Signatures of oceanic internal waves are frequently visible on Synthetic Aperture Radar images. The observed radar backscatter modulations are due to induced surface current variations, but the degree of modulation depend on several factors, such as the phase speed of the internal waves, the ambient wind and viewing geometry. Dimensional analysis is here used to derive simplified expressions relating the observed radar backscatter modulation to relevant geophysical parameters. The derived expressions agree well with simulations with a sophisticated radar imaging model, which is used to determine the free parameters.