Do you need a chain tool to fit chain from new?
kinda. Unless the chain is the right length out of the box (highly unlikely) you'll need to chain tool to shorten it to the correct length
Yes, you will need to remove some links to make it the right length for your bike.
OK - in that case, is there a rule for knowing the right length?
For a hardtail it's putting it in the larger ring at the front and smallest cog at the back. The mech hanger should be vertical.
For a full suss I think it's the same plus 2 links - may be wrong so someone will no doubt correct me
Big ring to big ring plus two links is the rule I've always used.
(ignoring the rear mech)
Cheers all.
With a full suss, it varies depending on design, some have lots of growth and others don't. So I check it by removing the air or spring from the shock and squashing it.
Cheers Nw - The chain is going on a HT but i'll remember that for future ref.