Define a parcel of fluid moving through a pipe with cross-sectional area A, the length of the parcel is d x, and the volume of the parcel A d x. If mass density is ρ, the mass of the parcel is density multiplied by its volume m = ρA d x. The change in pressure over distance d x is d p and flow velocity v = ⁠ d x / d t ⁠.