Abstract: A real-time mass flow measurement system that accurately measures fuel mass delivered to each cylinder per stroke of an injection pump or a fuel injector in an internal combustion engine. In accordance with preferred embodiment of the present invention, the mass flow measurement system and method includes a fuel measuring device that measures fuel delivered by one of the plurality of fuel injectors or fuel pumps. The preferred embodiment also includes a fuel transfer circuit for directing fuel flow to the fuel measuring device and a plurality of fuel diverting devices positioned along the fuel transfer circuit for diverting fuel flow from the plurality of fuel injectors or pumps into the fuel transfer circuit and a fuel routing device positioned along the fuel transfer circuit for routing the diverted fuel flow to either the fuel measuring device or a fuel drain.