Coke yields in fluid catalytic cracking units (FCCU's) can be accurately predicted by a recently developed reaction kinetic equation. The equation has been checked against actual coke yields in both pilot and commercial FCCU's. It is well known that coke yield is important in the heat balance of fluid catalytic crackers. The amount of coke produced in the FCCU can limit the extent of the cracking reaction and, in turn, affect the primary product yields.