Actuator disc theory is the basis of most rotor design methods. However, the off-design condition of a very low rotational speed Ω is still a topic for scientific discussions. A small modification in the momentum theory for Joukowsky discs with uniform circulation results in the efficiency going to 0 instead of ∞ when Ω goes to 0. For high values of Ω, the classical momentum theory is reproduced. The new theory is confirmed by a very good match with numerically obtained results.