WheelSignal

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Results printed from https://wheelsignal.com/compression-ratio-calculator. Nominal estimates; see assumptions below.

Compression ratio calculator

Calculate an engine’s static compression ratio from its geometry, and see how each volume contributes.

Static compression ratio

Cylinder
Unit: mm
Unit: mm
Volumes and clearances
Unit: cc
Unit: cc Dish/valve reliefs positive, dome negative
Unit: mm Positive = piston below deck
Unit: mm Blank = same as bore
Unit: mm
Shows the chamber cc needed

Unit choices are saved only if you tick the box, in this browser’s local storage. Measurements are never saved or sent anywhere. A shared link carries the inputs after the # in the address, which browsers don’t send to the server.

Results

Enter the engine dimensions and volumes, or press Try an example, then Calculate.

How to use the compression ratio calculator

  1. Enter bore and stroke and choose millimetres or inches. Deck clearance and gasket dimensions use the same unit.
  2. Enter the combustion chamber volume in cc, the piston’s dish (positive) or dome (negative) volume, and the deck clearance.
  3. Enter the head gasket’s bore and compressed thickness.
  4. Press Calculate. Optionally add a target ratio to see the chamber volume that would reach it.

Worked example

An 86 × 86 mm cylinder sweeps 499.6 cc. With a 45 cc chamber, a flat-top piston at zero deck height and a gasket of 87 mm bore and 1.0 mm thickness (5.94 cc), the clearance volume is 50.94 cc. The ratio is (499.6 + 50.94) ÷ 50.94 = 10.81:1.

Formulas used

swept volume = π ÷ 4 × bore² × stroke gasket volume = π ÷ 4 × gasket bore² × gasket thickness deck volume = π ÷ 4 × bore² × deck clearance clearance volume = chamber + gasket + deck + piston (dish +, dome −) compression ratio = (swept + clearance) ÷ clearance

This is the standard method, as set out for example by JE Pistons. Volumes in cc use 1 cc = 1,000 mm³.

Frequently asked questions

What is static compression ratio?

The total cylinder volume with the piston at the bottom of its stroke, divided by the volume left with the piston at the top. It’s fixed by the engine’s geometry. “Dynamic” compression, which depends on when the intake valve closes, is a different figure and isn’t calculated here.

Which sign do I use for piston volume?

A dish or valve reliefs add volume above the piston, so enter them as a positive number of cc. A dome takes volume away, so enter it as negative. A truly flat-top piston is 0.

What is deck clearance?

The distance from the top of the piston at top dead centre to the block’s deck surface. Positive means the piston sits below the deck (adding volume); negative means it sticks out above the deck (taking volume away).

Where do I get chamber volume?

From the cylinder head maker’s specification, or by measuring (“cc-ing”) the chamber with a burette and a flat plate. Published figures for modified or resurfaced heads may no longer be accurate.

What compression ratio should I run?

This calculator doesn’t recommend one. The right figure depends on fuel, cam timing, boost, ignition control and the engine maker’s or builder’s guidance.

Last updated .