Compression Ratio to PSI Calculator
Convert an engine's static compression ratio (e.g. 10:1) into the theoretical cylinder pressure in psi, bar and kPa. Free, private, runs entirely in your browser.
Theoretical cylinder pressure (ideal cycle)
– psi
P = atmospheric pressure × compression ratio (absolute pressure). Theoretical value — a real compression test reads lower.
Reference chart: common ratios → theoretical psi
Sea level (14.7 psi), ideal cycle, recomputed by this page's own engine:
| Compression ratio | Theoretical psi | bar | kPa |
|---|---|---|---|
| 8:1 | 117.60 | 8.11 | 810.82 |
| 8.5:1 | 124.95 | 8.62 | 861.50 |
| 9:1 | 132.30 | 9.12 | 912.18 |
| 9.5:1 | 139.65 | 9.63 | 962.85 |
| 10:1 | 147.00 | 10.14 | 1013.53 |
| 10.5:1 | 154.35 | 10.64 | 1064.21 |
| 11:1 | 161.70 | 11.15 | 1114.88 |
| 12:1 | 176.40 | 12.16 | 1216.24 |
| 13:1 | 191.10 | 13.18 | 1317.59 |
Frequently asked questions
Why is this number higher than my workshop compression test?
This calculator uses the ideal Otto cycle: pressure = atmospheric pressure × compression ratio, with no losses. A real engine loses pressure through the intake stroke (the throttle restricts filling), valve overlap and cam timing, heat loss, ring leakage, cranking speed, and the gauge itself. A healthy gasoline engine often cranks at 130–200 psi — always below the theoretical value, and that is normal.
What is the formula?
P = atmospheric pressure × compression ratio. At sea level (14.7 psi), a 10:1 engine gives 14.7 × 10 = 147.0 psi theoretical. The page shows the same result in bar and kPa and adjusts atmospheric pressure for altitude.
Does altitude change the result?
Yes. The calculator multiplies by your local atmospheric pressure, which falls with altitude (about 11.5 psi at 2,000 m). Select your altitude to get the correct theoretical value for your location.
Can I use this to diagnose engine problems?
No — not alone. A workshop compression test measures relative health across cylinders: a big spread between cylinders signals trouble even when every number looks "fine" against a chart. This tool estimates the theoretical ceiling; actual diagnosis needs a gauge, a cranking engine, and mechanical judgment. If in doubt, ask a qualified mechanic.
Is this gauge or absolute pressure?
Absolute. The theoretical value is atmospheric pressure multiplied by the ratio, so it already includes the atmosphere. Workshop gauges read relative to ambient (psig), which is another reason readings differ.
Does this work for diesel engines?
The math works for any compression ratio (a 20:1 diesel gives about 294.0 psi theoretical), but diesels rely on much higher ratios and the ideal-cycle simplification is even rougher there. Treat it as an educational estimate only.
Related pressure tools: psi conversion, kPa to psi, bar to psi, tire pressure calculator.
Free tool by NRTechStudio