How to check chain wear (and what 0.5 vs 0.75 means)
Checking chain wear takes about 30 seconds and is the cheapest insurance in cycling. A worn chain that keeps getting ridden grinds your cassette and chainrings down with it, so catching the wear early is the difference between a $40 fix and a $250 one. Here are the two reliable ways to measure it.
Method 1: a chain checker tool (easiest)
A chain checker is a small metal gauge that drops into the chain and tells you the elongation as a percentage. Park Tool, Shimano, and others make them for a few dollars.
- Make sure the chain is on a gear and under light tension (on the bike is fine).
- Hook one end of the tool into a chain link.
- Try to drop the other end in.
- If it doesn't seat — your chain is still good.
- If the 0.5 side drops in — you've hit 0.5% wear.
- If the 0.75 side drops in — you're at 0.75% and well past due on a modern drivetrain.
That's it. Check it every few hundred kilometres and you'll never be surprised.
Method 2: a ruler (free)
No tool? A ruler works because a new chain measures exactly 1 inch per link.
- Shift to the big chainring to pull the chain taut.
- Line the 12-inch mark of a ruler up with the center of a chain pin.
- Look at the pin nearest the 12-inch mark, 12 full links away.
- Pin dead-on 12": chain is fine.
- Pin past 12" by 1/16": ~0.5% worn — replace soon (sooner on 11/12-speed).
- Pin past 12" by 1/8": ~0.75%+ worn — replace now, and check your cassette.
What 0.5% and 0.75% actually mean
Those numbers are how far the chain has elongated from new:
- 0.5% is the replacement point for 11- and 12-speed drivetrains. The cogs are narrow and the tolerances tight, so they're less forgiving of a stretched chain.
- 0.75% is the limit for older, wider 8-, 9-, and 10-speed chains, which tolerate a little more before they start damaging the cassette.
Replace at the right number for your drivetrain and you stay in the cheap window. (For why the timing matters so much, see how long a chain lasts.)
How often to check
A sensible rhythm:
- Every ~500 km, or
- Monthly if you ride regularly, or
- After any especially wet or gritty week.
The catch is remembering — most people check once, forget, and rediscover the chain three months later at 1% wear with a ruined cassette. Tracking distance on each chain and getting a nudge at the check interval turns this from a thing you forget into a thing that just happens.