Compression Ratio Calculator
Work out an engine's static compression ratio from its bore, stroke, chamber, gasket, deck and piston.
Compression ratio
10.25:1
- Swept volume per cylinder
- 1,016.1 cm³
- Clearance volume
- 109.9 cm³
The static compression ratio is 10.25 to 1.
Calculation breakdown
| Step | Formula | Result |
|---|---|---|
| Swept volume per cylinder | π ÷ 4 × 4.31² × 4.25 × 16.387064 | 1,016.10 cm³ |
| Clearance volume | 118 cc chamber + 9.854 cc gasket + 0 cc deck + -18 cc piston | 109.85 cm³ |
| Compression ratio | (1,016.1 + 109.85) ÷ 109.85 | 10.25 |
How it works
Enter the measurements for one cylinder.
Worked example
A big-block V8 with a 4.310 in bore, 4.250 in stroke, 118 cc chambers, a 4.375 in by 0.040 in gasket, an 18 cc dome and zero deck clearance:
- Swept volume: π ÷ 4 × 4.31² × 4.25 × 16.387 = 1,016.09 cc
- Gasket: π ÷ 4 × 4.375² × 0.040 × 16.387 = 9.85 cc
- Clearance: 118 + 9.85 − 18 = 109.85 cc
- Ratio: (1,016.09 + 109.85) ÷ 109.85 = 10.25:1
With 0.020 in of deck clearance the same engine drops to 9.86:1.
Sources
Frequently asked questions
How do you calculate engine compression ratio?
Add the swept volume of one cylinder to the clearance volume, then divide by the clearance volume. Swept volume is π ÷ 4 × bore² × stroke. Clearance volume is everything left above the piston at the top.
What goes into clearance volume?
The combustion chamber, the compressed head gasket, the gap between the piston and the deck, and the piston crown. A dish or valve relief adds volume, and a dome subtracts it.
What units do I use?
Enter bore, stroke, gasket sizes and deck clearance in inches, and the chamber and piston volumes in cc. The calculator converts inches to cubic centimeters with 1 cubic inch = 16.387064 cc.
What does deck clearance do?
It is the distance from the top of the piston to the top of the block at the top of the stroke. A piston 0.020 inch below the deck adds volume and lowers the ratio.
Is this static or dynamic compression?
This is static compression ratio, based on geometry alone. Dynamic compression also depends on camshaft timing and is lower than the static value.
For engine building, measure your own parts and confirm with your engine builder. Small errors in measurements change the ratio.