PSI Calculator - Pressure Unit Converter for Medical Use
Convert between PSI, mmHg, kPa, bar, atm, cmH2O, Pa, and MPa for medical device calibration, respiratory therapy, and health monitoring.
Select source and target pressure units, enter a value, and get an instant conversion with configurable precision.
PSI Calculator - Pressure Unit Converter for Medical Use
Convert between PSI, mmHg, kPa, bar, atm, cmH2O, Pa, and MPa for medical device calibration, respiratory therapy, and health monitoring.
About the PSI pressure converter
Pressure is a fundamental physical quantity defined as force per unit area, and it appears throughout medicine and healthcare in contexts ranging from the familiar (blood pressure, measured in millimetres of mercury) to the highly specialised (ventilator peak inspiratory pressure in cmH2O, hyperbaric chamber pressure in atmospheres, and medical gas line pressure in bar or PSI). Healthcare professionals regularly need to convert between these units for equipment calibration, patient monitoring documentation, and inter-system communication, and a reliable pressure converter is an essential tool in clinical and biomedical engineering environments.
The SI unit of pressure is the pascal (Pa), defined as one newton per square metre (1 N/m²). Because the pascal is a very small unit, medical and engineering applications typically use multiples: kilopascals (kPa, equal to 1000 Pa) are common in modern ventilators and physiology textbooks, while megapascals (MPa, equal to one million Pa) are relevant to high-pressure medical gas systems. The pascal and its multiples are the preferred units in contemporary scientific literature.
Millimetres of mercury (mmHg) is the traditional medical pressure unit, descending from the mercury sphygmomanometer. Blood pressure (e.g., 120/80 mmHg), intraocular pressure, intracranial pressure, and pulmonary capillary wedge pressure are all expressed in mmHg by clinical convention, even in countries that have otherwise adopted the SI system. The unit is sometimes called a torr (after Evangelista Torricelli), and 1 mmHg = 1 torr = 133.322 Pa.
PSI (pounds per square inch) is the dominant pressure unit in the United States engineering context, including medical gas pipeline systems, compressed air for respiratory therapy, and many pieces of biomedical equipment designed for North American markets. The conversion factor 1 PSI = 6894.76 Pa is exact within SI definitions, giving rise to the other common relationships: 1 PSI = 51.7149 mmHg, 1 PSI = 6.89476 kPa, 1 PSI = 0.0689476 bar, and 1 PSI = 0.068046 atm.
Centimetres of water (cmH2O) is the standard unit for ventilator pressures, including peak inspiratory pressure, positive end-expiratory pressure (PEEP), and continuous positive airway pressure (CPAP). The unit reflects the historical use of water manometers and is convenient because typical ventilator pressures (5–35 cmH2O) map to easily understood clinical ranges. 1 cmH2O = 98.0665 Pa, making the conversion to mmHg approximately 0.7356 (so 10 cmH2O ≈ 7.36 mmHg).
Bar is an SI-adjacent unit widely used in European engineering and in some medical gas standards. One bar equals exactly 100,000 Pa (100 kPa), making it close but not identical to one standard atmosphere (1 atm = 101,325 Pa = 1.01325 bar). Atmospheric pressure (atm) is important in hyperbaric oxygen therapy, altitude physiology, and dissolved gas calculations for diving medicine.
This converter handles all eight units — PSI, mmHg, kPa, bar, atm, cmH2O, Pa, and MPa — using exact conversion factors referenced from the NIST and ISO standards. All conversions route through the pascal as an intermediate step. The precision selector (0–6 decimal places) allows the result to be displayed at the level of detail appropriate for the application, from rough clinical estimates to precise equipment calibration specifications.
Pressure conversion examples
Common medical pressure conversions encountered in clinical and biomedical engineering practice.
| Input | Result | Clinical context |
|---|---|---|
| 120 mmHg → PSI (3 decimal places) | 2.320 PSI | Systolic blood pressure conversion for equipment calibration. 120 × 0.019337 = 2.320 PSI. |
| 20 cmH2O → PSI (2 decimal places) | 0.28 PSI | Peak inspiratory pressure on a ventilator — useful when interfacing with PSI-calibrated gas regulators. |
| 1 atm → PSI (2 decimal places) | 14.70 PSI | Standard atmospheric pressure — baseline reference for hyperbaric and altitude calculations. |
| 2.5 bar → PSI (1 decimal place) | 36.3 PSI | Medical gas pipeline pressure — converting European bar specifications to PSI for North American equipment. |
How to use the PSI pressure converter
- Enter the numeric pressure value you want to convert in the Pressure Value field.
- Select the unit you are converting from in the From Unit dropdown (e.g., mmHg for a blood pressure reading).
- Select the target unit in the To Unit dropdown (e.g., PSI for equipment calibration).
- Set the number of decimal places you need for the result — use higher precision for calibration work and lower precision for clinical estimates.
- Click Convert. The converted pressure value appears instantly with its unit label.
Pressure conversion FAQ
What is the relationship between mmHg and PSI?
1 PSI equals approximately 51.71 mmHg, which means 1 mmHg equals about 0.01934 PSI. In clinical terms, a systolic blood pressure of 120 mmHg corresponds to roughly 2.32 PSI. The exact value is 1 PSI = 51.7149 mmHg, derived from the definition of standard gravity and the density of mercury at 0°C.
Why is blood pressure measured in mmHg rather than kPa?
Blood pressure has been measured in millimetres of mercury since the 19th-century invention of the mercury sphygmomanometer. Although the WHO and many scientific bodies recommend expressing blood pressure in kPa (1 mmHg ≈ 0.133 kPa), clinical practice has retained mmHg because of the enormous body of reference values, guidelines, and clinician training built around that unit. Most modern non-mercury devices still display mmHg for compatibility.
What units are used for ventilator pressures?
Ventilator pressures — including peak inspiratory pressure, PEEP (positive end-expiratory pressure), plateau pressure, and CPAP — are expressed in centimetres of water (cmH2O) in most countries. The conversion is 1 cmH2O = 0.098066 kPa. Some older US ventilators use PSI for gas supply pressure, which is why cmH2O-to-PSI conversion is frequently needed in respiratory therapy.
How does the converter handle very small or very large values?
The converter uses full double-precision floating-point arithmetic internally, which provides approximately 15–17 significant digits of accuracy. The displayed precision is controlled by the decimal places selector (0–6). For very small values (e.g., Pa in physiological ranges) or very large values (e.g., MPa in high-pressure systems), use more decimal places to avoid rounding away significant digits.
What is the difference between gauge pressure and absolute pressure?
Absolute pressure is measured relative to a perfect vacuum, while gauge pressure is measured relative to local atmospheric pressure (~14.70 PSI or 101.325 kPa at sea level). Blood pressure cuffs and most clinical monitors measure gauge pressure (so 0 mmHg gauge = atmospheric pressure). Many engineering specifications use gauge pressure (PSIG). This converter converts absolute pressure values — if you are working with gauge pressures, add atmospheric pressure before converting or interpreting the result.
Is bar the same as atmosphere?
No, though they are close. One standard atmosphere (atm) is defined as exactly 101,325 Pa, while one bar is defined as exactly 100,000 Pa. The difference is about 1.3%. At sea level, atmospheric pressure is approximately 1.01325 bar. In medical gas standards and diving physiology, both units are used and the distinction matters for precise calculations in hyperbaric therapy and respiratory gas management.