Pressure Units Explained: Pa, kPa, MPa, Bar, Psi and More

By Wu Peng, Senior Instrumentation Engineer · 20+ years in process instrumentation · Last reviewed July 10, 2026

Pressure units fall into three groups. The SI group builds on the pascal: Pa, kPa, MPa and the meteorologists’ hPa. The metric-industrial group runs on bar and millibar, plus the legacy kgf/cm² still stamped on older equipment across Asia. The imperial group uses psi, with inches of water and inches of mercury for low pressure and vacuum. One instrument spec can cross all three in a single project, and mixing them up is not a rounding error: writing 30 without saying mbar or inH2O changes the span by a factor of 2.5.

This guide defines each unit, gives the exact NIST conversion factors in one table, works three conversions you will actually meet, and finishes with how units should appear on an instrument datasheet. If you want the arithmetic done for you, our absolute and gauge pressure calculator converts between four units as it computes.

Contents

The pascal: the SI unit of pressure

The pascal (Pa) is the SI unit of pressure: one newton of force spread over one square meter, 1 Pa = 1 N/m². It is a deliberately small unit. Atmospheric pressure is about 101,325 Pa, so day-to-day engineering numbers in plain pascals get long fast, and the SI prefixes keep them readable:

  • kPa (kilopascal, 1,000 Pa): pneumatics, HVAC, low-range process measurement. Our low pressure transducer line lives entirely in the kPa band.
  • MPa (megapascal, 1,000,000 Pa): hydraulics, pipeline ratings, high-pressure process. Chinese and Japanese datasheets state most ranges in MPa.
  • hPa (hectopascal, 100 Pa): weather reporting. Numerically identical to the millibar.
  • Plain Pa: cleanroom and duct differentials, where a full span may be only 60 to 500 Pa.

The prefix rule of thumb: pick the prefix that keeps your working numbers between roughly 1 and 1,000. A boiler at 1.6 MPa reads better than 1,600,000 Pa; a filter differential of 250 Pa reads better than 0.00025 MPa.

Bar and millibar

The bar is metric but not SI. It survives because it is convenient: 1 bar = 100,000 Pa = 100 kPa, which is about 1.3 percent below standard atmospheric pressure. Tell a European engineer a pump delivers 10 bar and the mental picture (about ten atmospheres) is immediate. Western European industry, and most process instrumentation sold there, specifies in bar.

The millibar (mbar, 100 Pa) covers low pressure and vacuum work, and meteorology uses it interchangeably with hPa. One care point: a range written “0-30” on an inquiry means 3 kPa if the unit is mbar and 7.5 kPa if someone meant inches of water. Always attach the unit to the number.

Psi: the imperial unit

Psi is pound-force per square inch: 1 psi = 6,894.757 Pa = 6.895 kPa. It is the working unit of North American industry, from compressed air (a shop line runs about 90 to 125 psi) to hydraulics (3,000 and 5,000 psi systems) to tire pressure.

On a datasheet, psi almost always carries a reference suffix. Psig is gauge pressure, measured relative to the atmosphere around the instrument. Psia is absolute pressure, measured from full vacuum. The two differ by local atmospheric pressure, about 14.7 psi at sea level, and confusing them mis-ranges an instrument by exactly that amount. The full distinction, with altitude effects, is covered by our absolute-gauge converter and the absolute pressure transmitter page.

Atm, kgf/cm², torr and the water column units

Standard atmosphere (atm). A defined reference, 1 atm = 101,325 Pa = 14.696 psi = 760 torr. Used in physics, chemistry and compressor ratings (“free air delivery at 1 atm”), rarely as an instrument scale.

kgf/cm² (kilogram-force per square centimeter, sometimes written kg/cm²). The pre-SI metric standard: 1 kgf/cm² = 98,066.5 Pa = 0.980665 bar. Older equipment across Asia still carries gauges scaled in it: Chinese plants call it simply “kg”, and legacy Japanese and Korean machines are marked the same way. It sits about 2 percent below the bar; treating them as equal builds a 2 percent error into every reading.

Torr and mmHg. Vacuum units, 1 torr = 133.3224 Pa, numerically the same as the conventional millimeter of mercury. Vacuum systems, freeze dryers and analytical equipment quote in torr; medical blood pressure quotes in mmHg.

Water column units: inH2O, mmH2O. The natural units of very low pressure, from the era of water-filled manometers: 1 inH2O (conventional) = 249.0889 Pa and 1 mmH2O = 9.80665 Pa. HVAC duct static, furnace draft, cleanroom differentials and filter monitoring all quote in inches of water (also written inWG or WC). One psi is about 27.7 inH2O, which is why a duct-range transmitter needs a genuinely low-range sensor rather than a re-scaled general purpose one.

Master conversion table

Factors are NIST conversion values; the Pa column is the anchor. To convert any unit A to unit B, multiply by A’s Pa value and divide by B’s.

Unit Symbol Value in Pa In bar In psi
Pascal Pa 1 0.00001 0.000145
Kilopascal kPa 1,000 0.01 0.14504
Megapascal MPa 1,000,000 10 145.04
Bar bar 100,000 1 14.504
Millibar mbar 100 0.001 0.014504
Pound per square inch psi 6,894.757 0.068948 1
Standard atmosphere atm 101,325 1.01325 14.696
Kilogram-force per cm² kgf/cm² 98,066.5 0.98067 14.223
Torr (= mmHg conv.) torr 133.3224 0.0013332 0.019337
Inch of water (conv.) inH2O 249.0889 0.0024909 0.036127
Millimeter of water mmH2O 9.80665 0.000098 0.0014223
Inch of mercury (conv.) inHg 3,386.389 0.033864 0.49115

Handy anchors worth memorizing: 1 bar = 100 kPa = 14.5 psi; 1 atm = 101.325 kPa = 14.7 psi = 760 torr; 1 psi = 6.895 kPa = 27.7 inH2O; 1 MPa = 10 bar = 145 psi.

Worked conversions

Example 1: 10 bar to psi. 10 bar × 100,000 = 1,000,000 Pa. Divide by 6,894.757 Pa per psi: 145.04 psi. A European pump curve at 10 bar and an American spec at 145 psi describe the same machine.

Example 2: is the range 30 mbar or 30 inH2O? 30 mbar = 3,000 Pa = 0.44 psi. 30 inH2O = 30 × 249.0889 = 7,472.7 Pa = 1.08 psi. Same digits, 2.5 times apart. On a low-range differential instrument this is the difference between the right span and one that clips at 40 percent of the real process swing. Convert both candidates to pascals and confirm with the process data before ordering.

Example 3: 1.6 MPa flange rating to psi. 1.6 MPa = 1,600,000 Pa; divide by 6,894.757: 232.1 psi. That is why the Chinese PN16-class rating (1.6 MPa) and the ASME Class 150 rating do not line up one-to-one; check the actual pressure-temperature tables rather than converting the class number.

The universal recipe never changes: multiply into pascals, divide out into the target unit. Doing every conversion through Pa removes chained rounding errors, and for spans that also involve a gauge or absolute reference the calculator on our tools page handles the reference shift and the unit in one step.

Which unit each region and industry uses

Where / what Usual unit Note
North America, general industry psi (psig / psia) Compressed air, hydraulics, process
Western Europe bar, mbar Instrument scales and pump curves
China, Japan, Australia kPa, MPa SI-first documentation; legacy kgf/cm² gauges persist
HVAC ducts, cleanrooms, filters Pa, inH2O Very low differentials
Meteorology hPa / mbar, inHg Barometric reporting
Vacuum systems torr, mbar, Pa Pumps, freeze dryers, analytics
Hydraulics MPa, bar, psi 3,000 / 5,000 psi classes in the US
Medical, physiology mmHg Blood pressure convention

Pressure magnitude ladder from 1 Pa to 100 MPa on a log scale, showing typical applications and the unit normally used at each level Pressure (log scale) 100 MPa Ultra-high pressure service: intensifiers, test rigs MPa, ksi 10 MPa Hydraulic systems, 10 to 35 MPa (1,450 to 5,000 psi) MPa, bar, psi 1 MPa Steam boilers (1.6 MPa), compressed air (0.7 to 0.9 MPa) MPa, bar, psig 101.325 kPa Standard atmosphere (1 atm, 14.7 psia, 760 torr) kPa, bar, atm 10 kPa Fan and blower discharge, water column of about 1 m kPa, mbar, inH2O 250 Pa HVAC duct static, filter differential (about 1 inH2O) Pa, inH2O 1 to 60 Pa Cleanroom room-to-room differentials Pa

Pressure units on instrument datasheets

A pressure range on a datasheet needs two things: a unit and a reference. “0-10 bar” is incomplete; “0-10 barg” (gauge) or “0-10 bara” (absolute) is a specification. Every transmitter in our pressure instruments range is ordered against gauge, absolute or differential reference, and the suffix is what tells the factory which sensor goes in the housing. If the distinction is new territory, start with the pressure transmitters overview or the differential pressure transmitter page.

Three habits prevent most unit trouble on inquiries:

  • State the unit with every number, including zero points and overpressure. “Range 0-30, overload 60” invites the mbar-versus-inH2O trap from Example 2.
  • Keep one unit per document. If the P&ID is in kPa, do not quote alarm setpoints in psi in the same package. Convert once, at the boundary.
  • Match the unit to the span. Ranges under about 1 kPa read best in Pa or inH2O; 1 to 1,000 kPa in kPa or bar; above 1 MPa in MPa. This is also how the display resolution of a transmitter stays meaningful.

Application example

Instrumentation buyer, United Kingdom. An inquiry for averaging pitot tube flow elements on 2 to 6 inch lines arrived with the differential range written as “0/30” and both mbar and inches of water in the paperwork. The two readings are not close: 30 mbar is 3.0 kPa, while 30 inH2O is 7.5 kPa, 2.5 times more. Before quoting we converted both candidates to pascals, put them against the expected flow differential, and confirmed the intended span with the customer. Five minutes of unit checking is cheaper than a mis-ranged DP transmitter on site.

For the reference suffixes on a psi reading, gauge versus absolute versus differential, see PSI vs PSIA vs PSIG.

The atmosphere itself, 1013.25 hPa at sea level, is the anchor for most of these conversions. Our barometric pressure guide lays out the units it is reported in and how it is measured.

FAQ

Is 1 pascal equal to 1 N/m2?

Yes, by definition. One pascal is one newton of force distributed over one square meter, 1 Pa = 1 N/m². In SI base units that is 1 kg/(m·s²).

What is 5 units of pressure?

Five you will meet constantly in industry: pascal (with kPa and MPa), bar, psi, standard atmosphere (atm) and torr or mmHg for vacuum. The water column units, inH2O and mmH2O, make a strong sixth in HVAC and low-pressure work.

What are 3 units for pressure?

The three working units of industrial measurement are the pascal with its prefixes (kPa, MPa) in SI documentation, the bar in European practice, and psi in North American practice. All three convert exactly: 1 bar = 100 kPa = 14.504 psi.

What are the 4 types of pressure measurement?

Gauge, absolute, differential and sealed gauge. Gauge measures relative to the surrounding atmosphere, absolute measures from full vacuum, differential measures between two ports, and sealed gauge measures against a fixed reference sealed at manufacture. The unit can be identical in all four; the reference suffix (psig, psia, barg) carries the meaning.

Request a quote

If you are specifying a pressure transmitter, send the range with its unit and reference (for example 0-16 barg or 0-250 Pa differential), the medium and temperature, and the output you need. We quote in your unit and mark the equivalent on the order paperwork so nothing shifts in translation. You can also reach our application engineers directly.

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