Taking SCARY out of used bikes, one bike at a time.

When measuring a chain, match it up with the old one, only if it’s the original chain. If you don’t know, you’ll need to ensure it’s the right length. Oh, make sure the new one you are putting on is the right speed and matches or is compatible with the components.
To measure the chain, you can run the chain through the drivetrain on the small cogs of the cassette and chainring. Overlap two links and cut and install. Run the chain through all gears. Make sure you double-check that you can shift to a big cog on the cassette in the back and the big chainring in the front without binding. And small and small with some tension to the chain.
If still not correct or there are other issues, take it to your local shop to be safety checked. Always better to be safe than sorry.