Engine Compression Ratio Calculator
Calculate an engine's static compression ratio from cylinder bore, stroke, clearance volume, head gasket volume, and piston dome or dish volume.
Results
What is it?
Static compression ratio (CR) is the ratio of cylinder volume at bottom dead centre (BDC) to cylinder volume at top dead centre (TDC). Higher CR increases thermal efficiency and power, but demands higher-octane fuel to prevent detonation. CR = (swept volume + clearance volume) / clearance volume.
How to use
Enter bore and stroke in mm, then the measured combustion chamber volume (clearance volume) in cc. Add the head gasket compressed volume (calculate from bore area x gasket thickness). For piston shape: enter a positive number for a dished piston (adds clearance volume) or a negative number for a domed piston (reduces it).
Example scenario
An 86 mm bore x 86 mm stroke cylinder displaces 499.2 cc. With 40 cc combustion chamber, 8 cc head gasket, and flat-top piston (0 cc): total clearance = 48 cc. CR = (499.2 + 48) / 48 = 11.4:1 � appropriate for 91+ octane premium fuel in a naturally aspirated engine.
Pro tip
Turbocharged and supercharged engines run lower CR (8:1�9:1) because boost pressure multiplies the effective compression ratio at full throttle. A rough rule: each 1 psi of boost is equivalent to raising CR by ~0.3 points. Use this calculator to simulate the effect of milling the head (reducing clearance volume) or changing piston type.