Compression ratio is a measure of the amount that the intake charge of an engine is compressed before combustion takes place.

The geometric compression ratio is the volume above the piston at bottom dead centre of the compression stroke divided by the volume above the piston at top dead centre.

In a forced induction engine, the intake charge going into the cylinder has already been compressed by the turbocharger or supercharger. When this charge is compressed inside the cylinder, the effect is the same as in a naturally aspirated engine with a higher compression ratio. This is known as the dynamic compression ratio.

Higher compression ratios are desirable because they increase the thermal efficiency of the engine. However, higher compression ratios can also lead to knocking.