The erratic buckling behavior of cylindrical shells in axial compression is analyzed using the concept of equivalent imperfections. It is postulated that the sum of all departures from perfection in a real shell can be replaced by a hypothetical axisymmetric imperfection in a corresponding shell of infinite length. The equivalent imperfect is treated as a random variable with a normal distribution whose mean and variance are assumed to be directly proportional to the shell radius to thickness ratio. This leads to a design formula which is compared with the results of some 360 cylinder buckling experiments published elsewhere. An appropriate choice of proportionality constants yields a curve which gives a good lower bound on the experimental buckling loads over the whole range of radius to thickness ratios. An analogy is drawn between the Perry-Robertson strut formula and the one proposed here for cylinders.