A new analytical formula to describe the vertical stiffness of a rolling lobe spring has been established; the equation incorporates the rate of change of the effective area of the spring. Two rolling lobe air springs were studied by performing analytical calculations, and the results were verified by comparisons to finite element simulations and experimental data. The analytical formula can be used to qualitatively predict the vertical stiffness of a rolling lobe air spring. Precise prediction of the value of the vertical stiffness can be obtained using the formula in cases where the reservoir volume is not very large. The influence of geometric parameters on the vertical stiffness of a rolling lobe air spring has been studied using the proposed formula and a guideline for its design has been obtained by plotting a cause–effect chart.