Calculate Compression Ratio

An engine's compression ratio will dictate how it will perform. In general, if the volume of the cylinder when the piston is down is greater than the combustion chamber volume when the piston up, the space is calculated as a ratio of the volume. When the valves for exhaust and intake are closed, no air is let out, the piston raises, and the mixture of air and fuel is compressed. Compression ratio refers to how the cylinder's space changes from when the piston is down to when the piston is up. 5 factors define this equation: volume of cylinder displacement, volume of clearance, dome of piston, volume of head gasket and volume of chamber. To calculate the ratio for your vehicle, you need to use the formula, compression ration = volume at below dead center divided by volume at top dead center. This is not as simple as it sounds but if you know a few key measurements, you will be able to calculate the compression ratio.


  1. Locate your owner's manual; it will be necessary to help with measurements of parts of your engine.
  2. Clean your engine as much as possible with an engine de-greaser before beginning your work.
  3. Measure the bore of the cylinder with a bore gauge.
  4. Measure the distance of cylinder from the piston at the bottom to the top to find the stroke of the cylinder.
  5. Find the volume of the combustion chamber (from manufacturer or manual).
  6. Find the compression height of the piston (from the manufacturer).
  7. Find the dome/dish volume of the piston (from the manufacturer).
  8. Calculate the piston-to-deck clearance (bore x bore x 0.7854 x distance between piston and deck at top dead center).
  9. Measure your head gasket's thickness and bore.
  10. Use the following formula to calculate the compression ratio once you have all the above measurements: cylinder volume + clearance volume + piston volume + gasket volume + chamber volume divided by clearance volume + piston volume + gasket volume + chamber volume.
  11. Convert measurements to cubic centimeters if necessary from inches by multiplying inches by 16.387.



  • You can also plug your measurements into a compression ratio calculator found at many sites online instead of doing the calculations yourself.


  • Be sure to use the correct tools when working on your car's engine. An incorrect tool could slip or break and cause serious bodily injury to you or someone nearby.
  • Take proper precautions when working on your car's engine. Make sure that the engine parts are cool before beginning and wear protective gear such as safety glasses, close-toed shoes and gloves.

Things You'll Need

  • Measuring tape with metric and U.S. units
  • Bore gauge
  • Socket wrenches
  • De-greaser
  • Safety glasses
  • Close-toed shoes
  • Gloves
  • Rags or paper towels
  • Calculator

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Sources and Citations