A novel and extremely accurate near Gaussian approximation for the fundamental mode in single-mode fibers having arbitrary index profiles is proposed and verified by the variational method. The spot size calculated from our approximation matches closely the analytical and exact numerical results for step and parabolic index fibers over a large range of values of the normalized frequency V. For the low V region where the Gaussian approximation is not accurate, our results are suitable to accurately predict the propagation characteristics of devices such as directional couplers involving evanescent field coupling. (C) 1996 Society of Photo-Optical instrumentation Engineers.