What pressure measures
Pressure is a force spread over an area. The SI unit, the pascal, is one newton per square meter, and the US customary unit, the pound per square inch, is a pound of force on a square inch. Other units come from history: the atmosphere matches the average air pressure at sea level, and millimeters or inches of mercury come from the height of mercury in a barometer.
How to use this converter
Type a value in any field and all the other units update at once. Start with what your gauge, tire placard, weather report or data sheet shows. Each common pair below the converter has its own page with the formula, an example and a chart.
Key equivalents
- 1 atm = 101,325 Pa = 1013.25 hPa = 1.01325 bar
- 1 atm ≈ 14.696 psi = 760 mmHg ≈ 29.92 inHg
- 1 bar = 100 kPa ≈ 14.504 psi
- 1 psi ≈ 6.895 kPa ≈ 0.0689 bar
- 1 MPa = 10 bar ≈ 145 psi
Gauge or absolute pressure?
Most gauges read gauge pressure, the difference with the surrounding air. A tire at 32 psi on the gauge actually holds about 46.7 psi absolute at sea level. Scientific formulas, vacuum work and diving calculations often need absolute pressure, so add the atmospheric pressure when needed. This converter changes units only; it does not add or remove the atmosphere.
Where each unit is used
- Tires: psi in the US, bar in Europe, kPa in Canada
- Weather: hPa or millibars in most countries, inches of mercury in the US
- Blood pressure: mmHg everywhere
- Engineering and materials: MPa, ksi and bar
- Vacuum: torr and pascals
Tips
When comparing tire pressures across manuals, convert and then round to the nearest whole psi or tenth of a bar, the precision of most gauges. Always follow the placard on the driver’s door rather than the maximum pressure molded on the tire.
Pressure units compared
| Unit | Symbol | In pascals |
|---|---|---|
| Pascal | Pa | 1 |
| Hectopascal | hPa | 100 |
| Kilopascal | kPa | 1,000 |
| Megapascal | MPa | 1,000,000 |
| Bar | bar | 100,000 |
| Atmosphere | atm | 101,325 |
| Pound per square inch | psi | 6,894.76 |
| Millimeter of mercury | mmHg | 133.322 |
| Inch of mercury | inHg | 3,386.39 |
| Torr | Torr | 133.322 |
| Kilogram-force per square centimeter | kgf/cm² | 98,066.5 |
The units in detail
What is a pascal?
The pascal is the SI unit of pressure, one newton per square meter. It is a very small unit: the air around us presses at about 101,325 Pa, so multiples such as the hectopascal, kilopascal and megapascal are used in practice.
What is a hectopascal?
The hectopascal equals 100 pascals and is identical to the millibar. Weather maps and forecasts outside the US give air pressure in hectopascals; standard sea-level pressure is 1013.25 hPa, and a strong hurricane can drop below 950 hPa.
What is a kilopascal?
The kilopascal equals 1,000 pascals. Canadian and many European car manuals give tire pressures in kPa, for example 240 kPa, which is about 35 psi. It is also used in engineering and for blood pressure in some medical texts.
What is a megapascal?
The megapascal equals one million pascals, or 10 bar. Engineers use it for the strength of concrete and steel and for hydraulic systems: ordinary structural concrete reaches a compressive strength of 20 to 40 MPa.
What is a bar?
The bar equals 100,000 pascals, very close to the atmospheric pressure at sea level. Europeans inflate car tires to about 2.2 to 2.5 bar, an espresso machine brews at around 9 bar and a full scuba tank holds about 200 bar.
What is an atmosphere?
The standard atmosphere is defined as exactly 101,325 pascals, the average air pressure at sea level. Divers count roughly one extra atmosphere for every 10 meters, or 33 feet, of seawater.
What is a pound per square inch?
The pound per square inch is the US unit of pressure, about 6,894.76 pascals. Car tires are usually inflated to 30 to 35 psi, and sea-level air pressure is 14.7 psi. Gauges often read psig, the pressure above atmospheric.
What is a millimeter of mercury?
The millimeter of mercury, about 133.32 pascals, comes from mercury barometers. It remains the unit of blood pressure worldwide: a reading of 120/80 mmHg means a systolic pressure of 120 and a diastolic pressure of 80.
What is an inch of mercury?
The inch of mercury, about 3,386.39 pascals, is how US weather reports and aircraft altimeters express barometric pressure. Standard pressure is 29.92 inHg; readings above 30.2 usually mean fair weather.
What is a torr?
The torr is 1/760 of a standard atmosphere, about 133.32 pascals, and is almost identical to the millimeter of mercury. It is used for vacuum systems in laboratories and the semiconductor industry.
What is a kilogram-force per square centimeter?
The kilogram-force per square centimeter, often written kg/cm², equals exactly 98,066.5 pascals, slightly less than a bar. It survives on older pressure gauges and in Asian and Latin American technical documents.