Review on 🚲 Bikehand Bicycle Bike Chain Wear Indicator Tool Chain Checker – Compatible with Shimano Sram KMC and All 7-12 Speed Chains: A Must-Have for Bike Maintenance by Aaron Shaw

I agree it would be nice if there was a guide for this. I had to ask a friend, although descriptions of such tools can be found on the Internet. The bottom line is to place the notch on the link and see if the pick head fits comfortably between the links it touches. When your chain wears out, the head slides down easily. If your chain isn't worn out, then it isn't either. There are two different measurements 0.75 and 1.0 (probably 0.75% and 1% chain elongation). You should replace your chain if it stretches between those two percent. If you can reach 0.75" but not 1.0", consider replacing the chain. With the 1.0 sitting comfortably it is definitely time to replace your chain and possibly your cassette as more chain wear means more cassette wear. an indicator with a big name, which makes it a good buy.