Calculate gauge pressure from absolute pressure or fluid depth, and convert gauge pressure to absolute pressure using atmospheric pressure.
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Gauge Pressure Formula
The calculator uses one of three formulas depending on the mode you select.
Absolute to gauge:
Fluid height (hydrostatic):
Gauge to absolute:
- P_g — gauge pressure (Pa)
- P_abs — absolute pressure (Pa)
- P_atm — local atmospheric pressure (Pa)
- ρ — fluid density (kg/m³)
- g — gravitational acceleration, 9.80665 m/s² standard
- h — fluid column height or depth (m)
Gauge pressure is measured relative to local atmospheric pressure, not the standard sea-level value. A negative gauge reading means vacuum: pressure below ambient. The hydrostatic formula assumes an incompressible fluid and ignores any pressure applied to the surface above it. If a tank is sealed and pressurized, add that surface pressure to ρgh.
Reference Values
Atmospheric pressure drops with elevation. Use the value for your location, not sea level, when accuracy matters.
| Elevation | P_atm (kPa) | P_atm (psi) |
|---|---|---|
| Sea level | 101.325 | 14.696 |
| 500 m | 95.46 | 13.84 |
| 1,000 m | 89.88 | 13.04 |
| 1,500 m | 84.56 | 12.26 |
| 2,000 m | 79.50 | 11.53 |
Common fluid densities for the hydrostatic mode:
| Fluid | Density (kg/m³) | P_g per meter (kPa/m) |
|---|---|---|
| Fresh water | 1,000 | 9.81 |
| Seawater | 1,025 | 10.05 |
| Typical oil | 850 | 8.34 |
| Gasoline | 740 | 7.26 |
| Mercury | 13,595 | 133.32 |
Example and FAQ
Example: A car tire reads 32 psi on the gauge. The car is at sea level. Absolute pressure inside the tire is 32 + 14.696 = 46.7 psi.
Example: Pressure at the bottom of a 5 m fresh water tank open to air. P_g = 1000 × 9.80665 × 5 = 49,033 Pa, or about 49.0 kPa (7.11 psi).
Why does my tire pressure drop in winter? Cold air contracts. Gauge pressure falls roughly 1 psi for every 10 °F drop in temperature, even if no air leaks out.
Can gauge pressure be negative? Yes. Negative gauge pressure indicates vacuum. The lower limit is −P_atm, which corresponds to absolute zero pressure.
Does altitude affect a tire gauge reading? Yes, slightly. The same absolute pressure inside the tire reads higher on the gauge at high elevation because ambient pressure is lower.
When do I use absolute pressure instead? Use absolute pressure for gas law calculations (PV = nRT), thermodynamic properties, and anything involving vacuum systems. Use gauge pressure for tires, pipe systems, and most field instruments.
