The continuity equation is an extension of conservation of mass to fluid flow. Consider a fluid flowing through a fixed volume tank having one inlet and one outlet. If the flow is steady (no accumulation of fluid within the tank), then the rate of fluid flow at entry must be equal to the rate of fluid flow at the exit for mass conservation. If, at an entry (or exit) having a cross-sectional area A m, a fluid parcel travels a distance d L in time d t , then the volume flow rate ( V m ⋅ ⋅ s) is given by: V = A ⋅ ⋅ d L Δ Δ t