The core formula for airflow is very simple and widely used in ventilation engineering:
Q = V × A
where Q is the volume flow rate, V is the average air velocity and A is the cross sectional area through which the air flows. This relationship applies to ducts, mine roadways, shafts and any other passage where air movement is reasonably uniform.
In SI units, V is typically expressed in m/s and A in m2, giving Q in m3/s. To obtain m3/h, you multiply by 3600. In imperial units, V is often in ft/min and A in ft², giving airflow in CFM (cubic feet per minute). Regardless of units, the physical meaning is the same: how much air passes through a given area per unit time.
To use the formula accurately, you must pay attention to how velocity is measured. Air velocity is rarely uniform across a duct or roadway. It is usually higher in the centre and lower near the walls. For this reason, engineers take multiple readings at different points in the cross section using a vane anemometer, hot wire probe or Pitot tube, and then average these to obtain a representative V. This averaged velocity is then multiplied by the full cross sectional area to calculate Q.
In industrial ventilation, the airflow formula is applied to supply and exhaust ducts, air handling units and local exhaust hoods. Correct airflow ensures that contaminants are captured and that comfort or process conditions are maintained. In underground mining, the same formula is used for main airways, development headings and auxiliary ventilation ducts. By measuring velocity and geometry, engineers confirm that main fans and auxiliary fans are providing the minimum legal quantity of air to working areas.
Sometimes, airflow is also linked to pressure and fan characteristics. While Q = V × A remains the fundamental geometric relationship, fan curves and system resistance equations determine what velocity and flow can be achieved for a given fan and duct system. But the starting point is always the same: airflow equals average velocity multiplied by area.
In summary, the formula for airflow is Q = V × A. Combined with good velocity measurements and accurate area calculations, this simple equation underpins most airflow calculations in industrial plants and mining ventilation systems.
