Pressure Converter
Convert between pressure units — bar, pounds per square inch (psi), pascals, kilopascals, megapascals, atmospheres, torr, millimetres and inches of mercury, millibars, hectopascals and kgf/cm². Pick any two; it runs entirely in your browser.
Every factor here comes from a fixed definition rather than a measurement. A bar is exactly 100,000 pascals, one standard atmosphere is exactly 101,325 pascals, a torr is exactly one 760th of an atmosphere, a kilogram-force per square centimetre is exactly 98,066.5 pascals, and psi is built from the exact pound and the exact inch. So the live tool and the numbers on every page below always agree.
Which unit does your gauge use?
Pressure is the one quantity where the unit usually depends on where the equipment was made rather than on what it measures. The United States uses psi. Most of Europe and Asia uses bar. Formal engineering standards use kilopascals or megapascals. Older machinery from India, Russia, Japan and Europe is often marked kg/cm². Weather uses hectopascals or the identical millibar, except in North America, which uses inches of mercury. Medicine uses mmHg everywhere. That is why converting pressure is so often about reading somebody else's gauge.
Gauge pressure and absolute pressure
This is the trap that catches people, and no converter can fix it for you. Almost every gauge you will meet — tyre gauges, compressors, boiler dials — reads gauge pressure, meaning the amount above the surrounding air. That is why a flat tyre reads zero rather than reading the roughly one atmosphere of air actually inside it. Absolute pressure counts from a perfect vacuum instead, and is what physics and gas-law calculations need. Converting between units never changes which of the two you have: if you started with a gauge reading, you still have a gauge reading afterwards, just in different units.
Millibars and hectopascals are the same size
A millibar and a hectopascal are exactly equal — both are 100 pascals. When weather services moved from millibars to hectopascals they were not changing the numbers, only the name, so 1013 mb and 1013 hPa mean precisely the same air pressure. Old charts and new ones can be read side by side with no adjustment at all.
The torr is not quite a millimetre of mercury
These two are treated as interchangeable almost everywhere, and for almost every purpose that is fine — but they are not the same unit. A torr is defined from the atmosphere: exactly one 760th of 101,325 pascals. A millimetre of mercury is defined from a conventional density of mercury under standard gravity, which fixes it at exactly 133.322387415 pascals. The two differ by about one part in seven million. It matters only in precision vacuum metrology, but it is the reason this converter keeps them as separate units and gives each its own exact value instead of quietly rounding them together.
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Units in this converter
| Unit | What it is |
|---|---|
| Pascal (Pa) | The pascal is the SI unit of pressure — one newton spread over one square metre. It is a very small amount of pressure, so everyday figures are usually given in kilopascals or bars. |
| Hectopascal (hPa) | A hectopascal is 100 pascals. Weather forecasts worldwide give air pressure in hectopascals — and one hectopascal is exactly the same size as one millibar, which is why old and new weather charts show identical numbers. |
| Kilopascal (kPa) | A kilopascal is 1,000 pascals. It is the everyday metric pressure unit — tyre pressures, gas cylinders and engineering specs are commonly given in kPa. |
| Megapascal (MPa) | A megapascal is one million pascals, or 10 bar. Concrete strength, hydraulic systems and material stress are usually quoted in MPa. |
| Millibar (mbar) | A millibar is one thousandth of a bar. It is the traditional weather unit and is exactly equal to a hectopascal, so 1013 mbar and 1013 hPa mean the same air pressure. |
| Bar (bar) | A bar is exactly 100,000 pascals — deliberately set very close to average air pressure at sea level. Tyre pressures, diving gauges and pumps across Europe and Asia are given in bar. |
| Standard atmosphere (atm) | One standard atmosphere is exactly 101,325 pascals — the agreed reference for average air pressure at sea level. It is a fixed definition, not a measurement, so it never changes with the weather. |
| Torr (Torr) | A torr is exactly one 760th of a standard atmosphere, so 760 torr is one atmosphere by definition. It is used for vacuum work. It is very nearly — but not exactly — the same as a millimetre of mercury. |
| Millimetre of mercury (mmHg) | A millimetre of mercury is the pressure that lifts mercury 1 mm in a tube, fixed at exactly 133.322387415 pascals. Blood pressure is always given in mmHg. It differs from the torr by about one part in seven million. |
| Inch of mercury (inHg) | An inch of mercury is exactly 25.4 millimetres of mercury. US weather reports and aircraft altimeters give air pressure in inHg — around 29.92 inHg at sea level. |
| Millimetre of water (mmH₂O) | A millimetre of water is the pressure of a 1 mm column of water, fixed at exactly 9.80665 pascals. It is used for very low pressures — ventilation ducts, fan curves and drainage. |
| Inch of water (inH₂O) | An inch of water is exactly 25.4 millimetres of water. Heating and air-conditioning engineers use it for duct and gas pressures, often written "inches WC" for water column. |
| Pound per square inch (psi) | A psi is one pound of force spread over one square inch. It is the standard pressure unit in the United States — tyre pressures, air compressors and water systems are all given in psi. |
| Kilogram-force per square centimetre (kgf/cm²) | A kilogram-force per square centimetre is exactly 98,066.5 pascals, also called the technical atmosphere. It is close to one bar, and is still printed on machinery, pressure gauges and tyre charts across India and Europe. |