Vortex formation at intake is an important problem encountered in hydraulic engineering. When the submergence of the intake is not sufficient, air enters the intake by means of a free-surface vortex. The entrained air can cause difficulties in the operation, efficiency, and maintenance of the hydraulic machine and structures. The value of the intake's submergence when the vortex starts entraining air is known as "critical submergence." In this study a practical formula for the critical submergence of intakes in still water is presented.