Supposing I am putting a new chain on a bike and I do not have an old chain to judge it against, how do I work out how long the chain should be?

I think it’s…take it round the big ring and the largest gear on the block, but not thru the rear mech, and allow a two link overlap. Works for me.

Rule of thumb assuming you a normal triple on the front, a wide range cassette at the back and a long cage rear mech is to feed the chain around the biggest chaining and biggest sprocket MISSING the rear mech out, mark where the chain joins and the ADD two links (allow for powerlinks if using swam). This should give you the correct length. You can go shorter, but then when you do an emergency shift into big big it won’t work. There is a host of chain length stuff on the park tools website

If it’s a bouncy toy, don’t forget to swing the linkage and do this where the rear axle and bottom bracket are furthest apart or you might get big big and then rip the mech off at full compression!