Pressure Units Explained: Pascal, Bar, PSI, Atmosphere and mmHg

Pressure is force divided by area, and the world measures it in at least eight units: pascals, kilopascals, hectopascals, millibars, bars, psi, atmospheres, torr, millimeters of mercury and inches of mercury. They all describe the same quantity and convert with fixed factors, most of them exact. This guide defines each one correctly, shows where you actually meet it, and covers the distinction that causes more real mistakes than any conversion factor: gauge pressure versus absolute pressure.

The units, defined

Pascal (Pa), kilopascal and hectopascal

The pascal is the SI unit: one newton per square meter. The 14th General Conference on Weights and Measures (CGPM) adopted the special name "pascal" for the newton per square meter in 1971, after Blaise Pascal. One pascal is a very small pressure (standard sea-level air is 101,325 of them), so in practice you see multiples:

• kPa (1,000 Pa): tire placards (NHTSA notes US placards give kPa alongside psi) and engineering specs.
• hPa (100 Pa): weather maps. One hectopascal equals one millibar exactly, so a reading of 1,013 mbar is also 1,013 hPa.
• MPa (1,000,000 Pa): material strength and hydraulics.

Bar and millibar

One bar is exactly 100,000 Pa (100 kPa). It was introduced, with the millibar, by the Norwegian meteorologist Vilhelm Bjerknes. The bar is not an SI unit, but it has been legally recognized in the European Union since 2004 and is common for tire pressures and, outside the US, for scuba cylinder gauges. Oceanographers use the decibar because one decibar of seawater pressure corresponds to almost exactly one meter of depth (0.995 m with a seawater density of 1,025 kg/m³).

Standard atmosphere (atm)

The standard atmosphere is defined as exactly 101,325 Pa. The 10th CGPM adopted it in 1954. It was originally the pressure of a 760 mm column of mercury at 0 °C; the 0 °C refers to the temperature of the mercury, not of the air, and today the definition does not depend on mercury at all. The real air pressure at sea level varies with the weather, so "1 atm" is a reference value, not a measurement. Note that chemistry's "standard pressure" is no longer 1 atm: in 1982 IUPAC recommended exactly 100 kPa (1 bar).

Torr and millimeter of mercury

The torr is exactly 1/760 of a standard atmosphere, about 133.3224 Pa. It is named after Evangelista Torricelli, who explained the mercury barometer in 1644, and it is still used in high-vacuum work. The conventional millimeter of mercury is defined separately as 133.322387415 Pa. The two differ by less than one part in seven million, so for any practical reading they are interchangeable. Medicine still reports blood pressure in mmHg.

Pound per square inch (psi) and inch of mercury

One psi is one pound-force on one square inch. Because the pound-force (4.4482216152605 N) and the inch (0.0254 m) are both exactly defined, so is the psi: 6,894.757293… Pa. Psi is used mainly in the United States, where tire placards and scuba gauges read in it. The inch of mercury (inHg, conventionally 3,386.389 Pa) appears in US weather reports and in aviation altimeter settings across North America, where 29.92 inHg is the standard setting.

Exact values in pascals

To convert between any two units, divide the first value by the second. The most common pairs, computed from the values above:

Gauge vs absolute pressure

Every pressure reading is measured from a zero, and there are two zeros in common use:

• Absolute pressure counts from a perfect vacuum. Weather reports, physics and chemistry use it. Sea-level air is about 1,013 hPa absolute.
• Gauge pressure counts from the air around the instrument. Tire gauges, compressor dials, scuba gauges and blood-pressure cuffs read gauge pressure. A gauge open to the air reads zero.

In psi the difference is marked with a letter: psig for gauge, psia for absolute. A car tire at 2.2 bar on the gauge (31.9 psig) is at 3.21 bar absolute (46.6 psia). Blood pressure works the same way: a systolic reading of 120 mmHg means 120 mmHg above the surrounding air, which Wikipedia's mmHg article puts at 880 mmHg absolute when the air is at 760 mmHg. For a practical walk-through of gauge readings on tires, see our bar to psi tire pressure guide.

Where you meet each unit

Note that "200 bar or 3,000 psi" is PADI's round-number pairing, not a conversion: 200 bar is 2,901 psi, and 3,000 psi is 206.8 bar.

Worked examples

1. A deep low on the weather map

A storm's central pressure is reported as 950 hPa. Convert it for a US reader and compare it with the standard atmosphere:

1. 950 hPa = 95,000 Pa = 95 kPa.
2. To atm: 95,000 ÷ 101,325 = 0.9376 atm, about 6% below standard.
3. To inHg: 95,000 ÷ 3,386.389 = 28.05 inHg.
4. To psi: 95,000 ÷ 6,894.757 = 13.78 psi (absolute).

That is low pressure, not a vacuum: the air is still pushing with 94% of its standard force.

2. A diver at 30 meters

Seawater (density about 1,025 kg/m³) adds pressure in proportion to depth: ρ × g × depth.

1. Water alone: 1,025 × 9.80665 × 30 = 301,554 Pa, which is 3.02 bar gauge (43.7 psig).
2. Add the atmosphere: 301,554 + 101,325 = 402,879 Pa = 4.03 bar absolute.
3. In other units: 402,879 ÷ 101,325 = 3.98 atm, and 402,879 ÷ 6,894.757 = 58.4 psia.

Divers round this to "4 bar at 30 m" (PADI uses +1 bar per 10 m). That is a bar figure: in atmospheres it is slightly under 4, so mixing the two units gives a wrong psi answer.

Converting with SamemaS

The pressure converter shows eight units at once: pascal, kilopascal, megapascal, bar, atmosphere, psi, torr and mmHg. Type into any field and the rest update in your browser. It has no hPa or mbar field yet, so enter a weather reading as kPa (950 hPa = 95 kPa) or as pascals. It has no gauge or absolute setting either: it converts the number you enter, so keep track of which zero your reading uses. For the force side of the definition (newtons and pound-force), use the force converter.

The short version

• 1 bar = 100,000 Pa exactly; 1 atm = 101,325 Pa exactly; 1 Torr = 1/760 atm.
• 1 psi = 6,894.757 Pa, so 1 bar = 14.5038 psi and 1 atm = 14.6959 psi.
• 1 hPa = 1 mbar. Standard sea-level pressure is 1,013.25 hPa or 29.92 inHg.
• Gauge pressure counts from the surrounding air; absolute counts from vacuum. Add about 1.013 bar (14.7 psi) to go from gauge to absolute at sea level.
• Bar and atm are close but 1.3% apart. Don't swap them in precise work.

Frequently asked questions

Sources

1. Standard atmosphere (unit) — Wikipedia
2. Bar (unit) — Wikipedia
3. Torr — Wikipedia
4. NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity — National Institute of Standards and Technology
5. Declaration of the 14th CGPM (1971): the pascal — BIPM
6. Bar or PSI? — PADI