Compression Ratio Calculator

Calculate an engine’s static compression ratio or solve the chamber volume or compressed gasket thickness required for a target ratio.

Solve for

Dimension units

Cylinder bore (in)

Inside cylinder diameter.

Crankshaft stroke (in)

Total piston travel from TDC to BDC.

Number of cylinders

Affects total displacement, not the ratio itself.

Target compression ratio

Enter the first number in a ratio such as 10.00:1.

Clearance volume at top dead center

Combustion chamber volume (cc)

Measured cylinder-head chamber volume.

Head gasket bore (in)

Use the gasket opening, not cylinder bore.

Compressed gasket thickness (in)

Use the manufacturer’s compressed value.

Piston crown volume (cc)

Positive for a dish or relief; negative for a dome.

Deck clearance (in)

Positive below deck; negative above deck.

Compression geometry results

Static compression ratio—

Swept volume per cylinder—

Total engine displacement—

Clearance volume—

Volume at BDC—

Total displacement—

Clearance-volume breakdown

Component	Volume	Effect

Static compression ratio formula

Static compression ratio compares the cylinder volume with the piston at bottom dead center to its clearance volume at top dead center. The clearance volume includes the chamber, compressed head gasket, piston crown and signed deck-clearance volumes.

Swept volume = π ÷ 4 × bore² × stroke
Clearance volume = chamber + gasket + piston crown + deck clearance
Compression ratio = (swept volume + clearance volume) ÷ clearance volume

For piston crown volume, a dish or valve relief adds volume and uses a positive number; a dome removes volume and uses a negative number. For deck clearance, a piston below the deck uses a positive distance and a piston above the deck uses a negative distance.

Checked example

A 4.030-inch bore, 3.480-inch stroke, 64 cc chamber, 4.100 × 0.041-inch compressed gasket, 5 cc piston dish and 0.020-inch below-deck clearance produce about 727.41 cc swept volume and 82.05 cc clearance volume per cylinder. The resulting static compression ratio is about 9.87:1; eight cylinders total approximately 5.82 liters or 355.12 cubic inches.

Measurement and interpretation limits

This is a geometric static ratio, not a compression-test pressure in psi. It does not calculate dynamic compression ratio, which also depends on intake-valve closing timing and connecting-rod geometry. Actual measurements can change with machining, gasket choice, carbon deposits, piston location, production tolerances and measuring technique. Verify every component specification and required clearance with the engine and parts manufacturers before machining or assembly.

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Calculate bore, stroke and swept volume with the Engine Displacement Calculator, compare tire dimensions with the Tire Size Calculator, or calculate transmission, axle, RPM and road-speed relationships with the Gear Ratio Calculator.