Quick answer
Static compression ratio = (swept volume + clearance volume) ÷ clearance volume. An 86 mm bore and 86 mm stroke give 499.56 cc per cylinder; with a 45 cc combustion chamber the ratio is (499.56 + 45) ÷ 45 = 12.1:1.
Enter bore, stroke and the clearance volume per cylinder. The calculator returns the compression ratio and the swept volume.
Related tools: Engine Displacement Calculator: Displacement from bore, stroke and number of cylinders. Horsepower Calculator: Power from torque and engine speed. Carburetor Size Calculator: Carburetor airflow needed from displacement, peak rpm and volumetric efficiency.
How to use the Compression Ratio Calculator
- Enter bore (mm) (above 0).
- Enter stroke (mm) (above 0).
- Enter clearance volume per cylinder (cc) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Bore | A number, in mm | > 0 |
| Stroke | A number, in mm | > 0 |
| Clearance volume per cylinder | A number, in cc | > 0 |
Formula and method
Formula
CR = (swept volume + clearance volume) ÷ clearance volume
IEEE-754 double precision; shown to the decimals indicated.
Clearance volume includes the combustion chamber, head gasket, piston dish or dome and deck clearance.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Bore = 86; Stroke = 86; Clearance volume per cylinder = 45.
| Result | Value |
|---|---|
| Compression ratio | 12.1:1 |
| Swept volume per cylinder | 499.56 cc |
Example 2
Inputs: Bore = 101.6; Stroke = 88.9; Clearance volume per cylinder = 70.
| Result | Value |
|---|---|
| Compression ratio | 11.3:1 |
| Swept volume per cylinder | 720.74 cc |
Example 3
Inputs: Bore = 75; Stroke = 80; Clearance volume per cylinder = 30.
| Result | Value |
|---|---|
| Compression ratio | 12.78:1 |
| Swept volume per cylinder | 353.43 cc |
The first line is the headline result (compression ratio). Below it the compression Ratio Calculator lists swept volume per cylinder, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The arithmetic
Swept volume = π ÷ 4 × bore² × stroke = 0.7854 × 8.6² × 8.6 cm = 499.56 cc. Adding the 45 cc clearance volume gives the total cylinder volume with the piston at the bottom, 544.56 cc, and dividing by the 45 cc left at the top gives 12.1.
Static versus dynamic
This is the static ratio, based on geometry alone. The effective (dynamic) ratio is lower because the intake valve closes after the piston starts to rise. Fuel octane, cam timing and boost limit how high a ratio an engine tolerates, so use it as a design figure and check the engine's specification.
How the result changes with bore
Holding every other input at the first worked example, this table changes only bore. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Bore | Compression ratio | Swept volume per cylinder |
|---|---|---|
| 43 | 3.78:1 | 124.89 cc |
| 86 | 12.1:1 | 499.56 cc |
| 172 | 45.41:1 | 1,998.23 cc |
| 344 | 178.62:1 | 7,992.92 cc |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Bore: entering "0" shows "Bore must be greater than 0." beside that field and no result.
- Clearance volume per cylinder: entering "0" shows "Clearance volume per cylinder must be greater than 0." beside that field and no result.
Choosing between the Compression Ratio Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Engine Displacement Calculator](/automotive/engine-displacement-calculator/) | bore, stroke and cylinders | Displacement |
| [Horsepower Calculator](/automotive/horsepower-calculator/) | torque, torque unit and engine speed | Power |
| [Carburetor Size Calculator](/automotive/carburetor-size-calculator/) | displacement, peak engine speed and volumetric efficiency | Required airflow |
| [Ratio Calculator](/math/ratio-calculator/) | first number and second number | Simplified ratio |
| [K/D Ratio Calculator](/other/kd/) | kills and deaths | K/D ratio |
Background: vehicle sizing and speed
A taller tyre travels farther per revolution, so the speedometer reads low and the effective gear ratio becomes numerically lower; a shorter tyre does the opposite. True speed is roughly the speedometer reading times the new tyre diameter divided by the old one. Verify with GPS over a known distance before changing parts.
Assumptions and limitations
- Static, not dynamic, compression ratio
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for bore and clearance volume per cylinder to confirm this.
Frequently asked questions
What is clearance volume?
The space left above the piston at top dead centre, including the combustion chamber, head gasket and piston dish.
What compression ratio is typical?
Around 9:1 to 11:1 for many petrol engines and 14:1 to 22:1 for diesels.
Which units should I use?
Use millimetres for bore and stroke and cc for clearance volume, as in the example.
What does the Compression Ratio Calculator calculate?
Static compression ratio from bore, stroke and clearance volume. It returns Compression ratio and Swept volume per cylinder.
What formula does the Compression Ratio Calculator use?
CR = (swept volume + clearance volume) ÷ clearance volume. Clearance volume includes the combustion chamber, head gasket, piston dish or dome and deck clearance.
What do I need to enter in the compression Ratio Calculator?
You enter Bore (mm), Stroke (mm) and Clearance volume per cylinder (cc).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Static, not dynamic, compression ratio.
What is an example of a compression Ratio result?
With bore 86, stroke 86 and clearance volume per cylinder 45, the compression ratio is 12.1:1.
Related calculators
- Engine Displacement Calculator — Displacement from bore, stroke and number of cylinders.
- Horsepower Calculator — Power from torque and engine speed.
- Carburetor Size Calculator — Carburetor airflow needed from displacement, peak rpm and volumetric efficiency.
- Ratio Calculator — Simplify a ratio of two whole numbers.
- K/D Ratio Calculator — Kill/death ratio.
- Wheel Offset Calculator — Wheel offset from width and backspacing.
- Golf Altitude Adjustment Calculator — Estimate how far a golf shot carries at a different elevation, using the Titleist R&D rule of thumb.
Sources
- OpenStax, Prealgebra 2e (Rice University): fractions, decimals, percents, ratios and rounding
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international inch 25.4 mm, avoirdupois pound 0.45359237 kg)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.