liminfoCompression Ratio Calculator
Free web tool: Compression Ratio Calculator
CR = (Swept Volume + Clearance Volume) / Clearance Volume. All dimensions in mm, volumes in cc.
Compression Ratio
9.60:1
Swept Volume
499.7 cc
Clearance Volume
58.1 cc
Gasket Volume
8.1 cc
About Compression Ratio Calculator
The Engine Compression Ratio Calculator computes the compression ratio (CR) of an internal combustion engine using the standard formula: CR = (Swept Volume + Clearance Volume) / Clearance Volume. You enter bore and stroke in millimeters, combustion chamber volume in cc, gasket thickness and bore, deck height, and piston dome or dish volume. The tool instantly calculates swept volume, gasket volume, effective clearance volume, and the final compression ratio displayed to two decimal places.
This calculator is used by engine builders, automotive engineers, motorsport tuners, and hobbyists working on engine modifications or rebuilds. Whether you are blueprinting a race engine, checking the effect of a thicker head gasket on CR, or verifying that a rebuilt motor meets spec, this tool gives you precise answers without requiring expensive software. The gasket volume calculation uses the actual gasket bore rather than the cylinder bore, giving more accurate results than simplified formulas.
All computation happens entirely in your browser using JavaScript — no data is transmitted to any server. The tool supports dark mode and is fully responsive for use on desktop, tablet, and mobile. Inputs include sensible real-world defaults based on a common 87.5 mm bore / 83.1 mm stroke engine configuration, so you can see example results immediately and edit values to match your engine.
Key Features
- Calculates compression ratio from bore, stroke, chamber volume, gasket, deck height, and piston dome
- Uses the precise formula CR = (Swept Volume + Clearance Volume) / Clearance Volume
- Computes gasket volume from actual gasket bore diameter, not cylinder bore
- Accounts for piston dome (positive) or dish (negative) volume in clearance volume
- Shows swept volume and clearance volume alongside the final CR for full transparency
- Real-time recalculation on every input change — no submit button needed
- 100% client-side processing — your engine data never leaves the browser
- Pre-loaded with realistic default values so you can explore results immediately
Frequently Asked Questions
What is compression ratio and why does it matter?
Compression ratio (CR) is the ratio of the total cylinder volume (piston at bottom dead center) to the clearance volume (piston at top dead center). Higher CR generally means more power and efficiency, but also requires higher octane fuel to prevent knock. Typical street engines run 9:1 to 11:1; performance and turbo engines vary widely.
What inputs does the calculator require?
You need bore (cylinder diameter in mm), stroke (piston travel in mm), combustion chamber volume (cc, measured or from spec sheet), gasket thickness (mm), gasket bore diameter (mm), deck height (gap between piston crown and block deck at TDC, in mm), and piston dome volume (positive for dome, negative for dish, in cc).
How is swept volume calculated?
Swept volume = (π/4) × bore² × stroke / 1000, converting mm³ to cc. This represents the volume displaced by the piston in one full stroke from BDC to TDC.
How is clearance volume calculated?
Clearance volume = chamber volume + gasket volume + deck volume − piston dome volume. Gasket volume = (π/4) × gasket bore² × gasket thickness / 1000. Deck volume = (π/4) × bore² × deck height / 1000.
What is deck height in this context?
Deck height here refers to the piston-to-deck clearance — the distance between the piston crown and the top of the engine block when the piston is at TDC. A positive deck height means the piston sits below the deck, adding clearance volume. Zero means the piston is flush with the deck.
How does the piston dome affect compression ratio?
A domed piston reduces clearance volume, raising CR. A dished piston increases clearance volume, lowering CR. Enter dome volume as a positive number and dish volume as a negative number (or simply enter a negative value for dish). The tool subtracts piston dome volume from the total clearance volume calculation.
What is a typical compression ratio for street vs. race engines?
Street gasoline engines typically run 9:1 to 11:1 CR. Performance naturally aspirated engines may go 12:1 to 14:1. Turbocharged engines often use lower CR (8:1 to 9.5:1) to handle boost pressure. Diesel engines run very high CR, typically 15:1 to 23:1.
Why does gasket bore matter for the calculation?
The gasket bore is usually slightly larger than the cylinder bore to allow for cylinder bore tolerances and to ensure the gasket does not restrict combustion. Using the gasket bore for the gasket volume calculation gives a more accurate result than using the cylinder bore, which would underestimate the volume of the compressed gases in the gasket space.