What this calculator does
Gauge pressure and absolute pressure are measured from two different zero points. Gauge pressure (kPa g) is measured relative to the surrounding atmosphere, so a tyre reading 0 kPa g simply means it is at atmospheric pressure, not that it contains no air. Absolute pressure (kPa abs) is measured relative to a true vacuum, so it includes the atmosphere's own weight as part of the total.
Converting kpag to kpa absolute means adding atmospheric pressure to the gauge reading, since the gauge reading already has that atmosphere subtracted out. This is a genuinely different step from converting between two different pressure units at the same reference point, such as kPa to PSI, which is just a multiplication. Going from gauge to absolute in the same unit requires this addition, and going from absolute to gauge requires the reverse subtraction.
The formula
Add the atmospheric pressure to a gauge reading to get absolute pressure, or subtract it from an absolute reading to get gauge pressure. Standard atmospheric pressure at sea level is 101.325 kPa, but the true local value depends on altitude and weather, so it is left as an editable field rather than fixed.
| Term | Meaning |
|---|---|
| kPa g | Gauge pressure: pressure measured relative to the surrounding atmosphere. |
| kPa abs | Absolute pressure: pressure measured relative to a total vacuum, including the atmosphere itself. |
| Atmospheric pressure | The local ambient air pressure being used as the reference point, standard value 101.325 kPa at sea level. |
The inputs explained
| Field | What to enter |
|---|---|
| Convert | Choose whether the value you are entering is a gauge reading to convert to absolute, or an absolute reading to convert to gauge. |
| Pressure value | The pressure value to convert, in kPa. |
| Atmospheric pressure (kPa) | The atmospheric pressure to use as the reference point. Defaults to standard sea-level pressure, but lower at altitude and variable with weather. |
When to use it
Reading a pressure gauge on a tank or compressor
Most industrial and workshop gauges read in gauge pressure by design, since they are open to the atmosphere and zero themselves against it. Converting to absolute pressure is needed for gas law calculations, where the true total pressure matters.
Working through gas law problems
The ideal gas law and related equations require absolute pressure, not gauge pressure. A gauge reading taken from lab or industrial equipment needs this conversion before it can be plugged into PV = nRT or similar formulas.
Adjusting for altitude
Atmospheric pressure drops with elevation, so the same gauge reading corresponds to a different absolute pressure at altitude than at sea level. Editing the atmospheric pressure field lets the conversion reflect the actual local air pressure rather than assuming sea level.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does absolute pressure change across common gauge readings?
A range of gauge pressure readings converted to absolute, at standard atmospheric pressure.
| Gauge pressure | Absolute pressure |
|---|---|
| 0 kPa g | 101.325 kPa abs |
| 50 kPa g | 151.325 kPa abs |
| 100 kPa g | 201.325 kPa abs |
| 200 kPa g | 301.325 kPa abs |
| 300 kPa g | 401.325 kPa abs |
| 500 kPa g | 601.325 kPa abs |
How does the same gauge reading convert at different atmospheric pressures?
A fixed 200 kPa g reading converted to absolute pressure across a range of atmospheric pressures, from high altitude to standard sea level.
| Atmospheric pressure | Absolute pressure |
|---|---|
| 90 kPa | 290.000 kPa abs |
| 95 kPa | 295.000 kPa abs |
| 98 kPa | 298.000 kPa abs |
| 101.325 kPa | 301.325 kPa abs |
| 103 kPa | 303.000 kPa abs |
Questions
What does the "g" in kPag mean?
It marks the value as gauge pressure, measured relative to the surrounding atmosphere rather than a vacuum. Some fields write this as kPa(g) or kPag interchangeably, and absolute pressure is marked kPa(a), kPaa or kPa abs.
Why is atmospheric pressure not just a fixed 101.325 kPa?
That figure is the standard sea-level value, used as a default here, but real atmospheric pressure varies with altitude and weather. At higher elevation the air is thinner and atmospheric pressure is genuinely lower, which changes the conversion.
Is this the same as converting between kPa and PSI?
No. Converting between different pressure units, like kPa and PSI, at the same reference point is a straight multiplication by a fixed factor. Converting gauge to absolute in the same unit requires adding or subtracting atmospheric pressure instead, because the two scales start from different zero points.
Can gauge pressure be negative?
Yes. A gauge reading below atmospheric pressure, such as in a partial vacuum, shows as negative gauge pressure. Its absolute pressure is still positive as long as it is above a true vacuum, found the same way by adding atmospheric pressure to the negative gauge value.
For conversions between different pressure units at a fixed reference point, such as kPa to PSI or bar to atmospheres, see the pressure unit converter.