How to use it
- Enter the Bore (cylinder diameter), mm and the Stroke, mm from the engine data.
- Enter the Combustion chamber volume, cc from the head, then choose the Piston top and, for a dish or a dome, its Dish or dome volume, cc.
- Enter the Head gasket thickness, mm (compressed), the Head gasket bore, mm and the Deck clearance, mm, with the Spark plug and other volume, cc (optional) if you know it.
- Read the Static compression ratio, then check each line of The working, step by step.
How the static compression ratio is found
The static compression ratio compares the largest volume in a cylinder, the swept volume plus the chamber, with the smallest volume, which is the chamber alone. The swept volume comes from the bore and the stroke. The chamber is the volume in the head, plus what the head gasket, the deck and the piston top add.
Swept volume (cc) = π/4 × bore² × stroke ÷ 1000
Gasket volume (cc) = π/4 × gasket bore² × compressed thickness ÷ 1000
Deck volume (cc) = π/4 × bore² × deck clearance ÷ 1000
Chamber (cc) = chamber + gasket + deck ± piston + spark plug and other
Compression ratio = (swept + chamber) ÷ chamberWhat each part does to the ratio
- A thinner head gasket leaves less space in the chamber, so the ratio rises. On an 81 mm bore each 0.1 mm of compressed thickness changes the chamber by about 0.5 cc.
- A dish adds volume and lowers the ratio; a dome takes volume away and raises it.
- A piston below the deck adds volume to the chamber; a piston that stands above the deck takes volume away.
What this calculator does not include
- The dynamic ratio, which depends on when the inlet valve closes and on the camshaft timing.
- The shape of the chamber and of the valve pockets, unless they are part of the measured volume you enter.
- Gasket material outside the hole: only the area of the hole counts here.
Worked example: 81 mm bore, 77 mm stroke, 1.15 mm gasket
Swept volume: π/4 × 81² × 77 ÷ 1000 = 396.78 cc. Gasket: π/4 × 81² × 1.15 ÷ 1000 = 5.93 cc. With a 35.90 cc combustion chamber, a flat piston and no deck clearance, the chamber is 35.90 + 5.93 = 41.83 cc.
Compression ratio: (396.78 + 41.83) ÷ 41.83 = 10.5 : 1. A gasket 0.35 mm thinner (0.8 mm) takes 1.8 cc out of the chamber, which gives 40.02 cc and a ratio of 10.9 : 1.
| Result | |
|---|---|
| Swept volume of one cylinder | 396.8 cc |
| Chamber volume | 41.8 cc |
| Static compression ratio | 10.5 : 1 |
Frequently asked questions
What is the static compression ratio?
It is the cylinder volume at the bottom of the stroke divided by the volume at the top. A ratio of 10.5 : 1 means the gas is squeezed to about a tenth of its starting volume, and the figure comes from the geometry alone.
Where do I find the combustion chamber volume?
It is in the head data or the head maker's figures. It is usually measured by filling the chamber with liquid. A head that has been machined or fitted with a different gasket needs a new figure.
Does a thinner head gasket raise the compression ratio?
Yes. A thinner gasket leaves less space in the chamber, so the ratio rises. On an 81 mm bore each 0.1 mm of compressed thickness changes the chamber by about 0.5 cc.
Why does the deck clearance matter?
At the top of the stroke the piston top can sit below the block deck, which adds volume to the chamber, or above it, which takes volume away. That gap changes the ratio, so it is part of the calculation.
What is a good compression ratio?
Petrol cars are usually built between 8:1 and 12:1, and high ratios need high-octane fuel to resist knock. Diesel engines are usually between 14:1 and 23:1. Use the figure the maker gives for your engine.
Is the static ratio the same as the dynamic one?
No. The static ratio uses the full swept volume. The dynamic ratio also allows for when the inlet valve closes and for gas pushed back out, so it can differ. This calculator gives only the static ratio.
Does the calculator keep the numbers I type?
No. The calculation runs in your browser, and nothing you type is sent to a server or saved for later.
Sources
- Wikipedia: Compression ratio (CR = (Vd + Vc) ÷ Vc, Vd = π/4 × b² × s, typical petrol and diesel ratios)
- SQ Engineering: Calculated Compression Ratios (archived 2009; chamber with gasket, spark plug and dish or dome)
Figures and text last checked on Tuesday, November 10, 2026.