The well drainage pressure and radius are key parameters of real-time well and reservoir performance optimization, well test design and new wells' location identification. Currently, the primary method of estimating the well drainage radius is buildup tests and their subsequent well test analysis. Such buildup tests are conducted using wireline run quartz gauges for an extended well shut-in period resulting in deferred production and risky operations.A calculation method for predicting well/reservoir drainage pressure and radius is proposed based on a single downhole pressure gauge, flowing well parameters and pressure-volume-temperature (PVT) data. The proposed method uses a simple approach and applies established well testing equations on the flowing pressure and rates of a well to estimate its drainage parameters. This method of estimation is therefore not only desirable, but also necessary to eliminate shutting-in producing wells for extended periods; in addition to avoiding the cost and risk associated with the wireline operations. The results of this calculation method has been confirmed against measured downhole shut-in pressure using wireline run gauges as well as dual gauge completed wells, in addition to estimated well parameters from buildup tests.This paper covers the procedure of the real-time estimation of the well/reservoir drainage pressure and radius in addition to an error estimation method between the measured and calculated parameters. Furthermore, the paper shows the value, applicability and validity of this technique through multiple examples.