Compression Ratio Calculator (Static)
Static compression ratio compares the cylinder volume with the piston at the bottom of its stroke with the volume remaining when the piston is at the top. Enter the bore, stroke and clearance volumes to find it, or work backwards from a target ratio.
Static compression ratio
Lengths use the selected unit; chamber, gasket and piston volumes are in cc.
Positive if the piston sits below the deck; negative if above.
Positive for dishes and valve reliefs; negative for domes.
- Swept volume
- 716.62 cc
- Gasket volume
- 8.654 cc
- Deck volume
- 2.059 cc
- Piston volume
- 0.00 cc
- Chamber volume
- 64.00 cc
- Total clearance
- 74.713 cc
How the ratio is built up
The clearance volume is everything sitting above the piston at top dead centre: the combustion chamber, the compressed head gasket, the deck clearance and any dish, dome or valve relief in the piston.
In words: swept volume = (pi / 4) x bore^2 x stroke. Gasket volume = (pi / 4) x gasket bore^2 x gasket thickness. Deck volume = (pi / 4) x bore^2 x deck clearance. Total clearance volume = chamber + gasket + deck + piston volume. Compression ratio = (swept volume + clearance volume) / clearance volume.
Piston volume is positive for a dish or valve reliefs, because those add volume, and negative for a dome, because it removes volume. Deck clearance is positive when the piston stops below the deck and negative when it protrudes above it.
Because displacement grows with the square of the bore, an overbore also raises the swept volume and nudges the compression ratio up unless the chamber is enlarged to match.
Worked example
A 4.000 in bore, a 3.480 in stroke, a 64 cc chamber, a 4.100 in gasket that compresses to 0.040 in, 0.010 in of deck clearance and flat-top pistons.
- Swept volume
- 716.62 cc
- Gasket volume
- 8.654 cc
- Deck volume
- 2.059 cc
- Chamber volume
- 64.00 cc
- Piston volume
- 0.00 cc
- Total clearance
- 74.713 cc
- Compression ratio
- 10.59:1
Static, not dynamic
This figure is the static compression ratio, measured with the engine stopped. The dynamic, or running, ratio depends on when the intake valve closes and can be quite different, which is why two engines with the same static ratio can behave differently.
Compression ratio also interacts with boost, cam timing and the fuel the engine is built to run. Decisions like these belong with a machine shop or engine builder who can see the whole combination, so use this calculator as one input rather than the only answer.
Frequently asked questions
What is the difference between static and dynamic compression ratio?
Static compression is measured geometrically with the piston at the bottom and top of its stroke. Dynamic compression accounts for the fact that the intake valve closes part way up the stroke, so the effective trapped volume is smaller.
How do I measure combustion chamber volume?
Chambers are usually measured in cc with a burette and a flat plate sealed over the chamber. A machine shop can do this accurately, and it is worth doing because small errors in chamber volume move the ratio noticeably.
Does head gasket thickness change compression?
Yes. A thicker gasket adds clearance volume, which lowers the compression ratio, while a thinner gasket raises it. The change is modest but measurable, and it is one reason builders select gasket thickness carefully.
What does a dome or a dish do to the ratio?
A dome removes clearance volume and raises the ratio. A dish or valve reliefs add clearance volume and lower it. Enter domes as a negative piston volume and dishes as a positive number.
Is a higher compression ratio always better?
No. A higher ratio can improve efficiency up to a point, but it also increases the demands on fuel quality, combustion chamber shape and the rest of the build. The right number depends on the whole engine, not the ratio alone.
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Last updated: October 11, 2026