Work out chain length instantly with clear inputs, formula shown and shareable results.
The standard formula is twice the chainstay in inches plus half the teeth on the largest chainring and largest cog, plus one link for derailleur wrap, rounded up to an even number since chain links come in pairs.
Chain length
links = 2 x chainstay (in) + (chainring teeth)/2 + (largest cog)/2 + 1, rounded to an even number
A chain alternates inner and outer links. An odd count cannot close without a half link, which is only used on single-speed setups.
Shifting to big-big will destroy the derailleur. Always size for the largest chainring and largest cog even if that combination is not meant to be used.