An attempt to produce a simple equation for gas hold-up in stirred tank reactors has been made. This new semi-empirical equation, based on the mass balance for the gas phase, predicts the observed effects due to the tank and impeller diameters and physical properties reasonably well. It can be used both for ranks working with a single Rushton impeller as well as with multiple Rushton impeller systems. The equation has been validated using both data found in literature and from our experiments.