Engine Compression Ratio Calculator

Calculate static compression ratio from the bore, stroke, combustion chamber, head gasket, piston crown, and deck clearance. The calculator shows every per-cylinder volume so you can audit the result.

Static geometric ratio—not a pressure reading This tool models a conventional reciprocating piston at bottom and top dead center. It does not calculate dynamic compression, cranking pressure, octane requirement, detonation margin, or component clearances.

Engine measurements

Changing units converts all current length measurements. Volumes remain in cc.

Finished diameter of one cylinder.

Piston travel from TDC to BDC.

Affects total displacement, not the ratio.

Cylinder-head chamber for one cylinder.

Diameter of the gasket opening.

Use the installed compressed value.

Positive dish/reliefs; negative dome. Blank = 0.

Positive below deck; negative above. Blank = 0.

Privacy: measurements are calculated locally. They are not uploaded, stored, or included in analytics events.

Static compression ratio

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Compression-ratio formula

Static compression ratio compares the maximum cylinder volume at bottom dead center (BDC) with the minimum volume at top dead center (TDC):

CR = (swept volume + clearance volume) ÷ clearance volume

For one circular cylinder, swept volume = (π ÷ 4) × bore² × stroke. The calculator builds the TDC clearance volume as:

clearance = chamber + gasket + deck + piston crown effect

Calculated gasket volume uses the gasket opening as a short cylinder. Deck volume uses the engine bore and signed piston-to-deck distance. A dish or valve relief adds space; a dome occupies space and is therefore entered as a negative value.

How to get a useful result

  1. Use finished dimensions. Enter the final bore and actual crankshaft stroke in the same unit.
  2. Use per-cylinder volumes. Chamber, gasket, and piston values must each describe one cylinder—not the whole engine.
  3. Use compressed gasket data. Enter the gasket opening diameter and installed compressed thickness, or select known volume if the manufacturer supplies it.
  4. Check the signs. Add dishes, valve reliefs, and below-deck clearance; subtract domes and above-deck protrusion.
  5. Audit the breakdown. Verify each clearance component before relying on the displayed ratio.

Static ratio, dynamic ratio, and compression pressure

Static compression ratio is fixed by geometry and uses the full stroke. Dynamic compression ratio attempts to account for when the intake valve actually closes, so it needs cam timing and additional geometry. A compression-test reading is gauge pressure measured while cranking and also reflects sealing, speed, temperature, altitude, valve timing, and test procedure.

These three values are related but not interchangeable. This calculator returns only the static geometric ratio.

Engine compression ratio FAQs

What is the static compression-ratio formula?

For one cylinder, divide swept volume plus clearance volume by clearance volume: (Vs + Vc) ÷ Vc.

What is included in clearance volume?

This tool adds cylinder-head chamber volume, installed head-gasket volume, piston dish or dome effect, and the signed piston-to-deck volume at TDC.

Should piston dome volume be positive or negative?

Enter a dish or valve-relief volume as positive because it adds space. Enter a dome as negative because it occupies space. Follow the sign convention shown here even if a parts catalog uses another convention.

Which head-gasket thickness should I enter?

Use the installed compressed thickness, not the loose gasket thickness. Use the actual gasket opening diameter, which may differ from the cylinder bore.

What sign should deck clearance use?

Use a positive number when the piston is below the block deck at TDC. Use a negative number when the piston protrudes above the deck. Zero represents a piston crown level with the deck under this simplified model.

Why might my result differ from a published ratio?

Nominal dimensions, manufacturing tolerances, piston crown details, crevice volume, resurfacing, gasket compression, carbon deposits, and rounding can all affect the true assembled volume. Published figures may also be nominal.

Is this the same as a compression-test reading?

No. Static ratio is dimensionless geometry. A compression test measures cranking pressure and depends on valve timing, sealing, test conditions, and the gauge.

Are my engine measurements tracked?

No. The calculation runs locally in your browser. This tool does not upload, store, or attach input values to analytics events.

Limits and engine-building disclaimer

  • The model treats bores, gasket openings, and deck space as circular cylinders and assumes all cylinders share the entered geometry.
  • It does not model top-ring crevice, irregular chamber or piston shapes beyond their supplied cc values, thermal expansion, head or block flex, or two-stroke port timing.
  • Compression ratio alone cannot establish required fuel octane, ignition timing, boost limit, piston-to-valve clearance, quench clearance, emissions compliance, or safe operation.

Engineering disclaimer: This is an educational static-volume estimate, not an engine-build specification or safety approval. Verify every component dimension, clearance, and manufacturer requirement; consult a qualified engine builder or machinist for decisions that could damage an engine or create a safety hazard.

Methodology and sources

Last reviewed: August 2, 2026. The volume-ratio definition, cylinder geometry, component breakdown, and unit conversion were checked against these references:

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