Pressure Converter
Convert between pascals, bar, psi, atmospheres, torr and inches of mercury with exact definitions.
How to use this calculator
- 1Enter the value you want to convert.
- 2Pick the unit you are converting from and the one you want.
- 3The table below shows the same value in every other supported unit.
How the calculation works
result = (value × factor(from) ) / factor(to)- factor
- How many pascals one unit represents
Every value is converted to pascals first, then out to the target unit. This means n units need n factors rather than n² conversion pairs.
The pascal (one newton per square metre) is the SI unit, and every factor here scales from it. Bar and atmosphere are both defined exactly in pascals, which is why conversions between them never drift.
Torr and millimetre of mercury are very slightly different units, despite being used interchangeably: the Torr is defined as exactly 1/760 of a standard atmosphere, while the mmHg is defined from the density of mercury. They differ by about one part in seven million — far below the precision of any practical gauge, but this converter keeps them separate rather than pretending they are identical.
Worked example
32 pound per square inch to bar
- 1.One pound per square inch is 6,894.7573 pascals, so 32 psi is 220,632.23 pascals.
- 2.One bar is 100,000 pascals, so divide: 220,632.23 ÷ 100,000 = 2.2063223.
- 3.Put another way, 1 psi = 0.0689476 bar.
Result: 2.2063223 bar
What pressure actually measures
Pressure is force spread over an area — the same push concentrated into a smaller area produces a higher pressure, which is why a drawing pin pierces a board that a fingertip pressing just as hard does not. Because it is a ratio of two other quantities rather than a base quantity of its own, every pressure unit is really a force unit divided by an area unit: the pascal is one newton per square metre, and the psi is one pound-force per square inch.
That structure explains why so many pressure units exist. Any community that already had its own force and area units effectively inherited a pressure unit for free, and several of them — the atmosphere, the bar, the millimetre of mercury — were defined for convenience against a physical reference rather than derived from a coherent system.
Where each unit is normally used
The unit you meet depends almost entirely on the field, not on the country.
- Pascal and kilopascal — the SI unit and its usual practical multiple — used in science, engineering and meteorology. A pascal is a very small pressure, so raw pascals are rare outside calculations.
- Bar and millibar — convenient because one bar is very close to average sea-level air pressure. Millibars (identical to hectopascals) remain the standard in weather forecasting.
- psi — the everyday unit in the US and for tyres, compressed air and plumbing in much of the English-speaking world.
- Atmosphere — a reference unit defined as average sea-level pressure, used in chemistry and diving to express pressures as multiples of "normal".
- mmHg and inHg — survivals from mercury-column instruments. Blood pressure is still universally reported in mmHg, and aviation and US weather reporting still use inches of mercury for barometric pressure.
Why mercury columns are still used for blood pressure
A mercury barometer measures pressure by how far it pushes a column of mercury up a tube, so the height of that column — in millimetres or inches — became a direct readout of pressure. Mercury sphygmomanometers have largely been retired from clinical use for safety reasons, but the unit outlived the instrument: blood pressure is still reported in mmHg worldwide, and changing it now would invalidate decades of clinical thresholds and research literature that are all expressed in those numbers.
What this assumes, and where it stops
Assumptions
- Conversion factors follow the internationally agreed definitions of each unit.
Limitations
- Gauge pressure and absolute pressure are not distinguished. A tyre gauge reading "32 psi" means 32 psi above atmospheric pressure (about 46.7 psi absolute) — this converter simply converts the number you enter, so make sure both sides of the conversion mean the same kind of pressure.
- The mercury and water column units (inHg, mmHg, mmH₂O) depend on fluid density and therefore temperature. The standard reference temperatures are noted in each unit name.
Common questions
Why is my tyre pressure in both psi and bar?
Tyres are sold and rated internationally, so both units appear on placards and gauges. Roughly, 1 bar is about 14.5 psi — a typical car tyre at 32 psi is about 2.2 bar. The conversion here is exact, since both units are defined exactly in pascals.
What is the difference between gauge and absolute pressure?
Gauge pressure is measured relative to the surrounding air; absolute pressure is measured from a perfect vacuum. They differ by one atmosphere (about 14.7 psi or 1.013 bar). Most everyday gauges — tyre, blood pressure, boiler — read gauge pressure, so a tyre at "0 psi" is actually at atmospheric pressure, not a vacuum.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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