The residual strain in an a-Si:H layer has been directly determined with X-ray diffraction techniques from variations in the diffraction angle of the first amorphous peak using CuKα radiation. The layer was deposited by HW-CVD on glass substrates at a growth temperature of 300 o C, and is known from previous studies to be highly disordered. It was found to have an average compressive stress of 750 MPa, using the c-Si lattice parameter as a reference, and typical values of the elastic constants for a-Si:H, increasing strongly towards the surface.