Volumetric efficiency is a measure of an engine's ability to breathe.

During induction air is drawn into the cylinder, and if the intake system allows the cylinder to completely fill with air, volumetric efficiency is 100%.

Volumetric efficiency varies with engine speed and throttle opening, and is usually at is highest in the middle of the engine speed range.

Tuned length intake systems and forced induction can lead to volumetric efficiencies greater than 100%.