Engine compression ratio calculator
Useful when changing the head gasket or pistons or milling the head, to see the new compression ratio before assembly.
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How to use it
- Enter bore, stroke, combustion chamber volume, head gasket bore, compressed gasket thickness and deck clearance (positive if piston is below deck).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Swept volume = π ÷ 4 × bore² × stroke. Clearance volume = chamber + π ÷ 4 × gasket bore² × thickness + π ÷ 4 × bore² × deck clearance + piston volume. Ratio = (swept + clearance) ÷ clearance.
Example
Inputs: Bore = 86 mm, Stroke = 86 mm, Combustion chamber volume = 50 cc, Head gasket bore = 88 mm, Compressed gasket thickness = 1 mm. Result: Compression ratio: 9.91 :1; Swept volume per cylinder: 499.6 cc; Total clearance volume: 56.08 cc.
Limits and caveats
- This is static compression. Dynamic compression is lower and depends on intake valve closing time.
- Measure chamber and piston volumes by liquid (cc-ing) for good accuracy; published figures are approximate.
Questions about this tool
How does gasket thickness matter?
A thicker gasket adds clearance volume and lowers compression; a thinner one raises it.
When is piston volume negative?
When the piston has a dome that intrudes into the chamber, reducing volume and raising the ratio.
What ratio do petrol engines usually have?
Many modern naturally aspirated petrol engines run about 10 to 13:1, while turbo engines are usually lower.