What compression ratio means
Engine compression ratio compares the cylinder volume when the piston is at bottom dead center with the smaller volume remaining when the piston reaches top dead center. It is a key engine specification and depends on several physical dimensions.
Compression ratio formula
compression ratio = (swept volume + clearance volume) ÷ clearance volume
swept volume = π ÷ 4 × bore² × stroke
What counts as clearance volume?
Clearance volume can include the combustion chamber, piston dish or dome, head gasket volume, and deck clearance. A piston dish adds volume, while a dome reduces it. The signs matter, so enter the piston value accordingly.
Example
The calculator combines the cylinder swept volume with the entered chamber, piston, gasket, and deck volumes to produce the geometric compression ratio.
If you need to calculate compression ratio during an engine build, this page works as an engine compression ratio calculator using bore, stroke, chamber volume, piston volume, gasket dimensions, and deck clearance.
Frequently asked questions
Is this the same as dynamic compression ratio?
No. This is geometric or static compression ratio. Dynamic compression ratio also considers valve timing and the effective compression stroke.
What does a positive piston volume mean?
Enter a positive number for a piston dish that increases clearance volume. Enter a negative number for a dome that occupies clearance volume.
Can small measurement errors matter?
Yes. Bore, stroke, gasket thickness, chamber volume, and deck clearance all affect the result. Use measured values when accuracy matters.