The traditional material balance (pit) plot for gas pools requires fully built-up reservoir pressures, obtained by shutting-in in the wells. The procedure described in this presentation does not require shut-in of wells. Instead, it utilizes information normally obtained but not usually used by reservoir engineers to quantify the original gas-in-place-the daily gas production rates and flowing pressures. The classical pseudo-steady state analysis and its shortcomings are discussed. In addition, a new procedure called the "flowing" material balance is introduced. This procedure consists of a p/z plot of the "flowing" pressure (as opposed to the average "shut-in" reservoir pressure) versus cumulative production. A straight line drawn through the flowing pressure data and then, a parallel line, drawn through the initial reservoir pressure will give the original gas-in-place. A variation of this method, using wellhead pressures (tubing and casing) is discussed and a field example is included. The method is a very practical and powerful tool for the early quantification of reserves.