This article presents an analytical method to calculate the expansion of intumescent coatings under different heating and fire conditions, being the most critical step in quantifying their fire protection performance under different conditions. The proposed method extends that of Amon and Denson, originally developed for spherical bubbles in viscous fluid subject to increase in pressure within the bubbles, to intumescent coatings with non-uniform temperature field and temperature-dependent viscosity. The pressure increase inside the bubbles is a result of the conversion of intumescent coatings from melt to gases at high temperatures. The extended analytical method has been used to predict the expansion processes of intumescent coatings tested by Zhang et al. under cone calorimeter with different heating rates and under furnace fire condition with different temperature-time curves, and those of Muller under cone calorimeter heating. In these tests, intumescent coatings were applied to steel plates and the tests examined the effects of different coating thicknesses and steel plate thicknesses, therefore allowing the fire and cone calorimeter tests to encompass a wide range of temperatures and rates of heating. Comparison of the analytical calculation and test results indicates that the proposed method is suitable for quantifying the expansion process of intumescent coatings. POLYM. ENG. SCI., 56:798-809, 2016. (c) 2016 Society of Plastics Engineers