A mass air flow sensor (MAF) is used to find out the mass flowrate of air entering a fuel-injected internal combustion engine.

The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that mass flow sensors are more appropriate than volumetric flow sensors for determining the quantity of intake air in each piston stroke. 

There are two common types of mass airflow sensors in use on automotive engines. These are the hot film(Bosch Type) and the hot wire(Japanese cars). Neither design employs technology that measures air mass directly.