Pressure Unit Converter

How to Use the Pressure Unit Converter

1. 1. Enter the pressure. Type a number such as 101.325, 14.7 or 1e-3. A decimal point or comma both work.
2. 2. Choose the units. Pick the unit you have and the one you want. Swap units reverses them.
3. 3. Read the result. You get the converted value, the same in scientific notation, the exact conversion factor, and a note on whether it was rounded to the significant digits you chose.
4. 4. Compare. The table lists your pressure in every unit, so you can see a tyre rating in psi, bar and kPa together.

What Each Unit Is

The SI unit of pressure is the pascal, one newton of force per square metre; it is named for Blaise Pascal. Everyday pressures are large numbers of pascals, so the kilopascal and hectopascal are common. The bar is exactly 100 000 Pa, a little less than atmospheric pressure, and the millibar is the same as the hectopascal, which is why weather maps show 1013 hPa and 1013 mbar interchangeably. The standard atmosphere is defined as exactly 101 325 Pa, roughly the average sea-level pressure.

Mercury units come from barometers. The torr, named for Evangelista Torricelli, is defined as exactly 1/760 of a standard atmosphere, which is 133.3223684… Pa. The millimetre of mercury in conventional use is the pressure at the bottom of a 1 mm column of mercury of density 13 595.1 kg/m³ in standard gravity: 133.322387415 Pa, exactly. The two differ by about one part in seven million, so a blood pressure in mmHg and the same figure in Torr are the same to any practical precision, but the converter keeps them apart. Inches of mercury are 25.4 conventional millimetres, the unit used for aviation altimeter settings in the United States. Psi is a pound-force over a square inch, 6894.757293… Pa; the technical atmosphere is a kilogram-force per square centimetre, exactly 98 066.5 Pa.

Worked Examples

• 1 atm is 101.325 kPa, 1.01325 bar, 760 Torr and about 14.6959 psi.
• A tyre at 32 psi is about 220.63 kPa or 2.2063 bar.
• A blood pressure of 120/80 mmHg is about 16.0 kPa over 10.7 kPa.
• 760 mmHg is 101 325.0144 Pa — a hair above 1 atm, because mmHg and Torr are not quite the same unit.
• 1 bar is 0.98692 atm; IUPAC has used 1 bar (100 kPa) rather than 1 atm as standard pressure since 1982.

Accuracy and Limits

Every factor is a definition from the National Institute of Standards and Technology's Guide for the Use of the International System of Units (SP 811, 2008 edition), so conversions are exact and the converter rounds only when it writes the answer. Two caveats matter in practice. First, it converts units, not conditions: a gauge reading (psig, barg) is measured relative to the surrounding air and must be added to the local atmospheric pressure to give an absolute pressure. Second, millimetres of water and mercury columns depend on temperature and local gravity; only the conventional values are used here. For pressure as force over area, see the Pressure Calculator.

Frequently Asked Questions

How do I convert psi to kPa?

Multiply by 6.894757293. One psi is 6894.757293 Pa, so 14.7 psi is about 101.35 kPa and 32 psi is about 220.63 kPa. To convert kPa to psi, divide by 6.894757 or multiply by 0.1450377. The converter does it without rounding the factor.

What is the difference between bar and atm?

A bar is exactly 100 000 Pa, while a standard atmosphere is exactly 101 325 Pa, so 1 atm is 1.01325 bar and 1 bar is about 0.987 atm. The two are close enough to confuse in casual use but not in gas-law calculations, where mixing them gives a 1.3% error.

Are Torr and mmHg the same?

Almost. One Torr is exactly 1/760 of a standard atmosphere (133.3223684 Pa), while a conventional millimetre of mercury is 133.322387415 Pa. The difference is about 0.00001%, which is far below what any gauge or blood-pressure cuff can resolve, so the two are used interchangeably.

How do I convert mmHg to kPa?

Multiply by 0.133322387. For example, a blood pressure of 120 mmHg is 15.9987 kPa, and 760 mmHg is 101.325 kPa. Going the other way, one kilopascal is about 7.50062 mmHg.

What is gauge pressure?

Gauge pressure is the pressure above the surrounding atmosphere, which is what a tyre gauge shows. Absolute pressure is measured from a perfect vacuum, so absolute equals gauge plus atmospheric pressure. A tyre showing 32 psi gauge at sea level is about 46.7 psi absolute. This converter changes units only and does not add or remove the atmosphere.

What is standard atmospheric pressure?

The standard atmosphere is defined as exactly 101 325 Pa, which is 1013.25 hPa, 29.92 inHg or 760 Torr. It is a reference value approximating mean sea-level pressure, not the pressure on any given day, which varies by a few percent with the weather.