Ion channels can express multiple conductance levels that are not integer multiples of some unitary conductance, and that interconvert among one another. We report here that for 26 different types of multiple conductance channels, all allowed conductance levels can be calculated accurately using the geometric sequence gn = g(o) (3/2)n, where gn is a conductance level and n is an integer > or = 0. We refer to this relationship as the "3/2 Rule," because the value of any term in the sequence of conductances (gn) can be calculated as 3/2 times the value of the preceding term (gn-1). The experimentally determined average value for "3/2" is 1.491 +/- 0.095 (sample size = 37, average +/- SD). We also verify the choice of a 3/2 ratio on the basis of error analysis over the range of ratio values between 1.1 and 2.0. In an independent analysis using Marquardt's algorithm, we further verified the 3/2 ratio and the assignment of specific conductances to specific terms in the geometric sequence. Thus, irrespective of the open time probability, the allowed conductance levels of these channels can be described accurately to within approximately 6%. We anticipate that the "3/2 Rule" will simplify description of multiple conductance channels in a wide variety of biological systems and provide an organizing principle for channel heterogeneity and differential effects of channel blockers.