How to use it
- Choose What do you know?: bore and stroke to find the displacement, or the displacement to find the stroke.
- Enter the Bore (cylinder diameter), mm and the Stroke, mm from the engine specification, or the Displacement, cc in the other option.
- Enter the Number of cylinders, and the Clearance volume per cylinder, cc if you know it.
- Read the Displacement, litres and the Compression ratio, and check each line of The working, step by step.
How the displacement is calculated
The displacement, or swept volume, is the volume the pistons sweep through the cylinders. For one cylinder it is the piston area times the stroke. The piston area is pi times the square of half the bore. The engine's displacement is that volume times the number of cylinders. The combustion chamber above the piston is not included.
Piston area = π × (bore ÷ 2)²
Swept volume per cylinder = piston area × stroke
Displacement = swept volume per cylinder × number of cylinders
1 cc = 1,000 mm³; 1 litre = 1,000 ccThe compression ratio
The compression ratio compares the largest volume of a cylinder, which is the swept volume plus the clearance volume, with the smallest, which is the clearance volume. The clearance volume is the space left in the cylinder at the end of the compression stroke.
Compression ratio = (Vd + Vc) ÷ Vc
Vd: swept volume of one cylinder
Vc: clearance volume of one cylinderWhat this calculator does not include
- The shape of the combustion chamber, the valves and the piston crown, which change the real compression.
- Power and torque: displacement is a size, not a performance figure.
- Rounding in the published bore and stroke, which can make the maker's figure differ slightly.
Worked example: 79.5 mm bore, 80.5 mm stroke, four cylinders
Piston area: π × (79.5 ÷ 2)² = 4,963.9 mm². Swept volume of one cylinder: 4,963.9 × 80.5 = 399,595 mm³ = 399.6 cc.
Displacement: 399.6 × 4 = 1,598.4 cc, which is 1.598 litres. With a clearance volume of 50 cc, the compression ratio is (399.6 + 50) ÷ 50 = 9.0 : 1.
| Result | |
|---|---|
| Displacement | 1,598 cc (1.598 L) |
| Swept volume per cylinder | 399.6 cc |
| Compression ratio | 9.0 : 1 |
Frequently asked questions
What is engine displacement?
It is the volume that all the pistons sweep through their cylinders in one stroke, measured in cubic centimetres (cc) or litres. A 1,600 cc engine is 1.6 litres. It does not include the volume above the piston at the top of its stroke.
Where do I find the bore and stroke of my engine?
The engine specification or the workshop manual lists them, usually in millimetres. If you know only the displacement, use the second option of the calculator to find the stroke for a given bore and cylinder count.
Does a bigger displacement always mean more power?
Not on its own. Power also depends on the design of the engine, its valves, the fuel and any turbocharger. Displacement is a measure of size, not of performance.
What does the compression ratio tell me?
It shows how much the gas in a cylinder is squeezed on its compression stroke. The value comes from the design and the fuel the engine was built for, so use the maker's figure for anything that matters.
Why is the clearance volume needed for the compression ratio?
Without the clearance volume the ratio cannot be found, because it is the smallest volume of the cylinder at the top of the stroke. Enter it from the engine data, or leave it empty to skip the ratio.
Are the results exact?
They are exact for the numbers you enter. Published bore and stroke figures are rounded, so the displacement the maker quotes can differ a little from the calculation.
Sources
- Wikipedia: Engine displacement (definition: the volume swept by the pistons, excluding the combustion chambers)
- Wikipedia: Compression ratio (CR = (Vd + Vc) ÷ Vc)
Figures and text last checked on Tuesday, November 10, 2026.