Pressure Conversion Calculator
Enter any pressure value, choose its unit, and this calculator instantly converts it to 16 other pressure units at once, from Pascals and kilopascals to psi, bar, atmospheres, Torr, millimeters of mercury, and more. The step-by-step panel shows exactly how each conversion is performed, and the reference table below gives the factor for every unit.
Formula
Worked example
Convert 29.92 inHg to kPa: first convert to Pa: 29.92 x 3,386.389 = 101,328.6 Pa. Then to kPa: 101,328.6 / 1,000 = 101.33 kPa. This matches standard sea-level atmospheric pressure (101.325 kPa).
What is pressure and how are pressure units defined?
Pressure is the force applied perpendicular to the surface of an object per unit area. The SI unit is the Pascal (Pa), defined as one Newton per square metre (1 Pa = 1 N/m2). Because a Pascal is very small relative to everyday pressures, derived units are common: 1 kPa = 1,000 Pa, 1 MPa = 1,000,000 Pa. Many older and industry-specific units remain in wide use: the bar (100,000 Pa) is the European engineering standard, pounds per square inch (psi) is the dominant US unit, and millimeters of mercury (mmHg) is used worldwide for blood pressure and vacuum measurement. Meteorologists prefer hectopascals (hPa, identical to millibars) because typical atmospheric pressure is around 1,013 hPa.
How to convert between pressure units
All pressure conversions go through a common intermediary: the Pascal. Every unit has a fixed factor expressing how many Pascals equal one unit (for example, 1 bar = 100,000 Pa, 1 psi = 6,894.757 Pa, 1 atm = 101,325 Pa exactly). To convert from unit A to unit B: multiply the value by the Pa-factor for A, then divide by the Pa-factor for B. This calculator performs all 17-way conversions in a single step. For quick mental estimates, remember that 1 bar is roughly 14.5 psi, and 1 atm is roughly 14.7 psi or 29.92 inHg.
Absolute pressure versus gauge pressure
Most pressure instruments measure gauge pressure, which is the pressure above the local atmospheric reference (typically ~101,325 Pa at sea level). Absolute pressure is gauge pressure plus atmospheric pressure. A tyre inflated to 32 psi (gauge) has an absolute pressure of about 32 + 14.7 = 46.7 psi. Vacuum measurements are sometimes expressed as negative gauge pressure or as absolute pressure below atmospheric. This calculator works with absolute pressure values. To convert a gauge reading to absolute, add 101,325 Pa (1 atm) before using the converter, then subtract again after converting if gauge output is needed.
Common pressure values in context
Standard sea-level atmospheric pressure is 101,325 Pa (1 atm, 1013.25 hPa, 14.696 psi, 760 mmHg, 29.921 inHg). A car tyre is typically inflated to 200-250 kPa (29-36 psi). Blood pressure is measured in mmHg: normal systolic is below 120 mmHg and normal diastolic below 80 mmHg. Industrial hydraulic systems run at 10-35 MPa (1,450-5,076 psi). Scuba cylinders are filled to about 200-300 bar (2,900-4,350 psi). Deep-sea pressure at 10 m depth increases by roughly 1 atm, so at 100 m the pressure is about 11 atm (1,114 kPa).
Pressure unit conversion factors
| Unit | Symbol | Definition / Factor (Pa) | Common use |
|---|---|---|---|
| Pascal | Pa | 1 (SI base unit) | Science, engineering (SI standard) |
| Kilopascal | kPa | 1,000 | Meteorology, medicine, tyre pressure |
| Megapascal | MPa | 1,000,000 | Structural engineering, hydraulics |
| Hectopascal | hPa | 100 | Meteorology (same as mbar) |
| Millibar | mbar | 100 | Meteorology, weather maps |
| Bar | bar | 100,000 | Industry, scuba diving, pressure gauges |
| Standard Atmosphere | atm | 101,325 (exact) | Reference pressure, chemistry |
| Technical Atmosphere | at | 98,066.5 | Engineering (kgf/cm2 based) |
| Pound per Square Inch | psi | 6,894.757 | US/UK engineering, tyre pressure |
| Pound per Square Foot | psf | 47.880 | Structural loads, HVAC |
| Torr | Torr | 133.322 | Vacuum, laboratory science |
| Millimeter of Mercury | mmHg | 133.322 | Medicine (blood pressure), vacuum |
| Inch of Mercury | inHg | 3,386.389 | US aviation, barometric pressure |
| Inch of Water (4 degC) | inH2O | 249.089 | HVAC duct pressure, respirators |
| Millimeter of Water (4 degC) | mmH2O | 9.807 | Low-differential pressure, medical |
| Dyne per Square Centimeter | dyn/cm2 | 0.1 | CGS system (historic) |
| kgf per Square Centimeter | kgf/cm2 | 98,066.5 | Technical atmosphere, older industry |
Multiply the Pascal value by the factor below to convert to each unit, or divide by it to convert to Pascals. Factors from NIST and ISO 80000-4.
Frequently asked questions
How do I convert psi to bar?
Divide the psi value by 14.5037744. For example, 30 psi / 14.5037744 = 2.0684 bar. Alternatively, multiply by 0.06894757. This calculator does it instantly: enter 30, select psi as the from-unit, and read bar from the results.
What is the difference between atm and bar?
One standard atmosphere (atm) is exactly 101,325 Pa, the average pressure at sea level. One bar is exactly 100,000 Pa. The bar is slightly less than 1 atm (1 bar = 0.9869 atm). The bar is preferred in European engineering because it is a round number in SI, while the atmosphere is used as a physical reference in chemistry and science.
Why do weather apps use hPa or mbar?
Meteorologists settled on millibars (mbar) in the early 20th century because 1013 mbar is close to average sea-level pressure, giving convenient numbers without large exponents. When SI units were adopted, the millibar was renamed to hectopascal (hPa); the two are numerically identical (1 hPa = 1 mbar = 100 Pa). Weather maps and station reports use hPa worldwide.
How is blood pressure measured in mmHg?
The millimeter of mercury (mmHg) was the original unit for blood pressure because early sphygmomanometers were mercury columns. A reading of 120 mmHg means the pressure supports a column of mercury 120 mm tall. One mmHg = 133.322 Pa. Although mercury instruments are now rare for safety reasons, the unit remains the international medical standard (WHO, AHA) to maintain consistency with decades of clinical data.
What is Torr and how does it relate to mmHg?
The Torr was defined in 1954 as 1/760 of a standard atmosphere (101,325/760 = 133.3224 Pa). The mmHg is defined by the actual physics of a mercury column at standard gravity (133.3224 Pa). The two differ by about 0.000016%, so they are used interchangeably in most practical contexts. Torr is more common in vacuum science (laboratory pressure, semiconductor fabrication) while mmHg is preferred in medicine.
What does gauge pressure mean versus absolute pressure?
Absolute pressure is measured relative to perfect vacuum (zero pressure). Gauge pressure is measured relative to ambient atmospheric pressure, so it reads zero when open to the air. A car tyre at 35 psi gauge has an absolute pressure of 35 + 14.7 = 49.7 psi. Most industrial gauges measure gauge pressure; most scientific instruments measure absolute pressure. Add 14.696 psi (101.325 kPa) to a gauge reading to get absolute pressure.