Compression Ratio Calculator

This compression ratio calculator can be used to work out the compression ratio of your engine.

How to use this calculator: Simply fill in all the boxes below with the requested figures and click on "Calculate CR" to find your engine's compression ratio.

Definition of Compression Ratio

The compression ratio of an engine is a very important element in engine performance. The compression ratio is the ratio between two elements: the gas volume in the cylinder with the piston at its highest point (top dead center of the stroke, TDC), and the gas volume with the piston at its lowest point (bottom dead center of the stroke, BDC).

There are two ways of calculating the compression ratio for an engine. Firstly, you can make a math calculation as accurately as possible, or secondly - the more popular method - uses an empty spark plug socket with a pressure gauge inserted.

We're looking at the first of these ways here. This technique is suited to someone who is in the middle of putting an engine together and has the right tools, or someone whose engine is already in pieces.

CR Calculator


Compression Ratio:  to 1

Working it out

To find the compression ratio (CR) you divide the total swept volume with the total compressed volume. This is how you find out what these totals are:

Swept volume = chamber volume + piston volume + gasket volume + clearance volume + cylinder volume

Compressed volume = chamber volume + piston volume + gasket volume + clearance volume

All of these elements must be measured with the same units. If you're doing a manual calculation, that usually means using cubic centimeters (cc).

Calculating engine compression ratio, step-by-step

  • Find the owner's manual; this will help you to take your measurements
  • Use an engine degreaser to make your engine as clean as you can before starting
  • Use a bore gauge to measure the cylinder's bore
  • Find out the combustion chamber's volume (from the manufacturer or owner's manual)
  • Find out the piston's compression height (from the manufacturer)
  • Find the piston's dome/dish volume (from the manufacturer)
  • Calculate the clearance from piston to deck (bore × bore × 0.7854 × gap between piston and deck at top dead center (TDC))

- Measure the thickness of your head gasket and bore

- When you have all the figures, use this formula for the calculation of your engine's compression ratio:

CR = (cylinder volume + clearance volume + piston volume + gasket volume + chamber volume) / (clearance volume + piston volume + gasket volume + chamber volume)

You may also be interested in our Power to Weight Ratio Calculator

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