Grain-growth data, collected after isothermal treatment of four Si₃N₄ materials, which contained different metal oxides as densification agents, are revisited. Diffusion-controlled Ostwald ripening mechanism is generally recognized to be mainly responsible for grain coarsening upon post-densification anneal. Emphasis is placed on rationalizing the grain-growth rate in terms of viscosity of the residual intergranular glass. A simple phenomenological equation is given that allows to predict the size of the Si₃N₄ grains upon annealing process. The pre-exponential coefficient and the exponent of the equation were determined using both grain-growth and intergranular viscosity data of a model Si₃N₄ system, which contained only pure SiO₂ glass at grain boundaries. This general formulation allows to estimate the effective glass viscosity of Si₃N₄-based materials.