If A  denotes the area of free surface of a liquid and h  the depth of an orifice of area of cross-section a , below the liquid surface, then the velocity v  of flow through the orifice is given by :

Applying Bernoulli’s theorem, we have Pρ+12(v')2+gh=Pρ+12v2+0 Where v'  is the velocity of all surfaces of liquid and v  the velocity of efflux.Further, from continuity equation,Av'=av  or v'=avA ∴  12(avA)2+gh=12v2 ∴  v=2gh(A2A2−a2)