The calculation of the refractive index of methane hydrate is carried out using a modified Lorenz-Lorentz model. The occupancy of methane into the structure of the hydrate is taken into account. It is calculated from van der Waals and Platteuuw theory and shows small changes when the experimental pressure varies from 30 to 110 bars and when the experimental temperature varies from − 3 to + 12 °C. We have observed great changes of the relative index of refraction with the methane occupancy (the relative index of refraction is the ratio between the refractive index of the methane hydrate and of liquid water). The relative index of refraction characterizes the ability of the methane hydrate particles to scatter the light. Thus, the occupancy factor of methane must be taken into account when the calculation of size distribution uses the optical properties of particles.