A dimensionless group used in analysis of fluid flow that characterizes the ratio of viscous forces to surface or interfacial tension forces. It is usually denoted N_{C} in the oil field and Ca in chemical engineering. For a flowing liquid, if N_{C} >>1, then viscous forces dominate over interfacial forces; however if N_{C} <<1, then viscous forces are negligible compared with interfacial forces. Capillary numbers are usually large for high-speed flows and low for low-speed flows; thus, typically for flow through pores in the reservoirN_{C} is ~10^{−6}, and for flow in productiontubularsN_{C} is ~1.

Capillary number equation:

N_{C} = Ca = (μU) / σ

where N_{C} = Ca = capillary number
μ = fluid viscosity U = fluid velocity
σ = surface or interfacial tension.