One of the most important tasks of thermochemical codes for the calculation of detonation properties is the accurate description of the state of gaseous products within a rather wide range of pressures and temperatures – from several hundreds of kbar and several thousands of K to atmospheric pressure and temperature. Due to its simplicity and convenience, the Becker-Kistiakowski-Wilson (BKW) equation of state is used in many practical applications in the explosives field, despite its lack of rigorous theoretical background. The BKW EOS gives good agreement between calculated and experimentally obtained detonation parameters for many standard high explosives having densities in the range 1.2 – 2 g/cm3. However, it fails to predict accurately detonation properties at lower densities. To overcome this problem, we introduced the concept of density dependent molecular covolumes in the BKW EOS instead of invariant. The applicability of the approach is verified by comparing experimental and calculated values of detonation parameters for a series of explosives having different formulations and densities. It was found that by applying this approach the accuracy of the calculations for lower densities can be significantly improved.