Measuring for chain wear is done with a chain checking tool or accurate ruler/tape measure. Which method you should use is the cause of much internet debate.

Using a ruler, a new chain should measure exactly 12 inches across 12 links, from middle of pin to middle of pin. The number most commonly agreed on for a worn chain is one percent elongation between links. In reality though, you want to replace the chain before this point.

So therefore anything past 12, 1/16 inches (0.5 percent) would be the time to replace a chain. And anything past 12, 1/8inches (one percent) has been worn to death and so a new cassette is likely needed.

Holding a ruler perfectly straight while lining it up to measure 1/8in is difficult, and with this, chain checker tools provide a far simpler and quicker 'go or no-go' result. Whichever way you choose to measure a chain, be sure to not include any Masterlink, quicklink or 'Powerlock' which may be installed. 

Jones brings up the point that the parameters for chain wear have changed as drivetrains have. “Through the years, the replacement standards have changed," he says. "The chains are narrower now, and people are not weaker. Chains don’t last like they used to. Add to this rear sprockets are narrower and think don’t last like they used to. With six- to seven-speed (chains) a one percent wear was the norm, but that is no longer the case.”  

Countering Jones’ comment to some extent, Murdick claims the latest Shimano 11-speed chains are far more durable and resistant to length growth compared with previous generation 10-speed and earlier chains.