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Compression Ratio Calculator

Computes the static compression ratio of a piston engine from bore, stroke, combustion chamber volume and the optional piston dish/dome, deck clearance and head gasket volumes: CR = (swept + clearance) / clearance.

When to use

You are building or modifying an engine and need the compression ratio after changing heads, pistons, gaskets or deck height.

Do not use when: You need dynamic (effective) compression ratio, which depends on intake valve closing timing, or plain displacement (use engine-displacement).

Formula

swept = π/4 × bore² × stroke; deck_volume = π/4 × bore² × deck_clearance; gasket_volume = π/4 × gasket_bore² × gasket_thickness; clearance = chamber_volume_cc + piston_volume_cc + deck_volume + gasket_volume; compression_ratio = (swept + clearance) / clearance (all volumes in cc)

Static (geometric) ratio for one cylinder. Effective compression is lower because the intake valve closes after bottom dead centre; pump-fuel engines typically run 8:1–11:1 naturally aspirated and higher with direct injection.

Inputs

ParameterTypeUnitRequiredDescription
bore_mmnumbermmyesCylinder bore in millimetres (inches × 25.4). Range: > 0, ≤ 1000
stroke_mmnumbermmyesStroke in millimetres. Range: > 0, ≤ 1000
chamber_volume_ccnumberccyesVolume of the cylinder-head combustion chamber in cc (from the head specification or by cc-ing the head). Range: > 0, ≤ 5000
piston_volume_ccnumberccdefault 0Positive for a dish or valve reliefs (adds clearance volume), negative for a dome, 0 for flat-top. Range: ≥ -500, ≤ 500
deck_clearance_mmnumbermmdefault 0Distance the piston sits below the block deck at TDC (negative if it protrudes). Range: ≥ -5, ≤ 20
head_gasket_thickness_mmnumbermmdefault 0Compressed gasket thickness; 0 to ignore the gasket volume. Range: ≥ 0, ≤ 10
head_gasket_bore_mmnumbermmnoGasket opening diameter; defaults to the cylinder bore. Range: > 0, ≤ 1000

Outputs

OutputTypeUnitDescription
compression_rationumber(swept + clearance) / clearance.
compression_ratio_textstringFormatted as n:1.
swept_volume_ccnumberccπ/4 × bore² × stroke for one cylinder.
deck_volume_ccnumberccπ/4 × bore² × deck_clearance.
gasket_volume_ccnumberccπ/4 × gasket_bore² × gasket_thickness.
clearance_volume_ccnumberccchamber + piston + deck + gasket volumes: the volume above the piston at TDC.

Example

350 SBC: 4.00 × 3.48 in, 64 cc heads, 5 cc reliefs, 0.025 in deck, 0.040 in gasket at 4.100 in: {"bore_mm":101.6,"stroke_mm":88.392,"chamber_volume_cc":64,"piston_volume_cc":5,"deck_clearance_mm":0.635,"head_gasket_thickness_mm":1.016,"head_gasket_bore_mm":104.14} → {"compression_ratio":9.655,"compression_ratio_text":"9.65:1","swept_volume_cc":716.62,"deck_volume_cc":5.148,"gasket_volume_cc":8.654,"clearance_volume_cc":82.802}

86 × 86 mm, 45 cc chamber, 1 mm gasket at 87 mm, flat piston, zero deck: {"bore_mm":86,"stroke_mm":86,"chamber_volume_cc":45,"head_gasket_thickness_mm":1,"head_gasket_bore_mm":87} → {"compression_ratio":10.806,"swept_volume_cc":499.56,"gasket_volume_cc":5.945,"clearance_volume_cc":50.945}

GET https://tttkmbb.com/api/v1/calculate/compression-ratio?bore_mm=101.6&stroke_mm=88.392&chamber_volume_cc=64&piston_volume_cc=5&deck_clearance_mm=0.635&head_gasket_thickness_mm=1.016&head_gasket_bore_mm=104.14

Machine access

Sources

FAQ

How do I get the chamber volume?

From the cylinder-head specification, or measure it by filling the chamber with liquid from a burette (cc-ing) with the valves and spark plug installed.

Does a thicker head gasket lower compression?

Yes: it adds clearance volume. Each 0.25 mm (0.010 in) of gasket on a 100 mm bore adds about 2 cc, roughly 0.2 points of ratio on a 10:1 engine.

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