the Ideal Gas Law is an equation you may or may not remember from high school chemistry.

PV = nRT

• P = Pressure (must be an Absolute Pressure scale)
• V = Volume
• n = number of moles (number of atoms) gas
• R = ideal gas constant = 8.3145 J/mol K
• T = temperature (always in degrees Kelvin for this calculation - absolute scale)

The form of the ideal gas law most applicable to engine management is n = PV / RT By solving for n, we are solving for the actual airflow into the motor as estimated by pressure, temperature and volume factors measured elsewhere.

One random googled writeup from a college class: http://www.chm.davidson.edu/ChemistryApplets/GasLaws/GasConstant.html