Added sources and updated by the Pedalwise team

A chain checker is a small gauge that drops into the chain and shows how worn, or "stretched", it is. It takes seconds and tells you when the chain is due for replacement.

What it's used for

Park Tool explains that a worn chain meshes poorly with the cogs and chainrings, causing poor shifting and faster wear, and that replacing a cassette costs much more than replacing a chain (Park Tool chain wear guide). Checking regularly lets you swap the chain in time.

Park Tool's general guide to replacement points, which it says vary by maker (Park Tool):

• Single speed: 1.0%
• 10 speed or fewer: 0.75%
• 11 speed or more: 0.5%

What to look for

• A checker that reads the wear levels your drivetrain uses. Some show 0.5%, 0.75% and 1.0%, and some are simple go or no-go gauges for 0.5% and 0.75% (Park Tool).
• One that works with your chain's speed count. Park Tool notes some checkers cover 5 to 12 speed chains (Park Tool).
• No checker handy? Park Tool describes a ruler method: count 24 rivets from the zero mark. The last should sit at 12 inches, and more than 1/16 inch past that means replacement is due (Park Tool).

Using it safely

• Keep tension on the chain while you seat the gauge, as Park Tool advises for its three-point checker (Park Tool).
• If the chain slips under load, check shifting adjustment and chain cleanliness too (Park Tool).

Videos

Guides that use it

• Check and replace a chain
• Clean and lube a chain
• Adjust a rear derailleur

Sources

• When to replace a chain on a bicycle, Park Tool