An important part of a rear derailleur's job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 53/39 chainrings and a 12-23 cassette: (53 - 39 = 14) + (23 - 12 = 11) = 25-tooth chain-wrap capacity.