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This calculator converts between gauge pressure and absolute pressure in both directions. The math is one line: absolute pressure = gauge pressure + atmospheric pressure. Enter a value in kPa, bar, psi, MPa, mmHg or inHg, and the tool returns the result in four units at once. If your plant is not at sea level, type the altitude and the calculator estimates local atmospheric pressure from the standard atmosphere instead of assuming 101.325 kPa.
The distinction matters when you order instruments: a transmitter with a gauge reference reads zero at whatever the weather is doing that day, while an absolute reference is anchored to perfect vacuum. Below the tool you will find the formula, an altitude table, three worked examples and a short guide to choosing between an absolute pressure transmitter and a gauge pressure transmitter.
Calculator
Absolute / gauge pressure converter
kPa
Absolute pressure
− | − | − | −
The altitude estimate uses the ICAO standard atmosphere; real barometric pressure also swings about plus or minus 3 kPa with weather. Results are reference-shift math, not a calibration.
The formula
Every pressure reading is a difference against a reference. A gauge instrument uses the atmosphere around it as the reference; an absolute instrument uses a sealed vacuum cell. That gives two simple conversions:
- Pabs = Pgauge + Patm
- Pgauge = Pabs − Patm
At sea level, standard atmospheric pressure is 101.325 kPa, which is 1.01325 bar or 14.696 psi. The unit suffixes carry the reference: psig and barg are gauge, psia and bara are absolute. A vacuum shows up as a negative gauge pressure; the deepest possible vacuum at sea level is about −101.3 kPa gauge, which is 0 kPa absolute. An absolute pressure can never be negative.
Atmospheric pressure changes with altitude and weather
The conversion constant between gauge and absolute is only 101.325 kPa at sea level on a standard day. It falls with altitude:
| Altitude | Atmospheric pressure (ISA) | In psi |
|---|---|---|
| Sea level | 101.33 kPa | 14.70 psi |
| 500 m | 95.46 kPa | 13.85 psi |
| 1000 m | 89.87 kPa | 13.03 psi |
| 1500 m | 84.56 kPa | 12.26 psi |
| 2000 m | 79.50 kPa | 11.53 psi |
| 3000 m | 70.11 kPa | 10.17 psi |
The table uses the ICAO standard atmosphere, P = 101.325 × (1 − 2.25577 × 10-5 h)5.25588 kPa with h in meters, valid up to 11 km. On top of altitude, weather moves the barometer roughly plus or minus 3 kPa around the local mean. A gauge transmitter never notices either effect because its reference moves with the atmosphere; an absolute reading, or any conversion between the two, must use the real local value.
Worked examples
1. Compressed air header, 35 psig at sea level. Convert to kPa first: 35 × 6.8948 = 241.3 kPa gauge. Add atmosphere: 241.3 + 101.3 = 342.6 kPa absolute. In psi that is 35 + 14.7 = 49.7 psia. The compressor sees 49.7 psia even though the gauge on the receiver says 35.
2. An absolute range that sits below atmosphere. A transmitter ranged 0.1 to 10 bar absolute has a low end of 10 kPa absolute. As a gauge value that is 10 − 101.3 = −91.3 kPa, a fairly deep vacuum. You cannot cover that point with a 0 to 10 bar gauge instrument at all; the process spends part of its cycle below the gauge zero.
3. The same line at altitude. A 200 kPa gauge steam line at a plant 1600 m above sea level: the standard atmosphere there is about 83.5 kPa, so the absolute pressure is 283.5 kPa, not the 301.3 kPa you would compute with the sea-level constant. Using 101.325 kPa everywhere overstates the absolute value by 17.8 kPa, an error of 6.3 percent on this line.
Application example
Silicone fluid vapor, Germany. A distributor asked us to quote a pressure sensor for OMTS (a silicone heat-transfer fluid) vapor at 280 C media temperature, ranged 0.1 to 10 bar absolute. The absolute reference was not optional: the low end of that range is 91 kPa below atmosphere, so a gauge instrument would spend the bottom of the cycle reading a negative value that drifts with the weather. We quoted a high-temperature sensor with a true absolute cell, ranged exactly to the process, from one written specification.
Absolute or gauge: which transmitter to order
Order a gauge reference when the process result you care about is relative to the surroundings: filter DP alarms, pump discharge, tank blanketing, hydraulic circuits. Most plant measurements fall here, and our pressure transmitters line covers compound vacuum-to-gauge spans from −100 kPa up to 100 MPa with a gauge cell as the default build.
Order an absolute reference when the physics of the process is anchored to vacuum: distillation and evaporation under vacuum, condenser monitoring, vapor-pressure work like the silicone-fluid case above, and any measurement where a weather swing of 3 kPa would eat the accuracy budget. On a 50 kPa absolute condenser measurement, 3 kPa of barometric drift is a 6 percent error a gauge instrument cannot distinguish from the process; an absolute transmitter simply does not see it. One rule of thumb: if you ever find yourself wanting to correct a gauge reading with today’s barometer, the instrument should have been absolute.
Related guides and tools
To turn the converted pressure into a transmitter output current, use the 4-20 mA signal calculator. For ranging and selection help across gauge, absolute and differential builds, the pressure instruments overview walks through the full line.
For the full table of unit conversion factors and datasheet conventions, see our pressure units guide.
New to the suffixes? See PSI vs PSIA vs PSIG for what gauge, absolute, and differential mean before you convert.
If you want the theory behind these conversions, the absolute vs gauge pressure guide covers the two reference points and when each one is the right specification.
FAQ
How do you convert bar gauge to absolute pressure?
Add atmospheric pressure. At sea level: bara = barg + 1.01325. A line running at 1 bar gauge is therefore at about 2.013 bar absolute. At altitude, substitute the local atmospheric pressure; at 2000 m the constant drops to about 0.795 bar.
What is the relationship between absolute pressure and gauge pressure?
Absolute pressure equals gauge pressure plus atmospheric pressure. They describe the same physical pressure against different references: gauge is measured relative to the surrounding atmosphere, absolute relative to a perfect vacuum. Gauge zero moves with weather and altitude; absolute zero never moves.
What does an absolute pressure of 73 psi convert to in gauge pressure?
Subtract atmospheric pressure: 73 psia − 14.7 = 58.3 psig at sea level. On a mountain site at 2000 m, where the atmosphere is about 11.5 psi, the same 73 psia would read 61.5 psig.
What is 35 psi in absolute kPa?
If the 35 psi is a gauge reading: 35 × 6.8948 = 241.3 kPa gauge, plus 101.3 kPa atmosphere = 342.6 kPa absolute. If the 35 psi is already absolute, it converts directly to 241.3 kPa absolute.
Request a quote
Tell us the measuring range, the reference you need (gauge or absolute), the medium and its temperature, and the output signal. We will range the transmitter to the process and flag it if the numbers suggest the other reference would serve you better. You can also reach our application engineers directly.