Engine Compression Ratio Calculator

About this calculator

This calculator finds an engine's compression ratio: CR = (swept volume + clearance volume) / clearance volume, where swept volume comes from the cylinder bore and stroke (V = π·(bore/2)²·stroke) and clearance volume is the space left above the piston at top dead center. Higher compression ratios extract more work per combustion cycle but demand higher-octane fuel to avoid knock.

Engine builders, automotive machinists, and performance tuners use it when planning a build — choosing pistons, decking the block, or adding head gaskets of a specific thickness — to hit a target compression ratio for a given fuel and application (naturally aspirated vs. forced induction). Enter bore, stroke, and clearance volume to get the ratio and swept volume.