In this paper, we report the measurements of indium composition of thick InGaN epilayers by X-ray diffraction (XRD) and Rutherford backscattering spectroscopy (RBS). In order to account for the biaxial stress in the InGaN epilayers, we determined both a and c lattice parameters in a θ/2θ scan. Indium composition was determined by simultaneous application of Vegard's law to both lattice parameters and by considering the relationship between the lattice parameters under strain. These composition values are compared with values determined by RBS. The value of elastic constants used in these calculations is critical and we show that by careful choice we can obtain a good correlation between the XRD and RBS measurements.