Compression Ratio to PSI Calculator
Estimate the expected cranking compression pressure of an engine cylinder from its compression ratio.
What this calculator estimates
An engine’s static compression ratio (the volume swept by the piston compared to the volume left at top dead center) determines roughly how much pressure builds up in the cylinder during the compression stroke. This tool applies the widely used adiabatic approximation, Pressure = Atmospheric x (Ratio)^1.3, then applies a cranking efficiency factor to account for valve timing, ring seal, and cranking speed, since a starter motor never achieves the compression a running engine does.
How to use it
Enter your engine’s compression ratio (found in the manufacturer specs), local atmospheric pressure (14.7 psi at sea level), and an estimated cranking efficiency (80-90% is typical for a healthy engine). The result approximates what a compression tester would read at the spark plug hole while cranking.
This is an educational estimate only. Real cranking compression varies with engine wear, cam timing, altitude, battery/starter condition, and throttle position — always compare to your specific engine’s published compression test specification.