conspiratemus1

Just get a steel ruler and measure carefully, in a good light, along 24 rivets, as per @LV2TNDM. If distance is more than 12 1/16 inches, replace the chain. I find it easier to measure from the front or back edge of the rivets rather than trying to place the zero of the ruler exactly in the middle of a rivet. The chain should be under some tension so use the top run of chain with an assistant putting some gentle pressure on a pedal. Make sure the ruler doesn't slide or creep as you shift your attention from one end to the other. The ruler needs to be longer than 12", otherwise it's hard to tell if it is just under 12 1/16 (OK) or just over. In a pinch you can measure to 23 rivets at 11 1/2 "


Alternatively, get a good caliper and open it to 130 mm. Insert the inside-measurement jaws into the spaces between outer links and butt the jaws up against the rollers. If you get more than 132.60 mm anywhere along the chain, it is worn. This is the principle of the Park tool but has the benefit of being a tool that you can use for all kinds of other measurements, and it gives you an actual number, rather than your having to take Park's word for it.