A rule of thumb used by mechanics multiplies the compression ratio by atmospheric pressure (14.7 psi at sea level) to estimate compression pressure. Real cranking-gauge readings differ because air heats as it is compressed (raising pressure) while valve timing, leakage and cranking speed lower it; most healthy petrol engines read 130–190 psi.
How to use the Compression Ratio to PSI Calculator
- Compression ratio X (X : Y)
- Compression ratio Y
- Atmospheric pressure
- Every result field is editable: type into Theoretical pressure (absolute) and Compression ratio X (X : Y) plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the compression ratio (for 10:1 enter 10 and 1) and the local atmospheric pressure. The calculator returns the theoretical absolute and gauge pressures.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Compression ratio X (X : Y) | A number |
| Compression ratio Y | A number |
| Atmospheric pressure | Units: psi, bar, kPa |
What each result means
- Theoretical pressure (absolute) = [Compression ratio X (X : Y)]÷[Compression ratio Y]×[Atmospheric pressure]
- Gauge pressure = [Theoretical pressure (absolute)] – [Atmospheric pressure]
Example values
How the main result changes when Compression ratio X changes and everything else stays at the example values above:
| Compression ratio X (X : Y) | Theoretical pressure (absolute) (psi) |
|---|---|
| 5 | 73.5 |
| 7.5 | 110.2 |
| 10 | 147 |
| 12.5 | 183.7 |
| 15 | 220.4 |
| 20 | 293.9 |
Formula
Absolute pressure ≈ compression ratio × atmospheric pressureGauge pressure = absolute − atmosphericWorked example
Inputs — Compression ratio X (X : Y): 10; Compression ratio Y: 1; Atmospheric pressure: 14.6959 psi.
Results — Theoretical pressure (absolute) = 147 psi; Gauge pressure = 132.3 psi.
Frequently asked questions
What compression psi is good?
Most petrol engines show 130–190 psi on a cranking test; what matters most is that all cylinders are within about 10% of each other.
Why does my gauge read more than CR × 14.7?
Compression heats the air, which raises pressure above the simple isothermal estimate.
Does altitude affect compression readings?
Yes – lower atmospheric pressure at altitude means lower readings.
What formula does the Compression Ratio to PSI Calculator use?
Absolute pressure ≈ compression ratio × atmospheric pressure.