Bicycles/Maintenance and Repair/Chains/Checking chain wear

From Wikibooks, the open-content textbooks collection

Chains increase in length over time due to wear on the pins and bushings. Continuing to use a chain that is beyond a reasonable length can not only cause the chain to "skip" when pedalling under pressure but will also wear out the rest of your drivetrain as the teeth on the cassette/freewheel and chainrings try to match the length of the chain. If you catch a chain before it is too worn you can dramatically increase the life of the rest of your drivetrain.

To check the length of your chain there are two methods. Either the ruler method or by using a specific tool. If you have access to a tool, you simply put the pins of the tool into the chain and rotate a dial to obtain a reading indicating the length of your chain. On the park tool (CC-2) a reading of under 0.5 means the chain is ok, and over 0.5 means the chain needs to be replaced. The ruler method is done by lining up a pin with the zero mark on a standard ruler. Next, you look for the closest pin to the 12" mark. On a new chain the pin will line up exactly, if the pin is 1/8" or more past the 12" mark, the chain needs to be replaced.

If, when you replace your chain, it skips under pressure, this is an indicator that other parts of your drivetrain need to be replaced too. First of all the freewheel or cassette, or secondly, the chainrings.