The Borda–Carnot equation is
where
: ΔE is the fluid's mechanical energy loss,
: ξ is an empirical loss coefficient, which is dimensionless and has a value between zero and one, 0 ≤ ξ ≤ 1,
: ρ is the fluid density,
: v and v are the mean flow velocities before and after the expansion.
In case of an abrupt and wide expansion, the loss coefficient is equal to one. In other instances, the loss coefficient has to be determined by other means, most often from empirical formulae (based on data obtained by experiments). The Borda–Carnot loss equation is only valid for decreasing velocity, v > v, otherwise the loss ΔE is zero without mechanical work by additional external forces there cannot be a gain in mechanical energy of the fluid.
The loss coefficient ξ can be influenced by streamlining. For example, in case of a pipe expansion, the use of a gradual expanding diffuser can reduce the mechanical energy losses.