Compression Ratio Calculator
Calculate an engine's static compression ratio from the swept and clearance volumes of one cylinder.
Use it for build planning, head selection, and checking whether the chamber volume matches your target ratio.
Result
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Run the calculation to see the compression ratio.
Quick Answer
Compression ratio compares the cylinder volume at bottom dead center with the smaller clearance volume at top dead center.
How to Use It
- Enter the bore, stroke, cylinder count, and chamber measurements.
- Add gasket and deck-clearance values if they matter for your build.
- Click Calculate.
- Use the result to compare against your target ratio.
Formula
Compression ratio = (swept volume + clearance volume) / clearance volume
Swept volume = pi x bore^2 / 4 x stroke
Worked Examples
Example 1: A V8 with a 3.78 in bore, 3.62 in stroke, and 6.1 cu in chamber volume lands near 10.5:1.
Example 2: Increasing gasket thickness lowers the ratio because the clearance volume gets larger.
Example 3: A target of 11:1 requires a smaller clearance volume than 9.5:1.
How to Interpret Your Results
| Output | Meaning | Action |
|---|---|---|
| Higher ratio | More compression and usually more efficiency. | Check fuel octane and knock tolerance. |
| Lower ratio | More clearance volume or a smaller target build. | Review chamber and gasket dimensions. |
| Target clearance | The clearance volume needed to reach the chosen ratio. | Use it when matching heads or pistons. |
Frequently Asked Questions
Related Calculators
References
- OmniCalculator reference page
- The calculator uses the standard swept-volume and clearance-volume compression-ratio formula.
- Last reviewed: March 2026.