Compression ratio compares the volume in a cylinder when the piston is at the bottom of its stroke with the volume when it is at the top. It affects power, efficiency and the fuel octane an engine needs, so it matters whenever you change heads, gaskets or pistons.
The formula
Compression ratio = (Swept volume + Clearance volume) ÷ Clearance volume
- Swept volume: the volume the piston moves through from bottom to top.
- Clearance volume: everything left above the piston at top dead centre: combustion chamber, head gasket, deck clearance and any piston dish (minus any dome).
Step 1: Swept volume
Swept volume = π ÷ 4 × Bore² × Stroke
With bore and stroke in centimetres, the answer is in cubic centimetres (cc). For a bore of 86 mm (8.6 cm) and a stroke of 86 mm: 0.7854 × 8.6² × 8.6 ≈ 499.6 cc per cylinder.
Try it with your numbersCompression Ratio CalculatorCalculate static engine compression ratio from bore, stroke, chamber, piston, gasket and deck volumes.Open calculatorStep 2: Clearance volume
| Part | How to get it | Example |
|---|---|---|
| Combustion chamber | Measured (cc’d) or from the head specification | 45.0 cc |
| Head gasket | π ÷ 4 × gasket bore² × compressed thickness | 0.7854 × 8.7² × 0.07 ≈ 4.2 cc |
| Deck clearance | π ÷ 4 × bore² × piston-to-deck distance | 0.7854 × 8.6² × 0.05 ≈ 2.9 cc |
| Piston dish (or dome) | From the piston specification (add a dish, subtract a dome) | 3.0 cc |
| Total clearance volume | ≈ 55.1 cc |
Step 3: Put it together
(499.6 + 55.1) ÷ 55.1 ≈ 10.07, written as 10.07:1.
What changes the ratio
- A thinner head gasket reduces clearance volume and raises compression.
- Milling (skimming) the head shrinks the chamber and raises compression.
- Dished pistons lower compression; domed pistons raise it.
- A larger bore or longer stroke increases swept volume and raises compression if clearance stays the same.
Static vs dynamic compression
The figure above is static compression ratio, based only on geometry. Dynamic compression also accounts for when the intake valve closes, which reduces the effective stroke. Two engines with the same static ratio can behave differently because of camshaft timing.
Measuring accurately
Small errors matter: 1 cc of clearance volume changes the example above by about 0.17 points. Measure the chamber with a burette, use the compressed gasket thickness and measure deck height at top dead centre.
Calculate yours
The compression ratio calculator handles the unit conversions and every clearance component. See also the RPM calculator and other daily life calculators.
Frequently asked questions
Many modern petrol engines run roughly 9:1 to 12:1; diesel engines are much higher. The right figure depends on design and fuel.
Yes. It reduces clearance volume, so the ratio goes up.
No. Compression ratio is a ratio of volumes. Cylinder pressure from a compression test depends on many other factors.
One cylinder. All cylinders share the same geometry, so the ratio is the same.
