Enter the cylinder bore diameter, stroke length, and the minimum clearance at end of the compression stroke. The calculator will evaluate and display the compression ratio of the engine.

Compression Ratio Formula

The following formula can be used to calculate the compression ratio of an engine.

CR = [ (PI/4*b^2*s) + Vc ]/Vc
  • Where CR is the compression ratio
  • b is the cylinder bore diameter
  • s is the stroke length
  • Vs is the minimum clearance at the end of the compression stroke

Compression Ratio Definition

A compression ratio is defined as the ratio of total working volume to total compression volume.

Compression Ratio Example

How to calculate compression ratio

  1. First, measure the bore cylinder diameter

    This should be the inner diameter of the cylinder, not the outer diameter.

  2. Next, measure the stroke length

    Measure the stroke length of the cylinder.

  3. Next, determine the minimum clearance

    It’s important that the minimum clearance is the clearance when the compression stroke is at its highest point.

  4. Calculate the compression ratio

    Enter the information from steps 1-3 into the equation above to calculate the compression ratio.


What is a compression ratio?

A compression ratio is a fundamental specification in combustion engine design that measures the ratio of the total working volume of a piston and its compression volume.

What is a good compression ratio?

The higher the compression ratio the better the performance of a piston. The more compression the fuel undergoes, the more pressure it sees, and the higher the thermal efficiency of the engine.

What is thermal efficiency of an engine?

Thermal efficiency is the total mechanical energy that can be extracted from a set amount of fuel.