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Compression Ratio Calculator

Works out static compression ratio from the real geometry — bore, stroke, chamber, gasket, deck and piston — rather than from a nominal figure.

mm
mm
cc

Per cylinder, from the head. Measured by filling it with liquid from a burette if you want it exact.

mm
mm
mm

Piston crown below the block face at TDC. Negative if it protrudes.

cc

Volume of the dish in the crown. Enter a dome as negative. Valve reliefs count as dish.

Compression ratio
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Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.

As a number — : 1
Swept volume — cc
Clearance volume at TDC — cc
Reading this —

The formula

CR = (swept + clearance) ÷ clearance clearance = chamber + gasket volume + deck volume + piston dish − dome

Compression ratio is the cylinder volume at bottom dead centre over the volume at top dead centre. The TDC volume is small and made of four parts, each a few cc, which is why a thicker gasket or a shaved head changes the ratio more than intuition suggests.

What to watch out for

Where the numbers come from and why they matter:

  • Chamber volume is measured, not read off a spec sheet, if you have had the head skimmed. A burette and a perspex plate is the standard method.
  • Gasket is compressed thickness, not the loose figure on the packet.
  • Deck clearance adds volume if the piston sits below the block face, and subtracts if it protrudes.
  • Dish and dome are often stamped on the piston or in its spec; valve reliefs count as dish.

This is static compression. Dynamic compression — what the cylinder actually sees — depends on when the intake valve closes and is what really sets the knock limit.

Frequently Asked Questions

How do I calculate compression ratio?

Add up the clearance volume at TDC — chamber, gasket, deck and piston dish — then CR = (swept + clearance) ÷ clearance. An 86 × 86 bore and stroke with 45 cc chamber, 1 mm gasket, 0.5 mm deck and a 5 cc dish gives about 9.5:1.

What compression ratio for a turbo engine?

Typically 8.5–9.5:1 on older forced-induction engines; modern direct-injection turbos run 10–10.5:1 thanks to charge cooling. Higher static compression plus boost raises the knock risk, which is why boosted builds often lower it.

How much does skimming the head raise compression?

Each 0.1 mm off the head removes about 0.58 cc from an 86 mm bore — around 0.1 of a ratio point on a typical engine. A 1 mm skim can add a full point, which may need a thicker gasket to bring back.

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