For a heuristic argument, consider a thin airfoil of chord  and infinite span, moving through air of density . Let the airfoil be inclined to the oncoming flow to produce an air speed  on one side of the airfoil, and an air speed  on the other side. The circulation is then
The difference in pressure  between the two sides of the airfoil can be found by applying Bernoulli's equation:
so the downward force on the air, per unit span, is
and the upward force (lift) on the airfoil is 
A differential version of this theorem applies on each element of the plate and is the basis of thin-airfoil theory.