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Compression Ratio Calculator
Find static compression ratio from swept and clearance volumes.
Compression ratio
—
Static.
| As a fraction | — |
|---|---|
| Swept | — |
| Clearance | — |
| — |
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Enter your figures above to see the step-by-step working.
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How the Compression Ratio Calculator Works
Compression ratio compares the cylinder volume when the piston is at the bottom to when it’s at the top. It’s the swept (displaced) volume plus the clearance (combustion chamber) volume, all divided by the clearance volume. A 500 cc cylinder with a 50 cc chamber has an 11:1 ratio.
Higher compression extracts more power and efficiency but demands higher-octane fuel to avoid knock. Forced-induction engines usually run lower static ratios.
compression ratio = (swept + clearance) ÷ clearance
Frequently Asked Questions
How do you calculate compression ratio?
Add the swept and clearance volumes and divide by the clearance volume. (500 + 50) ÷ 50 = 11:1.
What is a good compression ratio?
Around 9:1 to 11:1 for pump-gas engines; higher needs premium fuel, and turbo engines often run lower.
Why does compression ratio matter?
Higher ratios give more power and efficiency but risk knock unless the fuel octane is high enough.
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