How to calculate the correct chain length

Fit the new chain over the biggest chainring on the front, and the biggest cog at the back. You don’t need to feed it through the rear mech. Pull the chain tight and match up the two ends, leaving some excess hanging from one end.

Find the two links that are the closest to matching, then slide your hand along the excess to add one complete link (one inner and one outer).

If you're joining your chain with a quick link, you only need to add half a link at this stage. The quick link itself is half a chain link, so it makes up the difference. Make sure that your chain ends with two inner links.

The point you've just found is the point at which you need to break the chain. Use your chain tool to push the pin out from the chain at that point.

Now your chain is the right length, you can thread it through your drivetrain. Make sure you put the chain through the rear derailleur correctly. There's a little plate between the two jockey wheels, and your chain needs to be on the right side of it – you'll know when it's right, because your chain will be able to move without any obstructions.

Once you've threaded it through the drivetrain correctly, join the chain with a quick link. Slide each half of the quick link through each end of the chain, then pop it into place using chain link joining pliers.