Compression Ratio Calculator

The compression ratio is one of the defining numbers of an internal combustion engine, telling you how much the fuel and air mixture is squeezed before it burns, and this calculator works it out from two volumes. When the piston sits at the bottom of its stroke the cylinder holds its largest volume, and when it rises to the top it holds its smallest; the compression ratio compares these two. In practice it is found by adding the swept volume, the space the piston sweeps through as it moves, to the clearance volume, the small space left above the piston at the top, and then dividing that total by the clearance volume alone. Enter those two figures and the calculator returns the compression ratio expressed in the usual form as a number to one, such as 10.5 to 1. The ratio matters because it strongly influences how an engine performs. A higher compression ratio extracts more energy from each combustion, giving more power and better efficiency, which is why performance and modern engines push it as high as they can. The limit is knock, the unwanted early ignition that happens when the mixture is squeezed too hard for the fuel's octane rating, so raising compression usually means needing higher-octane petrol. Diesel engines, which rely on compression alone to ignite the fuel, run far higher ratios than petrol engines. This makes the calculator genuinely useful for anyone building, modifying or rebuilding an engine, fitting a different head or pistons, skimming a head, or simply understanding a spec sheet, as well as for students learning engine theory. The formula and a worked example are explained clearly below, and you should always work to the engine maker's specification.

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Swept volume is the volume the piston sweeps (per cylinder). Clearance volume is the combustion chamber space at top dead centre. Work to the engine maker's specification.