Physics
Gauge, absolute and atmospheric pressure
Gauge pressure is measured relative to the surrounding air, so absolute pressure adds the atmosphere back on.
The Formula
P absolute = P gauge + P atmospheric
Gauge pressure is measured relative to the surrounding air, so absolute pressure adds the atmosphere back on.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| P abs | Absolute pressure | pascals | Measured from a perfect vacuum, so it is never negative. |
| P gauge | Gauge pressure | pascals | Measured relative to the surrounding atmosphere; it can be negative under vacuum. |
| P atm | Atmospheric pressure | pascals | About 101325 pascals at sea level under standard conditions. |
What Each Symbol Means
P abspascals
Absolute pressure
Measured from a perfect vacuum, so it is never negative.
P gaugepascals
Gauge pressure
Measured relative to the surrounding atmosphere; it can be negative under vacuum.
P atmpascals
Atmospheric pressure
About 101325 pascals at sea level under standard conditions.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate pressure page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate Pressure, filed under Science. Every number the guide produces comes from substituting into this one line.