🔧 Compression Ratio Calculator
Calculate your engine’s static compression ratio from swept (displacement) volume and combustion chamber clearance volume.
What is compression ratio?
Static compression ratio (CR) compares the total volume inside an engine cylinder when the piston is at the bottom of its stroke to the volume remaining when the piston is at the top. Higher compression ratios generally improve thermal efficiency and power but require higher-octane fuel to avoid knock.
How this calculator works
Compression Ratio = (Swept Volume + Clearance Volume) ÷ Clearance Volume. Swept volume is the volume displaced by the piston as it travels from bottom dead center to top dead center (essentially the cylinder’s displacement). Clearance volume is everything left above the piston at top dead center: the combustion chamber, head gasket volume, piston dish/dome, and deck clearance combined.
This calculator gives the static (geometric) compression ratio. It does not account for valve timing effects, which determine the engine’s dynamic compression ratio — a different and generally lower figure used in tuning.
Use this for general reference and educational engine-building purposes; always cross-check critical builds with precise volumetric measurements (e.g. burette method).