A dimensionless group used in analysis of fluid flow that characterizes the ratio of gravitational forces to surface or interfacial tension forces. It is usually denoted N_{b} in the oil field and Bo in chemical engineering. A value of N_{b} <<1 implies the flow in question is only weakly dependent on gravitational forces, whereas N_{b} >>1 implies gravitational forces dominate over interfacial forces.

Bond number equation:

N_{b} = Bo = (Δρ gb^{2}) / σ,

where N_{b} = Bo = Bond number
Δρ = density difference between the two phases g = acceleration due to gravity b = a characteristic length scale of the flow geometry
σ = surface or interfacial tension.