Compression Ratio Calculator

Enter cylinder dimensions and combustion chamber volume.

Bore (inches)

Stroke (inches)

Chamber volume (cc)

Head gasket thickness (inches)

Head gasket bore (inches)

Piston dome/dish volume (cc, negative=dish)

Compression ratio —

Swept volume per cylinder —

Total clearance volume —

CR = (swept + clearance) / clearance. Clearance = chamber + gasket + piston dish. Typical street: 9:1–10.5:1. High performance: 11:1–13:1. Forced induction: 8:1–9.5:1.

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How to Calculate Compression Ratio

Compression ratio is the ratio of the cylinder volume at BDC (bottom dead center) to the volume at TDC (top dead center). Higher CR generally produces more power but requires higher octane fuel.

Total clearance volume includes the combustion chamber, head gasket volume, and piston dish (or subtract dome volume). CR = (swept volume + clearance volume) / clearance volume.

Compression ratio = (swept volume + clearance volume) / clearance volume. Swept volume is the volume displaced by the piston from BDC to TDC; clearance volume is the remaining space at TDC (combustion chamber + head gasket + piston dish). Higher compression extracts more energy from the same fuel charge but requires higher octane fuel to prevent knock.

Typical compression ratios: stock gasoline engines 9:1–12:1; forced induction 7:1–9:1 (turbo/supercharger adds pressure equivalent to higher compression); diesel engines 14:1–25:1 (diesels ignite by compression heat, not spark). Modern direct-injection engines run 12:1–13:1 with 91–93 octane regular premium fuel.

Octane requirement increases with compression ratio: roughly every 1 point of compression ratio requires 3–4 octane numbers of additional fuel. A stock 10:1 engine running fine on 87 octane upgraded to 11:1 pistons would likely need 91–93 octane to prevent detonation under load.

Effective compression ratio for supercharged/turbocharged engines: multiply the mechanical CR by the boost pressure ratio. A 9:1 engine with 10 PSI boost has an effective CR of approximately 9 × (14.7+10)/14.7 = 9 × 1.68 = 15:1 — which is why boost and compression ratio must be balanced carefully to avoid knock.