Material balance calculations for determining oil- or gas-in-place require static reservoir pressures, which can only be obtained when the well is shut in. In a previous publication((1)) titled "The 'Flowing' Gas Material Balance," it was shown that the reservoir pressure could be obtained from the flowing pressure for wells producing at a constant rate. The "Dynamic Material Balance" is an extension of the "Flowing Material Balance" and can be applied to either constant or variable flow rates. Both methods are applicable for gas and oil. The "Dynamic Material Balance" is a procedure that converts the flowing pressure at any point in time to the average reservoir pressure that exists in the reservoir at that time. Once that is done, the classical material balance calculations become applicable, and a conventional material balance plot can be generated. The procedure is graphical and very straightforward: a) knowing the flow rate and flowing sandface pressure at any given point in time, convert the measured flowing pressure to the average pressure that exists in the reservoir at that time; and, b) use this calculated average reservoir pressure and the corresponding cumulative production, to calculate the original oil- or gas-in-place by traditional methods. The method is illustrated using data sets.