Compression Ratio Calculator

Find your engine’s static compression ratio from the chamber, piston, gasket and deck numbers.

Dish or valve reliefs: positive. Dome: negative.
Piston below deck at TDC: positive.
Fractions and decimals both work.
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Compression Ratio
Swept volume
Gasket volume
Deck volume
Total clearance
Quench (squish)
Fuel Note

Combinations

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ComboChamberGasketPistonCRRemove

How to Calculate Compression Ratio

Static compression ratio compares the cylinder's total volume with the piston at the bottom to the volume left with the piston at the top (the clearance volume).

CR = (swept volume + clearance volume) ÷ clearance volume
Swept volume = π ÷ 4 × bore² × stroke
Clearance = chamber + piston dish (− dome) + gasket volume + deck volume

Gasket volume = π ÷ 4 × gasket bore² × thickness; deck volume = π ÷ 4 × bore² × deck clearance. Convert cubic inches to cc by multiplying by 16.387.

Worked Example

Tips

Pump gas street engines with iron heads usually run about 9:1 to 10:1; aluminum heads tolerate a bit more. Dynamic compression, which depends on when the intake valve closes, matters as much as the static number for detonation. Measure chamber and piston volumes with a burette for an accurate build; catalog numbers can be off by a few cc.