Compression Ratio Calculator (bore, stroke, chamber, gasket & piston volume)

Understanding the engine compression ratio calculator — concept and principle

The static compression ratio compares the cylinder volume with the piston at the bottom of its stroke to the volume left at the top. This calculator adds up the clearance volume from the combustion chamber, head gasket, deck clearance and piston dish or dome, and gives the ratio.

Everything left above the piston at top dead centre is the clearance volume; total ÷ clearance gives 9.92:1.

How it is calculated

Swept volume Vd = π ÷ 4 × bore² × stroke.
Clearance volume Vc = chamber volume + gasket volume (π ÷ 4 × gasket bore² × thickness) + deck volume (π ÷ 4 × bore² × deck clearance) + dish volume (a dome subtracts).
Compression ratio rc = (Vd + Vc) ÷ Vc.
With dimensions in cm, volumes are in cm³ (cc).

The principle

At bottom dead centre the cylinder holds the swept volume plus the clearance volume; at top dead centre only the clearance volume remains. The ratio of the two is how many times the charge is squeezed. Small changes in clearance volume change the ratio noticeably, because Vc is small. A thinner gasket or a domed piston raises the ratio; a dished piston or a thicker gasket lowers it.

Worked example

Default inputs: bore 86 mm, stroke 86 mm, chamber 45 cc, gasket 0.8 mm thick with 88 mm bore, deck clearance 0.2 mm, piston dish 5 cc.

Swept volume = π ÷ 4 × 8.6² × 8.6 ≈ 499.6 cc.
Gasket volume = π ÷ 4 × 8.8² × 0.08 ≈ 4.866 cc; deck volume = π ÷ 4 × 8.6² × 0.02 ≈ 1.162 cc.
Clearance volume = 45 + 4.866 + 1.162 + 5 ≈ 56.03 cc.
Compression ratio = (499.6 + 56.03) ÷ 56.03 ≈ 9.92:1.

Each point is the calculator re-run with only “Bore (mm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Bore (mm)	Compression ratio
21.5	1.57
43	3.26
64.5	6.06
86	9.92
108	14.77
129	20.55
172	34.58

How to read the result

Higher compression improves thermal efficiency and power but raises the risk of knock, so it must suit the fuel's octane rating, ignition timing and any boost. The static ratio does not account for when the intake valve closes; the dynamic compression ratio does (see the compression-ratio-to-psi calculator).

Common mistakes

Forgetting the head gasket or deck clearance volumes.
Entering a dome as a positive dish volume.
Using catalogue chamber volumes instead of measured ("cc'd") values.
Mixing millimetres and centimetres.

Keep in mind: Engine building requires accurate measurement and qualified professionals. Incorrect compression can cause severe engine damage.

Frequently asked questions

How do you calculate compression ratio?

(Swept volume + clearance volume) ÷ clearance volume.

How does a thinner head gasket change compression?

It reduces the clearance volume and raises the ratio.

What compression ratio is typical?

Many modern petrol engines run roughly 9:1 to 13:1; diesels run much higher.

Sources: Heywood, J. B., Internal Combustion Engine Fundamentals, 2nd ed. McGraw-Hill (2018) (engine geometry) · SAE J604, Engine Terminology and Nomenclature

Checked 9 Nov 2026. C. Shih, “Internal Combustion Engines” lecture notes, FAMU–FSU College of Engineering: compression ratio r = V_BDC/V_TDC, displacement = π/4 × B² × S. Breakdown of the clearance volume (chamber, gasket, deck, piston) is standard engine-building practice.

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