Compression Ratio Calculator
Calculate geometric compression ratio from swept and clearance volume for piston engines, or from maximum and minimum chamber volume for rotary/Wankel engines.
Piston engine compression ratio
Rotary / Wankel compression ratio
CR = Vmax ÷ Vmin
Worked examples
Rotary: 380 cc maximum and 50 cc minimum chamber volume gives CR = 7.60:1.
Measurement assumptions
For a real piston engine, clearance volume should include the combustion chamber, piston dish or dome contribution, head-gasket volume, deck clearance and other volume remaining above the piston at top dead center. Measure consistently in cc when practical.
For a rotary engine, measure the same working chamber at minimum and maximum volume. Rotor count is relevant to engine displacement documentation but is not multiplied into the geometric compression-ratio equation.
Limitations
This is geometric compression ratio, not dynamic compression ratio or cylinder-pressure measurement. Valve timing, operating temperature, leakage, fuel, boost and other operating conditions can affect actual engine behavior.