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Compression Ratio Calculator

Enter the swept and clearance volume per cylinder to get the exact compression ratio.

How it is calculated

The tool applies CR = (Vs + Vc) / Vc. Vs is the swept volume of one cylinder in cc (the area of the bore times the stroke) and Vc is the clearance volume in cc, the space left above the piston at top dead centre. The ratio is shown to two decimals as x : 1, and the second output, Vs + Vc, is the total cylinder volume with the piston at the bottom. Both inputs must be above zero. The clearance figure is taken as one number, so you have to add up the combustion chamber, head gasket, piston dish or dome and deck gap yourself. The tool does not check that the result is sensible for an engine.

Worked example

One cylinder with a swept volume of 500 cc and a clearance volume of 55 cc. Total volume = 500 + 55 = 555.00 cc. Compression ratio = 555 / 55 = 10.09 : 1. The piston therefore squeezes the 555 cc of gas at the bottom of the stroke into 55 cc at the top, a little over ten to one.

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Common mistakes

  • Entering the whole engine displacement as the swept volume. The input is per cylinder, so divide a 2000 cc four-cylinder by four to get 500 cc.
  • Using only the combustion chamber volume as the clearance and leaving out the head gasket, the piston crown shape and the gap at the top of the block. These all add to Vc and lower the true ratio.
  • Entering the clearance volume in litres or the swept volume in cubic inches. Both fields are in cc, so convert first.
  • Dividing the swept volume by the clearance volume without adding the two. That gives a figure that is one lower than the compression ratio.
  • Treating the result as the effective ratio of a turbo or supercharged engine. This is the static ratio from geometry only, and boost is not part of it.

About this calculator

Compression ratio is the ratio of the total cylinder volume when the piston is at the bottom of its stroke to the volume remaining when it is at the top: CR = (swept volume + clearance volume) / clearance volume, where swept volume is the volume displaced by the piston's stroke and clearance volume is the combustion chamber volume left above the piston at top dead center. It is one of the most fundamental specifications defining how an engine performs.

Engine builders and tuners work with compression ratio because it drives a direct tradeoff: raising it improves thermal efficiency and power output, but it also raises the octane rating the fuel needs to avoid knock, and pushing it too high for the fuel and ignition timing available risks engine-damaging detonation — which is why forced-induction engines typically run lower static compression ratios than naturally aspirated ones tuned for the same fuel.

Add swept volume to clearance volume, divide by the clearance volume, and you have the compression ratio; enter the per-cylinder figures to check a build sheet or compare engines the way a tuner would.

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