Compression Ratio Calculator

About this calculator

Compression ratio is the ratio of the total cylinder volume when the piston is at the bottom of its stroke to the volume remaining when it is at the top: CR = (swept volume + clearance volume) / clearance volume, where swept volume is the volume displaced by the piston's stroke and clearance volume is the combustion chamber volume left above the piston at top dead center. It is one of the most fundamental specifications defining how an engine performs.

Engine builders and tuners work with compression ratio because it drives a direct tradeoff: raising it improves thermal efficiency and power output, but it also raises the octane rating the fuel needs to avoid knock, and pushing it too high for the fuel and ignition timing available risks engine-damaging detonation — which is why forced-induction engines typically run lower static compression ratios than naturally aspirated ones tuned for the same fuel.

This calculator takes the swept and clearance volume per cylinder and returns the exact compression ratio, so you can check a build's specs or compare engines on the same basis a tuner would.