Association Croquet - Handicap Calculator
Singles Handicap Calculator
Singles is pretty straight-forward to work out - simply subtract one handicap from the other, but this calculator will also tell you the steps between the players, the changes to their indexes afterwards, and the adjustments for different-sized lawns.
Doubles Handicap Calculator
Doubles can get a little tricky to work out, so let this tool do it for you!
Note that there are some other considerations when allocating bisques, particularly in relation to timed games. There's a lot more information about this on the Oxford Croquet website.