Calculate absolute pressure from gauge and atmospheric pressure, or find altitude from pressure using ISA air model and sea-level inputs.
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Absolute Pressure Formula
Absolute pressure is the pressure measured against a perfect vacuum. The calculator uses one of three formulas depending on the mode you choose.
Gauge reading mode:
P_abs = P_gauge + P_atm
Gauge + altitude mode:
P_abs = P_gauge + P_atm(h)
where the local atmospheric pressure at altitude h (in meters, h ≤ 11,000 m) is estimated from the barometric formula:
P_atm(h) = 101.325 * (1 - 0.0065*h/288.15)^5.255877
Vacuum reading mode:
P_abs = P_atm - P_vacuum
- P_abs = absolute pressure (kPa, psia, bar(a), etc.)
- P_gauge = pressure read on a gauge referenced to local atmosphere
- P_atm = local atmospheric (barometric) pressure
- P_vacuum = vacuum reading entered as a positive value below atmosphere
- h = altitude above sea level
Assumptions: the gauge reads zero at local atmospheric pressure, the vacuum reading is the magnitude of the drop below atmosphere (always positive), and the altitude estimate uses the standard atmosphere model. If you do not enter a barometric value, the calculator uses 101.325 kPa for sea level.
The three modes map directly to the formulas above. Use Gauge reading when you have both a gauge reading and a known atmospheric pressure. Use Gauge + altitude when you do not have a barometer but know your elevation. Use Vacuum reading when your instrument reports how far below atmosphere you are, such as a vacuum gauge in inHg.
Reference Tables
Standard atmospheric pressure expressed in common units:
| Unit | Value at sea level |
|---|---|
| kPa | 101.325 |
| psi | 14.696 |
| bar | 1.01325 |
| atm | 1.0000 |
| mmHg | 760.0 |
| inHg | 29.92 |
Approximate atmospheric pressure at altitude (standard atmosphere):
| Altitude | kPa | psi | inHg |
|---|---|---|---|
| Sea level | 101.3 | 14.70 | 29.92 |
| 1,000 ft | 97.7 | 14.17 | 28.86 |
| 2,500 ft | 92.6 | 13.43 | 27.34 |
| 5,000 ft | 84.3 | 12.23 | 24.90 |
| 7,500 ft | 76.6 | 11.12 | 22.63 |
| 10,000 ft | 69.7 | 10.11 | 20.58 |
| 15,000 ft | 57.2 | 8.29 | 16.89 |
Worked Examples and FAQ
Example 1: Tire pressure. A tire gauge reads 32 psig at sea level. Absolute pressure is 32 + 14.696 = 46.7 psia.
Example 2: Process tank in Denver. A vessel reads 50 psig in Denver at about 5,280 ft. Local atmosphere is roughly 12.2 psi. Absolute pressure is 50 + 12.2 = 62.2 psia. Using sea level instead would overstate the absolute value by about 2.5 psi.
Example 3: Vacuum line. A vacuum gauge reads 25 inHg below atmosphere at sea level. Convert: 25 inHg × 3.3864 = 84.66 kPa. Absolute pressure is 101.325 - 84.66 = 16.7 kPa(a).
Why use absolute pressure? Gas law calculations (PV = nRT), compressor ratios, boiling point lookups, and vacuum work all require absolute pressure. Gauge values shift with weather and elevation, so they cannot be used directly in those equations.
Is absolute pressure ever negative? No. Absolute pressure is referenced to a perfect vacuum, so the lowest possible value is zero. If the calculator returns an error about negative absolute pressure, your vacuum reading exceeds the local atmosphere or your gauge sign is wrong.
psig vs psia. psig is gauge pressure, with zero at local atmosphere. psia is absolute, with zero at vacuum. At sea level, psia = psig + 14.696.
How accurate is the altitude estimate? The calculator uses the International Standard Atmosphere. Real atmospheric pressure varies with weather by a few percent. If precision matters, enter a measured barometric value instead.
