This report describes the theoretical basis and validation of a computer program as NMIWAVE, which computes linearised wave loads, added masses, damping coefficients and rigid body response motions for fixed, free-floating or tethered structures. By combining diffraction theory for large diameter members with a linearised form of Morison’s equation for smaller members it computes the fluid forces more efficiently and accurately than either method alone. The program has been validated by comparison with published theoretical and experimental data.