A general equation is derived for the critical conversion at the gel point in free radical crosslinking polymerizations by omitting the equal reactivity assumption from the Stockmayer criterion of gelation. Calculations show that the monomer depletion, the volume contraction during polymerization and the initiator depletion have to be taken into account when the pre-gelation period is sufficiently large. The nucleation and growth of the gel molecule are described using kinetic differential rate equations. If the assumptions of equal vinyl group reactivity and random crosslinking are introduced, the resulting equations are equal to those of the statistical theories.