A physics based model using Wheelers incremental inductance rule for calculating the change in inductance due to variations in line width and thickness for planar circular spiral inductors is given. It is shown that the series resistance of an MMIC inductor can be used as a figure of merit for the robustness of the inductor against etching variations in line width during fabrication. Circular inductors are shown to have less inductance variation than rectangular inductors. This model can be evaluated quickly using a circuit simulator without the need for expensive EM analysis. In the electromagnetic modeling of MMIC inductors, a fine grid and several sheets are used to accurately model the current distribution and determine the resistance. Sonnet� is used to accurately model the 3D characteristics of thick conductors such as loss and effects of physically thick metal. A procedure based on the Richardson extrapolation method is used to extract the resistance values without long computation time. Applications include calculating the change in inductance due to overor under-etching of metal lines during fabrication. For 2 to 4 turn inductors with variations in line width of +/-20 of the nominal width, the average variation in modeled inductance is within 8 of the EM simulated variation. © 2011 ACES.