A great deal of research has been conducted on how humans reason about probability, yet it remains unknown what mental computations support this ability. Research on the development of the Approximate Number Sense (ANS) has shown that performance in a magnitude (i.e., estimations of integers) discrimination task is well fit by a psychophysical model (Halberda & Feigenson, 2008). Whether or not estimations of integers plays a role in probability judgments has yet to be investigated. In the present study we use data from two adult experiments as well as results from comparisons of two computational models to argue that simple probability judgments based on proportion may involve a division operation, a finding that could explain why previous research has yielded little evidence supporting the role of the ANS in probability judgments.