Absolute, Gauge and Vacuum Pressure Calculator
Convert between gauge, absolute and vacuum pressure. Atmospheric pressure can be entered directly or estimated from site altitude.
- Patm = 101.325 × (1 − 2.25577 × 10⁻⁵ × z)5.25588 = 94.776 kPa
- Pabs = Pgauge + Patm = 344.78 kPa a
- Vacuum = Patm − Pabs
Pressure Reference Formulas
Vacuum = Patm − Pabs
Patm = 101.325 × (1 − 2.25577 × 10⁻⁵ × z)5.25588 kPa
z is the altitude above sea level in metres (ISA standard atmosphere). Real barometric pressure also varies a few percent with weather, so use a barometer reading for precise work.
Worked example
At a site 560 m above sea level the standard atmosphere is about 94.8 kPa, so 2.5 barg is 3.448 bar a. A vacuum gauge reading 600 mmHg there means an absolute pressure of about 111 mmHg.
Frequently Asked Questions
What is the difference between gauge and absolute pressure?
Gauge pressure is measured relative to local atmosphere; absolute pressure is measured from a perfect vacuum.
How is vacuum expressed?
As the amount below atmosphere (vacuum gauge reading) or as absolute pressure. For deep vacuum, absolute units such as mbar a or torr are clearer.
Why does altitude matter?
Atmospheric pressure falls about 12 % per 1000 m, so the same gauge reading is a lower absolute pressure at altitude.
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