A method for measuring the thickness of free-standing films of Formvar in the range of 2000 Å to 5 μm is presented. The optical path difference of the film with reference to air is measured using a modified Jamin interferometer, and the refractive index is determined by a thin wedge technique. The method provides thickness measurements with an accuracy of ±300 Å. The results are found to be in good agreement with those obtained from a profilometer. This simple method can be very useful for thickness measurement of free-standing thin plastic films for extreme ultraviolet (XUV) soft x-ray research applications.