Pressure unit converter with gauge and absolute

How the calculation works

1. 01Every unit is held as a number of pascals, so any pair converts through one factor and the factors cannot disagree with each other.
2. 02The conventional units use their exact conventional definitions: mmHg from a mercury density of 13 595.1 kg/m³ and standard gravity 9.80665 m/s², water columns at 9.80665 Pa per millimetre, 1 atm as exactly 101 325 Pa and 1 psi from the pound-force and the inch.
3. 03Absolute pressure is gauge pressure plus the atmospheric pressure at the time and place of the reading: P_abs = P_gauge + P_atm.

Limitations

• Standard atmosphere is a convention. At 1000 m the real atmosphere is about 11 % lower, and weather moves it by several percent, so use the local value for real work.
• Water and mercury columns depend on temperature and local gravity in practice. The conventional figures here assume the definitions above; a precision manometer needs its own corrections.
• It converts units. It does not tell you whether a reading is gauge or absolute: check the instrument.

Frequently asked questions

How many psi is one bar?

1 bar is 14.5038 psi, because 1 bar is exactly 100 000 Pa and 1 psi is 6894.757 Pa.

What is the difference between gauge and absolute pressure?

Gauge pressure is measured from the surrounding atmosphere, absolute from a perfect vacuum. A tyre reading 2.2 bar gauge is about 3.2 bar absolute. Mixing them up is the commonest error in vacuum and low-pressure work, and in setting up a pressure calibration.

Is mmHg the same as Torr?

Almost. 1 Torr is exactly 1/760 of a standard atmosphere, while the conventional mmHg is defined through the density of mercury. They differ by about 0.00001 %, which is below any practical gauge but is why the two are listed separately.

What does psig or psia mean?

psig is gauge pressure in psi and psia is absolute pressure in psi. If a datasheet gives only psi with no letter, find out which it is before you use the number.