain are 25.4 cm, and fifteen links are 38.1 cm.² The same method applies, and the same reference gives matching limits for each length.

A different count, a different limit

Another reference measures in half-links, which is each inner or outer plate pair counted separately. Twenty half-links in a new chain measure 10 inches (254 mm).¹ That reference recommends replacing before the old chain measures 10 1/16 inches, which it calls 0.7% wear. It also names a more conservative limit, where 24 half-links in the old chain measure 12 1/16 inches, or 0.5% wear.¹

Sources pick different cutoffs. Does your chain's maker publish its own limit? Then use that one, and treat the numbers above as background only.

The gauge method

A chain wear tool has a small tooth about the size of one on a sprocket. You place it on the chain under light load. One general reference says that if the tooth drops in all the way, the chain should be replaced.¹ Read that with the caution below, because gauges are not all equally reliable.

A bike-tool maker's repair guide also tells readers to check the chain for wear with a chain wear indicator before cleaning. If it's worn beyond its service life, the guide says it's best to skip cleaning and replace it.³

One repair reference calls special wear tools a bit more convenient but by no means necessary. It says most of them are inaccurate because roller play confounds the measurement of link-pin wear. The ruler check, held under some tension, is the one it calls accurate.² Don't trust a gauge blindly. Follow the gauge maker's instructions closely, and then check whatever result it gives you against the ruler, since the ruler is the method that source trusts.

Before you buy parts

• Write down the reading with the date, so you can compare next time.
• If the chain is past the first limit, look at the cogs and rings as well. See signs of a worn cassette and chainring.
• Ask a shop or check your manual about which chain suits your drivetrain.