Compression Ratio Calculator
Calculate engine compression ratio from bore, stroke, and combustion chamber volume.
How to use this tool
- Enter the cylinder bore diameter and stroke length in millimeters.
- Enter the combustion chamber (clearance) volume in cc from the cylinder head specs.
- The calculator returns swept volume and compression ratio.
Calculate compression ratio to gauge engine efficiency and fuel octane requirements.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Swept volume (cc) = (π ÷ 4) × bore2 × stroke ÷ 1000
Compression Ratio = (Swept volume + Clearance volume) ÷ Clearance volume
Bore and stroke in mm; clearance volume in cc.
How it works
Compression ratio compares the maximum cylinder volume (piston at BDC) to the minimum volume (piston at TDC, the clearance volume). It is calculated by adding the piston's swept volume to the combustion chamber clearance volume, then dividing by that clearance volume alone. Higher compression ratios improve thermal efficiency but require higher octane fuel to prevent detonation; the formula does not account for dynamic effects such as valve timing.
Worked example
- Bore = 100 mm, Stroke = 100 mm, Clearance volume = 50 cc.
- Swept volume = (π ÷ 4) × 100² × 100 ÷ 1000 = 785.398 cc.
- Compression Ratio = (785.398 + 50) ÷ 50 = 835.398 ÷ 50 = 16.708.
Swept volume = 785.40 cc; Compression ratio = 16.71 : 1
Common mistakes to avoid
- Entering clearance volume in cubic inches instead of cubic centimeters -- the formula expects cc, so using the wrong unit inflates the ratio dramatically.
- Ignoring head gasket compressed thickness and bore when estimating clearance volume, which can shift the actual ratio by 0.3-0.5 points.
- Confusing static compression ratio with dynamic (effective) compression ratio, which accounts for intake valve closing timing and is always lower.
Key terms
- Clearance volume
- The volume remaining in the combustion chamber when the piston is at top dead centre (TDC), including the piston dish, head gasket, and chamber shape.
- Compression ratio
- The ratio of total cylinder volume at BDC to the clearance volume at TDC; a key determinant of engine efficiency and required fuel octane.
- Top dead centre (TDC)
- The piston position at the top of its stroke, where cylinder volume is at its minimum.
- Bottom dead centre (BDC)
- The piston position at the bottom of its stroke, where cylinder volume is at its maximum.
- Detonation
- Uncontrolled auto-ignition of the air-fuel mixture before the spark plug fires, caused by excessive heat or compression; higher octane fuel resists it.
Frequently asked questions
- What is compression ratio?
- Compression ratio is the ratio of the cylinder volume at bottom dead center to the volume at top dead center. Higher ratios increase thermal efficiency but require higher-octane fuel.
- What compression ratio is best?
- Most naturally aspirated gasoline engines run 9:1 to 12:1. High-performance engines reach 13:1+. Diesel engines typically use 14:1 to 25:1 for compression ignition.