As @Crankycrank suggests, if you're not changing the gear ratios, use the old chain to determine the length of the new chain. And be prepared to replace rear sprockets as well. Worn sprockets, especially the smallest ones, may not mesh properly with a new chain. If you change the gear ratios when you replace the sprockets, you may also need to adjust the chain length, so if you have any plans to change your gearing, now is the time to do it.