Pressure Converter
Pressure, defined as force per unit area, is measured in a bewildering array of units across different fields and countries, reflecting centuries of independent development in various industries. The SI unit is the pascal (Pa), equal to one newton per square metre, but this is very small: atmospheric pressure at sea level is 101,325 pascals. This leads to larger units: kilopascals (kPa), hectopascals (hPa), and bars. The bar and millibar are used in meteorology for weather and storm systems. Pound per square inch (PSI) is the standard unit in the United States for tire pressure, compressed air systems, and pneumatic equipment. Mercury-based units (millimetres of mercury, inches of mercury, torr) are historical but persist in medical contexts (blood pressure) and barometry. One atmosphere is defined as standard atmospheric pressure at sea level: 101,325 pascals, 1,013.25 hectopascals, approximately 1 bar, 760 millimetres of mercury, 14.696 PSI. Converting between these units is essential when reading weather reports from different countries, inflating tires to correct pressure, calibrating diving equipment, interpreting medical measurements, or engineering hydraulic and pneumatic systems. This calculator displays eleven pressure units: pascals through torr. Enter a value in any field and all others update instantly with NIST precision.
NIST conversion factors
All conversions below use standard NIST-defined factors, with all units expressed relative to the pascal.
| Unit | Symbol | Pascals |
|---|---|---|
| Pascal | Pa | 1 |
| Hectopascal | hPa | 100 |
| Kilopascal | kPa | 1,000 |
| Megapascal | MPa | 1,000,000 |
| Bar | bar | 100,000 |
| Millibar | mbar | 100 |
| Atmosphere | atm | 101,325 |
| Millimetre of mercury | mmHg | 133.322 |
| Inch of mercury | inHg | 3,386.389 |
| Pound per square inch | psi | 6,894.757 |
| Torr | Torr | 133.322 |
Pressure converter: frequently asked questions
What is the SI unit of pressure?
The pascal (Pa) is the SI unit of pressure, defined as one newton per square metre. The pascal is quite small; atmospheric pressure at sea level is about 101,325 pascals, so kilopascals (kPa) and hectopascals (hPa) are more commonly used.
What is atmospheric pressure?
Standard atmospheric pressure at sea level is defined as exactly 101,325 pascals or 101.325 kilopascals. This is equivalent to 1 atmosphere (atm), 1,013.25 hectopascals (hPa), 1 bar (approximately), 760 millimetres of mercury (mmHg), 14.696 psi, and 101,325 Pa. Weather services often use hectopascals (hPa).
What is PSI and where is it used?
PSI stands for pounds per square inch, the imperial unit of pressure. It is commonly used in the United States for tire pressure, compressed air systems, and hydraulic pressure. Standard atmospheric pressure is about 14.696 psi. Most vehicle tire pressures range from 25 to 35 psi.
What is the difference between pressure and gauge pressure?
Absolute pressure includes atmospheric pressure in the measurement. Gauge pressure is measured relative to atmospheric pressure, so gauge pressure plus atmospheric pressure equals absolute pressure. Most practical pressure measurements use gauge pressure, but conversions here use absolute pressure values.
Why are there so many pressure units?
Different industries and countries developed different pressure units historically. Weather uses hectopascals or millibars. Tire and pneumatic systems use PSI or kPa. Diving uses atmospheres. Barometers use inches or millimetres of mercury. Converting ensures clarity across different fields and countries.
Official sources
- NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices.
- NIST Special Publication 330: The International System of Units.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.