What this calculator does
Pressure is force spread over an area, and the units differ mainly in which force and which area they started from. The pascal is one newton per square metre, which is a very small pressure, so kilopascals and bar do most of the practical work. A bar is 100,000 pascals and was chosen to sit close to atmospheric pressure without being tied to it.
The two that catch people out are atmospheres and millimetres of mercury. One atmosphere is 101,325 pascals exactly, which is slightly more than a bar, so the two are near enough to be confused and far enough apart to matter. Millimetres of mercury come from the height of a mercury column the pressure would support, which is why blood pressure and barometric readings are still quoted that way.
The formula
Every unit has a fixed factor expressing it in pascals. Converting multiplies by the factor of the unit you are coming from to reach pascals, then divides by the factor of the unit you want. The full ladder is shown alongside because a pressure figure is often needed in several units at once, particularly when a specification and an instrument disagree about which to use.
| Term | Meaning |
|---|---|
| Pascal (Pa) | One newton per square metre, the SI unit. Small enough that everyday pressures run into the tens of thousands. |
| Bar | Exactly 100,000 pascals, close to average atmospheric pressure at sea level but defined independently of it. |
| Atmosphere (atm) | Exactly 101,325 pascals, the standard reference for sea-level atmospheric pressure. |
| mmHg | The pressure supporting a one millimetre column of mercury, about 133.3 pascals. Also called the torr. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The pressure value to convert. |
| From | The unit the value is currently in. |
| To | The unit you want the answer in. Every other unit is listed in the table below regardless. |
When to use it
Checking a tyre or equipment pressure
Tyre placards, pumps and gauges rarely agree on units, with psi common on gauges and kPa on placards in metric countries. Converting settles what the gauge should actually read.
Reading a weather or altitude figure
Barometric pressure appears as hectopascals, millibars and inches of mercury depending on the country and the instrument. A hectopascal and a millibar are the same thing, which the ladder makes obvious.
Matching an engineering specification
Design codes, datasheets and test certificates use whichever unit their home standard prefers, and a conversion error on a pressure rating is not a small mistake.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What is one of each pressure unit in kilopascals?
A single unit of each kind, converted to kilopascals so the relative sizes can be compared directly.
| From | In kilopascals |
|---|---|
| Pascal | 0.001 kPa |
| Bar | 100 kPa |
| Pound per square inch | 6.89476 kPa |
| Atmosphere | 101.325 kPa |
| Millimetre of mercury | 0.133322 kPa |
| Inch of mercury | 3.38639 kPa |
Converting 1 bar to the other pressure units
A single bar expressed in each of the other units in turn.
| To | Result |
|---|---|
| Pascals | 100,000 Pa |
| Kilopascals | 100 kPa |
| PSI | 14.50376808 psi |
| Atmospheres | 0.98692327 atm |
| mmHg | 750.0637554 mmHg |
Questions
Is a bar the same as an atmosphere?
No, though they are close. A bar is exactly 100,000 pascals and an atmosphere is exactly 101,325, so an atmosphere is about 1.3% larger. Both are defined exactly, so the difference is not a measurement uncertainty but a real gap between two separately chosen references.
How many psi in a bar?
About 14.504. The reverse, one psi, is about 0.0689 bar or 6.895 kPa. Neither direction gives a round number, because the pound-force and the square inch have nothing to do with the metre or the newton.
Are hectopascals and millibars the same?
Yes, exactly. One hectopascal is 100 pascals and one millibar is a thousandth of a bar, which is also 100 pascals. Weather services switched from millibars to hectopascals without any of the numbers changing, which is why older and newer charts read identically.
What is the difference between gauge and absolute pressure?
Gauge pressure is measured relative to the surrounding atmosphere, absolute pressure relative to a vacuum. A tyre at 220 kPa gauge is at about 321 kPa absolute. This converter handles the units, not the reference point, so make sure both figures you are comparing use the same one.
For converting a stress figure between ksi and MPa specifically, see the ksi to MPa converter. For tyre pressures between kPa and psi, see the kPa to psi tyre pressure converter.