🎯 Compression Ratio Calculator
Enter the swept volume and the clearance (combustion chamber) volume per cylinder to get the compression ratio.
Compression ratio
11:1
Compression ratio = (swept volume + clearance volume) ÷ clearance volume. The clearance volume is the total space above the piston at top dead centre — combustion chamber, head gasket, deck and piston-dish volumes combined. Higher ratios extract more power but need higher-octane fuel. 🔒 In your browser.
How the compression ratio calculator works
The compression ratio compares the cylinder’s volume with the piston at the bottom of its stroke to its volume at the top. It’s (swept volume + clearance volume) ÷ clearance volume. The clearance volume is the total space left above the piston at top dead centre — the combustion chamber, head-gasket, deck and any piston-dish volumes added together.
Higher compression extracts more energy from each combustion cycle, improving power and efficiency, but it needs higher-octane fuel to avoid knock. Shaving the head, thinner gaskets or dished/domed pistons all change the clearance volume and therefore the ratio.
Frequently asked questions
How do I calculate compression ratio?
Compression ratio = (swept volume + clearance volume) ÷ clearance volume, per cylinder. If the swept volume is 500 cc and the clearance volume is 50 cc, the ratio is (500 + 50) ÷ 50 = 11:1.
What is the clearance volume?
The total volume above the piston when it’s at top dead centre: the combustion chamber in the head, plus the head-gasket volume, the deck clearance and any dish or dome in the piston crown, added together.
Does higher compression need higher-octane fuel?
Usually yes. Higher compression raises cylinder pressures and temperatures, which can cause knock (detonation) on low-octane fuel. Higher-octane fuel resists knock, so high-compression and boosted engines need it.
What is a typical compression ratio?
Many naturally-aspirated petrol engines run around 10:1 to 12:1; forced-induction engines often run lower (8:1–10:1) to manage cylinder pressure; diesels are far higher, roughly 14:1–23:1.
How do I raise my engine’s compression?
Reduce the clearance volume — mill the cylinder head, fit a thinner head gasket, or use domed pistons. Each lowers the volume above the piston at top dead centre, raising the ratio. Recalculate to stay within safe limits for your fuel.