A dimensionless number used in heat transfer in general and free convection calculations in particular. Proportional to (Buoyancy force) / (Viscous force).

Gr = (L³ rho² g beta (T₂-T₁)) / mu²

Gr = (L³ g beta (T₂-T₁)) / vu²

beta: coefficient of expansion, (T₂-T₁): temperature difference, g: gravitational acceleration, L: characteristic length, mu: viscosity, nu: kinematic viscosity, rho: density