Pressure Converter
Convert pressure between pascals, hPa, kPa, MPa, bar, psi, atmospheres, torr, mmHg and inches of mercury.
Updated Formula covered by automated tests
Convert pressure between pascals, hectopascals, kilopascals, megapascals, bar, psi, standard atmospheres, torr, millimeters of mercury and inches of mercury. Few quantities have so many units in daily use, because each industry adopted whatever its instruments measured: tire gauges read psi or bar, weather maps print hectopascals or inches of mercury, blood-pressure cuffs show millimeters of mercury, and engineering datasheets give megapascals.
Typical reasons to convert are checking a car’s recommended tire pressure on a gauge marked in different units, reading a barometer bought abroad, matching a pressure washer or air compressor rating to a hose spec, or translating a vacuum pump’s rating in torr for a lab protocol written in pascals.
The formula
- Pa
- Pascal, one newton of force spread over one square meter; the SI unit
- psi
- Pound-force per square inch: the force of gravity on one pound mass, over one square inch
- atm
- Standard atmosphere, a reference value close to average sea-level air pressure
- mmHg
- Pressure that supports a 1 mm column of mercury under standard gravity, about 133.322 Pa
Pressure is force divided by area, so every pressure unit is some force unit over some area unit. The pascal is tiny (roughly the pressure of a sheet of paper lying on a table), which is why kilopascals, bar and megapascals are more common in practice. The bar was chosen as a round 100,000 Pa that is close to atmospheric pressure, and the hectopascal used by meteorologists is numerically identical to the older millibar.
The mercury units come from the barometer Evangelista Torricelli built in 1643, where air pressure holds up a column of mercury about 760 mm tall. The torr is defined as exactly 1/760 of a standard atmosphere, while the millimeter of mercury is defined from the density of mercury and standard gravity; the two differ by about one part in seven million, so they are interchangeable for medicine and weather but listed separately here for completeness.
Worked example
Marcus bought a used car in the US whose door sticker says 35 psi, but the digital gauge at a gas station in Germany shows bar and kPa.
- 1 Multiply by the pascals in one psi: 35 × 6,894.757 = 241,316.5 Pa.
- 2 Divide by 1,000 for kilopascals: 241,316.5 ÷ 1,000 = 241.3 kPa.
- 3 Divide by 100,000 for bar: 241,316.5 ÷ 100,000 = 2.41 bar.
- 4 For comparison with the atmosphere: 241,316.5 ÷ 101,325 = 2.38 atm.
Result: 35 psi is 2.41 bar or 241 kPa. Marcus inflates to 2.4 bar when the tires are cold, since that is the condition the sticker refers to.
Before you rely on the result
- The converter does not distinguish gauge from absolute pressure; it converts whichever kind you enter into the same kind.
- Pressure readings depend on temperature and altitude; tire pressures in particular should be compared cold, and barometric readings at elevation need correction to sea level.
- Negative values are not accepted, so vacuum readings below gauge zero should be converted as absolute pressures.
Frequently asked questions
What is the difference between psig and psia?
Gauge pressure (psig) is measured relative to the surrounding air, so a flat tire reads 0 even though the air inside is at about 14.7 psi absolute. Absolute pressure (psia) counts from a perfect vacuum. Add about 14.7 psi, or 1.013 bar, to a gauge reading at sea level to get absolute pressure. Tire, hose and compressor figures are almost always gauge pressure.
Why do weather reports show 1013 hPa or 29.92 inHg?
Both are the standard atmosphere, 101,325 Pa, written in the unit each country’s forecasters use. Stations correct their readings to sea level so maps can be compared; that is why the barometric pressure on a forecast is usually higher than what a home barometer reads at elevation unless it has been adjusted.
Why is blood pressure still measured in mmHg?
Early sphygmomanometers used a mercury column, and decades of clinical guidelines, thresholds and research are written in mmHg. Modern digital cuffs contain no mercury but keep the unit so readings stay comparable. A reading of 120 mmHg is about 16 kPa or 2.3 psi above atmospheric pressure.
Is a kilogram per square centimeter the same as a bar?
Close but not equal. One kgf/cm² (the "technical atmosphere" still printed on some older gauges) is 98,066.5 Pa, about 2 percent less than a bar. The converter does not list it, but you can enter the value as 0.980665 bar per kgf/cm².