A method and apparatus for dispensing compressed natural gas and for maximizing the mass of compressed gas dispensed into a gas storage cylinder is disclosed. Pressure and temperature transducers are provided as a part of the apparatus to emit data signals to a control processor of the pressure and temperature of a supply of compressed gas delivered to a gas dispenser, as well as the ambient temperature at the dispenser and the pressure of the compressed gas within the cylinder, respectively. A mass flow meter is also provided for emitting a data signal to the control processor of the mass of compressed gas injected into the storage cylinder. The control processor includes a dispenser control program which processes the emitted data signals to automatically maximize the mass of compressed gas injected into the cylinder by performing at least a two-stage fill process for computing at least two dynamic estimates of the storage cylinder volume during the gas dispensing process, and for determining the maximum mass of compressed gas that can be safely injected into the gas storage cylinder in response thereto. Additional fill stages may be performed in order to calculate additional estimates of the storage cylinder volume in the control processor, if so desired, for even more accurately determining the mass of compressed gas that may be injected into the cylinder for maximizing the gas injection into the gas storage cylinder.