Water-Cooled VW Performance Handbook

Compression ratio calculator

You need a couple of measurements that are not easy to get in order to calculate your compression ratio, but once you have your figures, here’s the calculator. Note that for domed pistons, the volume will be a negative number.

Bore   mm
Stroke   mm
Head volume   cc
Piston volume   cc
Gasket thickness   mm
Cylinder volume   cc
Gasket volume   cc
Compression ratio     :1

For example, a four-cylinder motor with a bore of 79.5 mm, a stroke of 86.4 mm, a head volume of 27 cc, a piston volume of 26 cc, and a 1.75 mm thick head gasket will have a compression ratio of 7.95:1.

The formula for compression ratio is:

\[Compression\ ratio = \frac{CV + PV + HV + HGV}{PV + HV + HGV}\]


  • CD = cylinder volume
  • PV = piston volume
  • HV = cylinder head volume
  • HGV = head gasket volume

Compression ratio is an important parameter that compares the volume of the cylinder when the piston is at the bottom of its stroke to the volume of the cylinder when the piston is at the top of its stroke. A higher compression ratio generally results in better engine performance, but it also increases the risk of engine knocking and requires higher octane fuel.