Flory and Stockmayer used a statistical approach to derive an expression to predict the gel point by calculating when  approaches infinite size. The statistical approach assumes that (1) the reactivity of the functional groups of the same type is the same and independent of the molecular size and (2) there are no intramolecular reactions between the functional groups on the same molecule.
Consider the polymerization of bifunctional molecules ,  and multifunctional , where  is the functionality. The extends of the functional groups are  and , respectively. The ratio of all A groups, both reacted and unreacted, that are part of branched units, to the total number of A groups in the mixture is defined as . This will lead to the following reaction
The probability of obtaining the product of the reaction above is given by , since the probability that a B group reach with a branched unit is  and the probability that a B group react with non-branched A is .
This relation yields to an expression for the extent of reaction of A functional groups at the gel point
where r is the ratio of all A groups to all B groups. If more than one type of multifunctional branch unit is present and average  value is used for all monomer molecules with functionality greater than 2.
Note that the relation does not apply for reaction systems containing monofunctional reactants and/or both A and B type of branch units.