Pressure Converter

Pressure is one of the most frequently converted quantities in science and engineering because no single unit dominates every field. Meteorologists publish hectopascals, tyre gauges read psi, vacuum technologists swear by millitorr, and divers think in bar. The Pressure Converter on this page lets you enter a value in any supported unit and instantly see that same pressure expressed in every other supported unit at once, so you never have to chain conversions by hand.

All conversions run through a single canonical base, the pascal (Pa), the SI unit of pressure defined as one newton per square metre. Each unit is represented by an exact factor in pascals, and the displayed values are recomputed whenever you type, change the source unit, or adjust the significant-figure selector. The tool is purely client-side, so your values never leave your browser.

How to Use

1. Enter a numeric pressure value in the Pressure value field. The input accepts integers and decimals, including a leading minus sign for vacuum (sub-atmospheric) values.
2. Pick the source unit in the From unit dropdown, every supported unit is listed by abbreviation with its full name underneath.
3. The table below updates immediately and shows the same pressure converted into every other supported unit, with the source row highlighted.
4. Choose a precision level from the Significant figures dropdown (4, 6, or 8 sig figs) to control how the results are rounded. The canonical pascal value is also displayed so you can audit the intermediate step.
5. Use the preset row to load real-world examples, sea-level atmosphere, car and bicycle tyres, espresso pressure, a scuba tank, a typical systolic blood pressure, a near-vacuum, and the pressure at the bottom of the Mariana Trench.
6. If you type something that isn't a number, a red validation message appears and the table is hidden until the input is corrected.

The Unit Definitions

The pascal is the SI coherent unit. One pascal equals one newton of force spread over one square metre of area. Because the newton itself is defined in terms of kilograms, metres, and seconds, the pascal has a fully mechanical definition: 1 Pa = 1 kg·m⁻¹·s⁻². A pascal is a very small pressure, about the weight of a sheet of paper resting flat on your hand, so most practical pressures are quoted in kilopascals, megapascals, or in non-SI units.

The bar was introduced in 1909 as a convenient unit close to atmospheric pressure. By definition, 1 bar = 100,000 Pa exactly. The bar is not an SI unit, but it is accepted for limited use alongside SI. The closely related millibar (1 mbar = 100 Pa) is the traditional unit of atmospheric pressure in meteorology; modern forecasts use the equivalent hectopascal (1 hPa = 100 Pa).

The standard atmosphere (atm) is a legacy unit defined as exactly 101,325 Pa, the average sea-level pressure of the 1954 Tenth General Conference on Weights and Measures. The bar and the atmosphere are close (1 bar ≈ 0.987 atm) but they are not equal, and that ~1.3% gap matters when you mix sources.

The torr (and the identical mmHg) was historically defined as the pressure produced by a one-millimetre column of mercury under standard gravity. The modern definition makes 1 torr = 101,325 / 760 Pa exactly, matching the millimetre of mercury by construction.

The inch of mercury (inHg) is the corresponding imperial column-inch. The commonly used factor is 1 inHg = 3,386.389 Pa, derived from the same mercury density and standard gravity.

The psi (pound-force per square inch) is the everyday imperial unit. The exact factor used here is 1 psi = 6,894.757293168361 Pa, the value given in NIST SP 811.

The kgf/cm² (kilogram-force per square centimetre, also written kg/cm²) is a gravimetric metric unit still seen in older European pressure gauges and in some Asian specifications. 1 kgf/cm² = 98,066.5 Pa, the pressure produced by one kilogram of force acting through standard Earth gravity over a one-square-centimetre area.

An essential concept behind all of these is the difference between absolute and gauge pressure. Absolute pressure is measured from a perfect vacuum, zero Pa. Gauge pressure is measured from local atmospheric pressure; a tyre gauge that reads "32 psi" is showing the pressure above the ambient atmosphere, so the absolute pressure inside the tyre is roughly 32 + 14.7 ≈ 46.7 psi. This converter treats all inputs as absolute pressures; for gauge work, add or subtract the local atmospheric pressure explicitly before converting.

Worked Examples

The numbers below were produced from the same factors the tool uses.

• 1 atm → 101,325 Pa. Exactly, by definition of the standard atmosphere.
• 1 atm → 14.6959 psi (4 decimal places). Divide 101,325 Pa by 6,894.757293168361 Pa/psi.
• 1 bar → 750.062 torr (3 decimal places). Multiply 100,000 Pa by 760 / 101,325.
• 760 torr → 101,325 Pa. Exactly, because the torr is defined so that 760 of them equal one atmosphere.
• 32 psi → 2.20632 bar (5 decimal places). 32 × 6,894.757293168361 Pa = 220,632.23 Pa; divide by 100,000 Pa/bar.
• 120 mmHg (typical systolic blood pressure) → 15.9987 kPa. 120 × (101,325 / 760) = 15,998.68 Pa, then divide by 1,000.
• 9 bar (espresso machine line pressure) → 130.533 psi. 9 × 100,000 Pa = 900,000 Pa; divide by 6,894.757293168361.

Where It Shows Up

Tyres and road vehicles. Cars run roughly 30 to 35 psi (≈ 2.0 to 2.4 bar) of gauge pressure; bicycles run 60 to 130 psi depending on the tyre and rim; heavy trucks use kPa (typically 600 to 900 kPa). Workshop gauges and roadside inflators still mix psi and bar, so quick conversion matters.

Weather and atmospheric science. Surface pressure on weather maps is given in hectopascals (which equal millibars). The standard sea-level pressure is 1,013.25 hPa. Storm systems are described by their central pressure in the same unit.

Diving and breathing gases. Recreational scuba cylinders are filled to roughly 200 bar (≈ 2,900 psi). Partial pressures of oxygen and nitrogen are tracked in bar or atmospheres to avoid decompression sickness.

Hydraulics and machinery. Industrial hydraulic systems operate at 100 to 350 bar; cutting and forming presses at several hundred bar; water-jet cutters at 3,000 to 4,000 bar. Megapascal and bar are the everyday units.

Medicine and physiology. Arterial blood pressure is measured in mmHg because the sphygmomanometer descends from a mercury manometer. Intraocular pressure is in mmHg too, while ventilator pressures are in cmH₂O or mbar.

Vacuum technology. Rough vacuum is quoted in mbar or torr; high vacuum in millitorr or pascals; ultra-high vacuum in nanopascals. The unit choice changes between pump manufacturers, which makes a multi-unit table indispensable.

Common Mistakes

Confusing gauge and absolute pressure. A reading of "0 bar" on a tyre gauge is not zero pressure, it is one atmosphere. Two pressures that differ by exactly one atmosphere look identical on a gauge. Always state whether your input is absolute or gauge.

Treating bar and atm as interchangeable. They differ by about 1.3%. A 1 bar spec called out as "1 atm" is roughly 0.987 atm, fine for rough checks, wrong for metrology.

Using torr and mmHg loosely. Numerically they are the same in modern definitions, but "torr" usually implies an absolute pressure while "mmHg" sometimes appears in clinical contexts alongside cmH₂O and kPa. Don't add them to each other without converting first.

psi vs psig. "psig" is gauge psi above atmosphere; "psia" is absolute psi. A 30 psig reading is about 44.7 psia. This converter is absolute, adjust first.

Forcing SI prefixes. 1 kPa = 1,000 Pa, not 100 Pa. A common slip is treating "milli-" prefixes as decimal places without re-checking the exponent.

Mixing mass-based and force-based units. kgf/cm² is a force unit (gravity acting on a kilogram). It is close to bar but not equal. When in doubt, convert both into Pa and compare.

Frequently Asked Questions

What unit should I use for scientific work?

Use the pascal or one of its decimal multiples (kPa, MPa). SI units are required in peer-reviewed publications, legal metrology, and most engineering standards. Bar and atm are tolerated in industry but should be flagged as non-SI.

Why does the converter refuse my minus sign?

It doesn't, a leading minus sign is accepted and is the natural way to express gauge vacuum below ambient. The validator only blocks non-numeric characters and stray punctuation such as spaces or commas inside the digits.

Is the standard atmosphere the same as sea-level pressure everywhere?

No. The standard atmosphere is a defined value of 101,325 Pa, used for specification and conversion. Real sea-level pressure varies day to day between roughly 980 and 1,040 hPa depending on weather. Use the standard only when a fixed reference is required.

How accurate are the conversion factors?

The bar, atm, torr, mmHg, kgf/cm², kPa, MPa, and mbar factors are exact by definition. The psi and inHg factors are conventional values published by NIST in SP 811 and trace back to standard gravity and the density of mercury. Differences in the last digit of psi (in the 7th significant figure) appear in older references but are not meaningful for engineering work.

Can I use this for vacuum calculations?

Yes, but remember this tool returns absolute pressure. A vacuum gauge that reads "−760 torr" (i.e., 0 absolute torr) should be entered as 0 torr here; if your instrument gives a positive gauge reading, subtract it from local atmospheric pressure first to get absolute, then convert.

What is the difference between torr and mmHg?

Historically, 1 torr ≈ 0.999 999 857 6 mmHg, because they were defined from slightly different mercury densities. Modern definitions fix 1 torr = 101,325 / 760 Pa exactly, and the millimetre of mercury is defined the same way. The two are now numerically identical and interchangeable.

References

• BIPM, The International System of Units (SI) Brochure, 9th edition. For the Pressure Converter, Bureau International des Poids et Mesures, Sèvres.
• NIST, Guide for the Use of the International System of Units (SI), NIST Special Publication 811. U.S. Department of Commerce, For the Pressure Converter, National Institute of Standards and Technology.
• ISO 80000-4:2006, Quantities and units, Part 4: Mechanics. International Organization for Standardization, Geneva.
• IUPAC, Compendium of Chemical Terminology ("Gold Book"), entries on pressure and standard atmosphere.