Planar optical waveguides were fabricated by using an electric-field-assisted potassium-ion exchange in soda-lime glass substrates. Mode index measurements for both the TE and TM modes were made for the purpose of establishing formulas for the effective diffusion constant, guide depth, and refractive-index change, given the electric field, the diffusion temperature, and the time. The index profile could be accurately modeled by a modified Fermi function, with significant improvement over the step-index model.