Pressure Converter

What this converter answers

What a pressure is in another unit, how exact that answer is, and whether it is measured from the surrounding air or from a vacuum. Type one number and it appears at once in pascals, hectopascals, millibars, kilopascals, bars, megapascals, pounds per square inch, standard atmospheres, torr, millimeters of mercury and inches of mercury. Choose Gauge or Absolute and the page also adds or takes away the atmosphere, so a tire reading of 32 psi shows as 32 psig and as 46.7 psia.

How to use it

• Pressure: the number to convert, in the unit you pick under From. Commas and e-notation both work: 1,013.25, 2.5e6. A negative number only makes sense as a gauge reading below the surrounding air.
• From and To: the unit your gauge or document uses and the unit you want it in. Hectopascals and millibars sit next to each other because they are the same size.
• Gauge or absolute: leave Units only for a plain conversion. Choose Gauge for tire, pump, compressor and most dial gauges, which read 0 in open air. Choose Absolute for a pressure measured from a vacuum, such as a barometer reading or a figure given in psia.
• Atmosphere (with Gauge or Absolute): the air pressure around the gauge. Standard atmosphere uses 101.325 kPa, the sea-level reference. Estimate from altitude takes your height above sea level. Enter the air pressure takes a barometer reading where you are.
• Significant figures: Auto shows up to 12. Choose 3 to 12 for a shorter answer; only the display changes.

Under the answer, Swap turns the conversion around, and each unit name in the table makes that unit the main result. The four Try buttons load a weather report, a blood-pressure reading, a vacuum gauge and a tire gauge at 5,280 ft, each explained below.

How to convert psi to bar (and bar to psi)

Multiply psi by 0.0689475729317 to get bar, or multiply bar by 14.503773773 to get psi. Both factors follow from exact definitions: a bar is 100,000 Pa and a psi is about 6,894.757 Pa, so a bar is a little more than 14.5 psi. The same readings in kilopascals are 100 times the bar value.

Values are rounded to 4 significant figures, the same as choosing 4 under Significant figures.

Pressure conversion formula

Every unit has a fixed size in pascals, $f$ (the pascal is one newton per square meter). To convert a pressure $p$ from one unit to another:

• $p_{\text{from}}$ is the pressure you have and $p_{\text{to}}$ the pressure in the new unit.
• $f_{\text{from}}$ and $f_{\text{to}}$ are one of each unit in pascals: 100,000 for a bar, 101,325 for a standard atmosphere, about 6,894.757 for a psi.

Gauge and absolute pressure differ by the air pressure around the gauge, $p_{\text{atm}}$, in the same unit:

Worked example: a 32 psi tire gauge reading

A tire gauge reads 32 psi. Here it is in bar, and as the absolute pressure inside the tire at sea level:

1. Pounds per square inch to pascals. A psi is one pound-force (4.4482216152605 N) on one square inch (0.00064516 m²), so 32 × 4.4482216152605 ÷ 0.00064516 = 220,632.233381 Pa.
2. Pascals to bars: 220,632.233381 ÷ 100,000 = 2.20632233381, shown as 2.206 barg with 4 significant figures. The same reading is 220.6 kPa.
3. Add the standard atmosphere: 220,632.233381 Pa + 101,325 Pa = 321,957.233381 Pa absolute.
4. In bars that is 3.21957233381, shown as 3.22 bara. In psi it is 46.6959487755, shown as 46.7 psia: the 32 psi from the gauge plus 14.6959487755 psi of air.

The table under the result gives the same gauge reading as 2,206 hPa, 0.2206 MPa, 2.177 atm, 1,655 Torr and 65.15 inHg. At 5,280 ft (the Tire at 5,280 ft button) the air around the tire is about 83,418 Pa, so the same gauge reading is 44.1 psia, or 3.041 bara.

Gauge vs absolute pressure (psig and psia)

Gauge pressure is measured from the air around the gauge, and absolute pressure from a perfect vacuum, so absolute pressure = gauge pressure + atmospheric pressure. A tire gauge reads zero on a flat tire even though the air inside still presses at about 101 kPa: its 32 psi means 32 psi more than the air outside. A letter makes the kind explicit: psig and barg for gauge, psia and bara for absolute.

• Gauge pressure can be negative. A vacuum gauge showing 20 inHg of vacuum reads −20 inHg gauge. At the standard atmosphere that is about 9.92 inHg absolute, or 33.6 kPa.
• Absolute pressure can’t be negative. Zero is a perfect vacuum, so the converter refuses a negative absolute pressure, and a gauge reading lower than minus the atmosphere.
• The atmosphere changes. It falls with altitude and rises and falls with the weather. For the altitude part, the page uses the National Weather Service station-pressure formula with 101.325 kPa at sea level:

A gauge pressure converts to another unit the same way whatever the atmosphere, because the atmosphere cancels out. It matters only when you need the absolute pressure itself: for vacuum work, or to compare a gauge reading with a pressure given in psia.

Tire pressure in psi, bar and kPa

Tire gauges read gauge pressure, so the pressure a tire placard asks for is a gauge pressure, and changing its unit needs no atmosphere. Some placards and pumps give bar:

A 2.5 bar placard is 36.26 psig, which is 50.96 psia at the standard atmosphere. Under any result, the Nearby values table works as a ready-made tire chart: 2.5 bar comes with every value from 1.9 to 3.1 bar in steps of 0.1 bar, in psi and as absolute pressure. This page changes units only; how tire pressure changes with temperature or load is a different calculation.

Barometric pressure: hPa, millibars and inches of mercury

A hectopascal and a millibar are the same size, 100 Pa, so a weather map in either unit shows the same number. The standard sea-level pressure, 1,013.25 hPa, is 29.92 inHg.

The pressure in a weather report is corrected to sea level. Above sea level the air around you is at a lower pressure, the station pressure, and that is the value to type under Enter the air pressure when you convert between gauge and absolute. The Weather (hPa) button shows the conversion step: 101,325 Pa ÷ 3,386.388640341 = 29.9212555797 inHg.

Blood pressure units: mmHg and kPa

Blood pressure is given in millimeters of mercury, and some references use kilopascals. Multiply mmHg by 0.133322387415 to get kPa: 120 mmHg is 15.9986864898 kPa (about 16 kPa) and 80 mmHg is 10.6657909932 kPa (about 10.67 kPa). These are exact because the converter uses the full conventional millimeter of mercury, 133.322387415 Pa, which NIST lists as 133.3224 Pa. Like a tire reading, a blood-pressure reading is measured relative to the surrounding air. The converter changes the unit only and says nothing about what a reading means for health.

Torr vs mmHg

Nearly the same, but not identical. A torr is exactly 1/760 of a standard atmosphere, about 133.322368421 Pa, while the conventional millimeter of mercury is 133.322387415 Pa. They differ by about 0.14 parts per million, so 760 Torr is exactly 1 atm and 1 atm is 759.999891726 mmHg. For any gauge you can read, the two are interchangeable; the converter keeps them apart so that each conversion stays exact.

kPa, MPa and pascals

The pascal is a small pressure: the standard atmosphere is 101,325 Pa. Everyday pressures are therefore given in kilopascals (1 kPa = 1,000 Pa), and hydraulic, pipe and material ratings in megapascals: 1 MPa = 1,000,000 Pa = 10 bar, which is about 145.03773773 psi.

Exact conversion factors

So the standard atmosphere is 101,325 Pa = 1,013.25 hPa = 101.325 kPa = 1.01325 bar = 14.6959487755 psi = 760 Torr = 759.999891726 mmHg = 29.9212555797 inHg. Conversions between pascal units, bars and atmospheres come out exact; anything involving psi or torr usually doesn’t, because their pascal values are fractions whose decimals never end. The converter marks every value Exact or Rounded for that reason.

Reading the result

• The headline is the pressure in the To unit, followed by the equation and whether it is exact or how it was rounded. With Gauge or Absolute chosen, every label carries the kind: psig, bara, kPa gauge.
• The factor line gives psi in bar and bar in psi for the example: 1 psi ≈ 0.0689475729317 bar and 1 bar ≈ 14.503773773 psi. Both are rounded, hence the ≈; a defined factor, such as bar to pascals, reads “exactly”.
• The tiles (with Gauge or Absolute) show the other kind of pressure in both units, the atmosphere used and where it came from, and the reading as entered.
• How this was calculated shows the steps through pascals with your numbers, including the atmosphere.
• The conversion table lists every unit, each marked Exact or Rounded. Download CSV and Copy table take the whole table.
• Nearby values converts round numbers around yours, in steps of up to a twenty-fifth of your value, and adds the absolute or gauge column when you chose Gauge or Absolute. Its Download CSV gives a printable chart.
• Save for comparison keeps up to three conversions side by side, with the change in pascals.

Common pressure conversion mistakes

• Treating a gauge reading as absolute. 32 psi in atmospheres is 2.177 atm, but the air in the tire is at 3.177 atm absolute, because the gauge leaves out the atmosphere. The page says so when you convert into atmospheres with Units only.
• Taking 1 bar as 1 atmosphere. A standard atmosphere is 1.01325 bar, 1.325% more, and 1 bar is 0.986923266716 atm.
• Rounding the factor too far. 32 psi × 0.07 gives 2.24 bar, 1.53% high; the full factor gives 2.206 bar.
• Using the weather report’s pressure as the air around you. It is corrected to sea level. At 5,280 ft the estimate is 83.42 kPa, not 101.3 kPa, which changes an absolute pressure by almost 18 kPa.
• Reading a vacuum gauge’s number as absolute. “20 inHg” on a vacuum gauge is 20 inHg below the air, about 9.92 inHg absolute at sea level.
• Taking kgf/cm² as bar. A kilogram-force per square centimeter is 0.980665 bar, about 2% less.

Assumptions and limitations

• The factors are the exact definitions above; results are rounded only for display. A converted value is still no more precise than the gauge: a tire gauge marked in whole psi gives 32 psi give or take 0.5 psi, about ±0.03 bar, whatever digits the conversion shows.
• Millimeters and inches of mercury are the conventional units, not mercury at a stated temperature. Water columns, kgf/cm² and ksi aren’t in the list; the questions below give their pascal values.
• The standard atmosphere is a reference, not the air pressure where you are. The altitude estimate describes an average atmosphere and is used from −500 m to 11,000 m (about −1,640 ft to 36,089 ft); on a given day the weather moves the real value.
• Pressures go from −10¹⁵ to 10¹⁵ in the unit typed. An absolute pressure must be 0 or more and a gauge reading no lower than minus the atmosphere; the page explains which limit a number breaks instead of converting an impossible pressure.