We introduce a technique to measure transparent glass slab thickness. It employs a very simple setup combining two interferometers: a forward-going beam scheme and a self-mixing readout of the beam reflected back to the laser source. Interestingly, the difference of the two readouts provides a quantity related to thickness measurement, irrespective of refractive index. We demonstrate this method using samples on a range of thickness from 30 to 1000microm.