Compression Ratio Calculator

It compares the cylinder volume with the piston at bottom dead centre to the volume at top dead centre. A 10:1 ratio means the fuel and air charge is squeezed into one tenth of its starting space before the spark.

Static CR equals the swept volume plus the clearance volume, all divided by the clearance volume: (swept + clearance) / clearance. Swept volume alone never appears in the ratio without the clearance added on top.

It uses the cylinder formula pi/4 times bore squared times stroke, divided by 1000 to convert cubic millimetres to cc. An 86 mm bore with an 86 mm stroke gives about 499.56 cc per cylinder.

The space left above the piston at top dead centre: mainly the combustion chamber, plus the head gasket volume, any deck clearance, and a piston dish that adds volume or a dome that subtracts it. It is the smaller number and it swings the ratio the most.

Static, from geometry alone. Dynamic compression also depends on when the intake valve closes on the compression stroke, so it is almost always lower than the static figure and cannot be found from volumes by themselves.

Usually. A higher ratio raises peak cylinder pressure and temperature, which makes the charge more likely to knock, so high static CR engines often call for premium fuel, richer mixtures or careful ignition timing.

Per cylinder. Compression ratio is the same for every identical cylinder, so entering the total swept volume of a multi-cylinder engine along with one chamber would give a wrong, inflated number.

Dividing by a zero clearance volume is undefined and physically impossible, so the tool asks for a positive clearance value. The same guard applies to swept volume, bore and stroke.

No. The math runs entirely in your browser with plain arithmetic, so your engine specs never leave your device and nothing is uploaded or stored.