What this calculator does
Engine compression ratio works out static compression ratio for an engine cylinder from bore, stroke and clearance volume. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaDisplacement Vd = π/4 × bore² × stroke; CR = (Vd + Vc) / Vc, where Vc is the clearance (combustion chamber) volume
The inputs explained
| Field | What to enter |
|---|---|
| Cylinder bore (in) | A number, measured in in. Starts at 4. |
| Piston stroke (in) | A number, measured in in. Starts at 3.48. |
| Clearance volume (combustion chamber + gasket + dish/dome, at TDC) (cc) | A number, measured in cc. Starts at 75. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with cylinder bore
Every other input is held at the calculator’s starting values while cylinder bore varies. Select any row to load that scenario into the calculator.