Never fear, traveler, your educational odyssey learning the how’s and why’s and who’s of chain wear is nearly at an end. You’re now ready to know when to replace your chain.
It’s actually pretty simple: get yourself a chain checker tool from Parktool (CC-3.2 7/8/9/10/11/ some 12 speed chains or a CC-3.4) and or Pedros (Chain Checker Plus II for basically everything including Ekar 13 sp).
Parktool’s CC-3.2 is a “go/no-go gauge” that tells you when a chain reaches .5% and .75% "stretch". This is when most manufacturers say it is time to replace. For 9 and 10-speed chains, replace as soon as chain just hits .75% stretch. For 11 and 12-speed chain it’s time to go chain shopping when the tool just slides in on the .5% side.
Pedros Chain Checker Plus II has a whole glorious .PDF for you to read with instructions on how to use their tool. They are adamant that their 3 point measuring design leaves the poxy 2 point alternatives from ‘the other guys’ in the dust.
I have both tools and I usually reach for the Parktool CC-3.2 or 11 speed and below and the Pedros Chain Check Plus II for 12 sp and above. For what it’s worth.
Happy chain-checking!