Flushing is a property of each coastal waterbody and is most often described by a timescale characteristic. A number of techniques have been developed to estimate residence times, but some are overly simplistic and not very accurate, while others are difficult to apply due to data requirements. The main objective of this study is the development of a simplified formula relating flushing characteristics to physical properties of tidally flushed estuaries. Numerical models of several dissimilar estuaries were developed and the modelling results were then used to compute average residence times for the waterbodies. Stepwise linear regression was applied to develop relationships between basic physical properties of the estuaries and the computed average residence times. For tidally flushed estuaries, analysis shows that a statistically high correlation exists between average residence times and combined physical properties of the waterbodies. This paper also compares a newly developed residence time formula with other widely used methods. The new method predicts more accurate estimates of average residence times than the tidal prism models frequently used by researchers. It also compares favourably with complex methods such as the jet-sink circulation model and a method based on salinity.