Compression ratio calculator
Compression ratio from swept and chamber volume.
Fill in the fields and the result will appear here automatically.
The geometric compression ratio compares one cylinder’s volume at bottom dead centre with its volume at top dead centre. Enter that cylinder’s swept volume and the total clearance volume above the piston at TDC. Clearance includes the head chamber, gasket, piston position and piston shape. The dimensionless ratio is not a compression pressure and alone does not determine power, fuel octane or permissible boost.
How it works
Formula and logic
CR = (Vₛ + V꜀)/V꜀ = 1 + Vₛ/V꜀. Both volumes are positive and in cm³ for one cylinder; V꜀ is the total clearance at TDC. For a fixed Vₛ, reducing V꜀ increases the ratio. Displayed values are rounded; unrepresentable output quantities cause an error.
Example
A 454.17 cm³ cylinder with a 45 cm³ chamber gives 11.093.
Fields and units
- Cylinder swept volume, cm³ — cm³
- Combustion chamber volume, cm³ — cm³
How to use
- — Enter one cylinder’s swept volume in cm³, not the whole engine’s.
- — Enter total clearance above the piston at TDC in cm³, including all its geometric contributions.
- — Read CR and total volume; do not interpret CR as a compression-tester pressure.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- NASA Glenn: piston-cylinder volume ratio during compression, not fuel selection
- Limitation
- Read CR and total volume; do not interpret CR as a compression-tester pressure.
FAQ
What does a higher compression ratio buy?
The ratio describes compression geometry. It can affect engine thermodynamics, but actual power and fuel requirements also depend on filling, valve timing, ignition, temperature and design. The calculator does not select fuel.
Why does skimming the head raise it so much?
Reducing clearance makes the denominator smaller. At Vₛ = 454.17 cm³, changing V꜀ from 45 to 42 cm³ raises CR from 11.093 to 11.8136. The tool does not convert a machining depth into volume or check mechanical clearances.
How does geometric differ from effective compression?
The geometric ratio uses the piston’s extreme positions. Effective compression also depends on intake-valve closing and cylinder filling. A compression tester reports a pressure with its own unit, not the CR number.
What about forced induction?
Boost does not change the specified geometric volumes, but changes the charge conditions. The tool does not assess pressure, knock or a safe boost compression ratio; the particular engine’s data and limits are required.