What compression ratio is
The static compression ratio compares the volume in one cylinder when the piston is at the bottom of its stroke with the volume left when it is at the top. A ratio of 10:1 means the air-fuel mixture is squeezed into one tenth of its original volume before it is ignited. Higher compression extracts more energy from the fuel, up to the point where the mixture ignites on its own (knock or detonation), which damages engines.
The formula
Swept volume (Vs) = π ÷ 4 × bore² × stroke ÷ 1000
Clearance volume (Vc) = chamber + gasket + deck + dish (dome counts as negative)
Compression ratio = (Vs + Vc) ÷ Vc
All volumes are for a single cylinder, in cubic centimetres. The swept volume is the same as the per-cylinder figure from the engine displacement calculator.
- Gasket volume = π ÷ 4 × gasket bore² × compressed thickness ÷ 1000.
- Deck volume = π ÷ 4 × cylinder bore² × deck clearance ÷ 1000. If the piston sticks out above the deck, the clearance is negative.
- Dish or valve reliefs in the piston add volume; a dome takes volume away, so enter it as a negative number.
Worked example
One cylinder of an 86 × 86 mm engine, with a 45 cc chamber, a 1.0 mm gasket with an 86.5 mm bore, the piston 0.5 mm below the deck and a 5 cc dish:
- Swept volume: 0.7854 × 86² × 86 ÷ 1000 = 499.6 cc
- Gasket: 0.7854 × 86.5² × 1.0 ÷ 1000 = 5.88 cc
- Deck: 0.7854 × 86² × 0.5 ÷ 1000 = 2.90 cc
- Clearance: 45 + 5.88 + 2.90 + 5 = 58.78 cc
- Compression ratio: (499.6 + 58.78) ÷ 58.78 = 9.5 : 1
Replacing the 1.0 mm gasket with a 0.5 mm one removes about 2.9 cc and raises the ratio to roughly 10:1. Small changes in clearance volume have a big effect, which is why head skimming and gasket choice are done carefully.
Typical compression ratios
| Engine type | Typical static ratio |
|---|---|
| Older carburettor petrol engines | 8 : 1 – 9.5 : 1 |
| Modern port-injected petrol | 9.5 : 1 – 11 : 1 |
| Modern direct-injection petrol | 10.5 : 1 – 13 : 1 |
| Turbocharged petrol | 8.5 : 1 – 10.5 : 1 |
| Diesel | 14 : 1 – 22 : 1 |
The safe ratio depends on fuel octane, cooling, combustion-chamber design and ignition timing, so treat these as typical values, not limits.
Two-stroke engines
In a piston-ported two-stroke, compression only really starts once the piston closes the exhaust port. Tuners therefore also quote a trapped (or corrected) compression ratio, calculated from the swept volume above the exhaust port instead of the full stroke. It is always lower than the geometric figure.
Measuring chamber volume
Chamber volume is measured by "cc-ing" the head: a flat clear plate with a small hole is sealed over the chamber with grease, and a burette fills the chamber with a thin liquid until full. The volume used is the chamber volume. Manufacturers' figures are a good starting point, but machining and carbon deposits change the real value.
Static versus dynamic compression
The static ratio assumes compression starts at bottom dead centre. In reality the intake valve closes some degrees later, so the effective (dynamic) compression is lower and depends on camshaft timing. Engine builders use dynamic compression to judge how much octane an engine will need.
Common mistakes
- Entering a dome as positive. A dome reduces clearance volume; enter it as a negative number.
- Using the uncompressed gasket thickness. Use the compressed thickness quoted by the gasket maker.
- Forgetting the deck volume. Half a millimetre on an 86 mm bore is almost 3 cc.
Frequently asked questions
How do you calculate compression ratio?
Add the swept volume of one cylinder to the clearance volume (chamber + gasket + deck + dish) and divide by the clearance volume.
What is a good compression ratio for a petrol engine?
Most modern naturally aspirated petrol engines run about 10:1 to 13:1, turbocharged ones about 8.5:1 to 10.5:1. The right figure depends on fuel octane and engine design.
Does a thinner head gasket raise compression?
Yes. It reduces the clearance volume. On an 86 mm bore, every 0.5 mm of gasket is about 2.9 cc.
Is a piston dome positive or negative?
A dome takes up space in the chamber, so it reduces clearance volume and is entered as a negative volume. A dish adds volume and is positive.
Sources
- Heywood JB. Internal Combustion Engine Fundamentals, 2nd ed. McGraw-Hill, 2018 (compression ratio and engine geometry).
- Bell AG. Two-Stroke Performance Tuning. Haynes, 1999 (trapped compression ratio).
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.