Compression Ratio Calculator

Calculate static compression ratio with optional gasket, piston and deck-volume adjustments

Use a positive piston value for a dish (adds clearance) and a negative value for a dome (reduces clearance).

Your result will appear here.

About the Compression Ratio Calculator

Compression ratio compares the cylinder volume when the piston is at bottom dead centre with the volume remaining at top dead centre. It is a geometric ratio based on swept volume and clearance volume.

Higher compression can improve thermal efficiency in some engines, but practical limits depend on fuel, combustion design, boost, temperature and engine durability.

How to Use the Compression Ratio Calculator

  1. Enter swept volume per cylinder or the dimensions needed to derive it.
  2. Enter clearance volume at top dead centre.
  3. Calculate the compression ratio.
  4. Use measured or manufacturer chamber data when making engine-build decisions rather than relying on rough estimates.

How the Calculation Works

Compression ratio CR = (swept volume + clearance volume) ÷ clearance volume.

Swept volume comes from bore and stroke geometry, while clearance volume includes the combustion chamber and any other volume remaining above the piston at top dead centre.

Worked example

Compression Ratio Calculator Worked Example

500 cc swept volume and 50 cc clearance volume

Compression ratio = (500+50)÷50 = 550÷50 = 11.0, commonly written 11:1.

Result guide

Understanding Your Compression Ratio Calculator Result

The ratio shows how much the cylinder contents are geometrically compressed. It does not directly predict cylinder pressure because valve timing, boost and thermodynamics also matter.

Before you rely on the answer

Assumptions and Limitations

  • Swept and clearance volumes are measured on the same unit basis.
  • Clearance volume includes all relevant top-dead-centre spaces.
  • Dynamic compression effects from valve timing are excluded.

Practical tip

Using This Result in Practice

For engine-building work, small changes in gasket thickness, piston dish or chamber volume can materially change the ratio, so use measured volumes where possible.

Related calculations

Useful Calculations to Continue With

If swept volume is not already known, calculate it from bore, stroke and cylinder count with the Engine Displacement Calculator.

If you are evaluating engine operating speed rather than geometry, use the Engine RPM Calculator.

Common questions

Compression Ratio Calculator FAQ

What does 10:1 compression ratio mean?

The cylinder volume at bottom dead centre is ten times the clearance volume at top dead centre.

Is compression ratio the same as cylinder pressure?

No. Pressure also depends on valve timing, temperature, boost and other operating conditions.

Does turbocharging change static compression ratio?

No. Static compression ratio is geometric, although boost changes the pressure entering the cylinder.