Compression Ratio Calculator
Find an engine cylinder's geometric static compression ratio. Use swept and clearance volumes from the same cylinder to obtain its ratio to one.
How the result changes
Only swept volume changes; all other inputs stay at your current values. The line illustrates the formula across the shown range. This is not a forecast of real-world outcomes.
About this calculation
Static compression ratio compares cylinder volume at bottom and top dead centre. This calculator uses swept displacement and clearance chamber volume, reporting a dimensionless ratio of the larger volume to the smaller. Calculate compression ratio when planning a trip or vehicle expense.
Formula
static compression ratio = (swept volume + clearance volume) ÷ clearance volume
Worked example
With Swept volume: 500 cm³, Clearance volume: 50 cm³, static compression ratio is 11 :1.
Inputs and units
Enter both volumes in cubic centimetres for one cylinder or for a consistently measured whole engine. Swept volume is the piston displacement; clearance volume includes the chamber and any relevant gasket, deck, and piston-dish space.
- Swept volume (cm³): Use the cylinder displacement from bottom to top dead centre.
- Clearance volume (cm³): Use the remaining chamber volume when the piston is at top dead centre.
Calculation and worked example
At bottom dead centre, volume equals swept plus clearance volume. At top dead centre, only clearance volume remains. Dividing the former by the latter gives a ratio conventionally written as a number to one In compact notation, static compression ratio = (swept volume + clearance volume) ÷ clearance volume.
With 500 cm³ swept volume and 50 cm³ clearance volume, bottom volume is 550 cm³. Dividing 550 by 50 gives a static compression ratio of 11:1.
How to use the answer
A larger ratio means a greater geometric volume reduction between piston positions. It does not directly give cylinder pressure, required fuel grade, engine power, or dynamic compression, all of which depend on additional operating conditions.
Check the effect of a proposed change in chamber volume by holding swept volume fixed and varying clearance. For a multicylinder engine, compare per-cylinder values rather than combining a single cylinder's chamber volume with total engine displacement.
Assumptions and checks
Volumes are measured accurately at mechanical dead centres, with no intake-valve timing or gas leakage modelled. This is a static geometric ratio rather than a pressure calculation.
Using swept volume divided by clearance alone would give 10:1 for the example, missing the clearance volume present at bottom dead centre. Zero or negative clearance volume is physically invalid and rejected.
If swept and clearance volumes are equal, the ratio should be 2:1. Doubling swept volume while holding clearance fixed increases the ratio by exactly swept/clearance.
Common questions
What does the static compression ratio result represent?
It is the bottom-dead-centre cylinder volume divided by the top-dead-centre clearance volume, expressed as a ratio to one.
What should I check before relying on this result?
Verify that the two volumes refer to the same cylinder and include gasket and piston geometry consistently. Do not equate this ratio with measured pressure.
Inputs
- Swept volume (cm³)
- Clearance volume (cm³)