A dimensionless number, Ga, used in the study of viscous liquids. It is the relationship between the force due to molecular friction and the force of gravity in a flowing viscous fluid:

where l is the characteristic dimension of length, g is the gravitational acceleration, and ρ and μ are the density and viscosity of the viscous liquid. Ga is related to the Grashof number:

where β is the coefficient of bulk expansion (1/T) and ΔT is the temperature difference. It is used in calculations involving thermal expansion such as in the design of heat exchangers. The Galileo number is also referred to as the Galilei number and is named after the Italian scientist Galileo Galilei (1564–1642).