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Engine Compression Ratio Calculator

Find the compression ratio and swept volume of a cylinder.

How it is calculated

The tool works on a single cylinder. Swept volume is Vs = π ÷ 4 × bore² × stroke ÷ 1000, with bore and stroke in millimetres; dividing by 1000 turns cubic millimetres into cubic centimetres (cc). The compression ratio is CR = (Vs + Vc) ÷ Vc, where Vc is the clearance volume in cc: the space above the piston at top dead centre, including the combustion chamber, head gasket, piston dish or dome and deck gap. All three inputs must be above zero. The result is the geometric (static) ratio. It does not account for valve timing, boost or the cylinder count, so multiply the swept volume by the number of cylinders for the engine displacement.

Worked example

A cylinder has a bore of 86 mm, a stroke of 86 mm and a clearance volume of 52 cc. The piston area is π ÷ 4 × 86² ≈ 5,808.8 mm². Multiplied by the 86 mm stroke it gives about 499,557 mm³, so the swept volume is 499.56 cc. Add the clearance: 499.56 + 52 = 551.56. Divide by 52: 10.61 : 1.

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Common mistakes

  • Entering the clearance volume in cubic inches or litres. The field is in cc.
  • Entering bore and stroke in centimetres or inches. Both must be in millimetres.
  • Entering the whole engine displacement as swept volume or using the total clearance of all cylinders. Everything is per cylinder.
  • Forgetting the head gasket volume, the piston dish or dome and the deck clearance when working out the clearance volume, so that the ratio looks too high.
  • Confusing static with effective (dynamic) compression, which is lower because the intake valve closes after the piston starts upward.

About this calculator

This calculator finds an engine's compression ratio: CR = (swept volume + clearance volume) / clearance volume, where swept volume comes from the cylinder bore and stroke (V = π·(bore/2)²·stroke) and clearance volume is the space left above the piston at top dead center. Higher compression ratios extract more work per combustion cycle but demand higher-octane fuel to avoid knock.

Engine builders, automotive machinists, and performance tuners use it when planning a build — choosing pistons, decking the block, or adding head gaskets of a specific thickness — to hit a target compression ratio for a given fuel and application (naturally aspirated vs. forced induction).

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Comments (1)

  • Editorial team

    This is the static ratio from geometry, so use it to compare builds rather than predict detonation: cam timing, boost and operating temperature change the pressure in the cylinder. An 86 mm bore, 86 mm stroke and 50 cc clearance gives 10.99 : 1, with 499.56 cc swept. Measure the clearance volume with liquid if you can. What bore, stroke and chamber volumes are you comparing?

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