Methods and apparatus utilize a rate of drop in pressure upstream of a gas flow controller (GFC) to accurately measure a rate of flow through the GFC. Measurement of the gas flow through the many gas flow controllers in production use today is enabled, without requiring any special or sophisticated pressure regulators or other special components. Various provisions ensure that none of the changes in pressure that occur during or after the measurement perturb the constant flow of gas through the GFC under test. A pressure regulator is coupled to a gas source. The GFC is positioned downstream of the pressure regulator. A pressure transducer is measuring pressure in a volume between the pressure regulator and the GFC, wherein means are provided for increasing the pressure in the volume.