Mass flow rate of solids is probably the most important parameter in pneumatic conveying systems. It is a challenging task to measure this parameter in gas-solid flows. A new and simple model is presented and described to calculate the mass flow rate of solids in pneumatic conveying systems, based on air flow and pressure measurements. The principle of the model is conservation of mass, and it is applied in a horizontal straight pipe section. Two tests need to be performed on the actual conveying rig to be used for calibration of the model. Due to the kind of sensors used to carry out the necessary measurements, application of the model is rather inexpensive and non-intrusive. The model has been validated successfully with data from two different single blow tank conveying systems and in dense and dilute phase conveying. Four test materials were used for validation of the model: alumina, baryte, cement and dextrose. Provided that the requirements to apply the air mass balance model are fulfilled, its effective widespread application on real industrial systems seems plausible. (C) 2010 Elsevier B.V. All rights reserved.