Pressure Conversion Guide
Choose the right pressure unit, convert with verified factors, and preserve the measurement reference across medicine, weather, aviation, vacuum, and engineering.
Start with the pascal factor
Convert the source value to pascals, then divide by the target unit's pascal factor.
- Meteorology
- 1 mbar = 1 hPa
- Standard atmosphere
- 1 atm = 101,325 Pa
- Mercury units
- 1 inHg = 25.4 mmHg
What pressure units measure
Pressure is force applied per unit area. The pascal is the coherent SI unit: one pascal equals one newton per square metre. The BIPM SI Brochure defines the SI framework, while the NIST pressure conversion table publishes the factors used here.
A unit conversion changes the numeric scale, not the reference point. Absolute pressure, gauge pressure, and differential pressure describe different references. Confirm that two values use the same reference before comparing or converting them.
Pressure unit reference table
The pascal factors follow NIST values. Torr and conventional mmHg use the same conversion factor in this calculator, consistent with the conventional relationship used in standard conversion tables.
| Unit | Name | One unit in Pa | Common context |
|---|---|---|---|
| Pa | pascal | 1 | SI calculations, acoustics, and low-pressure work |
| kPa | kilopascal | 1,000 | Engineering, tires, and medical equipment |
| bar | bar | 100,000 | Industrial equipment, diving, and European tires |
| mbar | millibar | 100 | Weather reporting and vacuum systems |
| atm | standard atmosphere | 101,325 | Reference conditions, chemistry, and diving |
| psi | pound-force per square inch | 6,894.757293168 | US tires, hydraulics, and compressed gases |
| Torr | torr | 133.322368421 | Laboratory and vacuum measurement |
| mmHg | conventional millimetre of mercury | 133.322368421 | Blood pressure and historical barometry |
| inHg | conventional inch of mercury | 3,386.388640341 | US aviation and weather reporting |
One formula for every pressure conversion
target value = source value × source Pa factor ÷ target Pa factor
Identify the reference
Confirm whether the reading is absolute, gauge, or differential pressure.
Find both factors
Read the source and target pascal factors from the reference table.
Calculate once
Multiply by the source factor, divide by the target factor, and keep full precision.
Round at the end
Match the result to the precision of the instrument, specification, or reporting standard.
Worked pressure conversions
Blood-pressure units: 120 mmHg to kPa
120 × 133.322368421 ÷ 1,00015.998684 kPaThe calculated value is 15.998684 kPa. For a simple unit comparison, 16.0 kPa is usually sufficient.
Convert mmHg to kPaWeather units: 29.92 inHg to mbar
29.92 × 3,386.388640341 ÷ 1001013.20748 mbarThis is close to standard atmospheric pressure. Keep two decimal places when comparing altimeter or weather values.
Convert inHg to mbarVacuum units: 0.001 Torr to Pa
0.001 × 133.322368421 ÷ 10.13332237 PaLow-pressure work often needs scientific notation or several significant figures. Preserve the instrument's reported precision.
Convert torr to PaEngineering units: 250 kPa to psi
250 × 1,000 ÷ 6,894.75729316836.2594344 psiDo the calculation with the full factor, then round once to the precision supported by the gauge or specification.
Convert kPa to psiChoose the unit for the context
The correct unit is often set by the field, instrument, or reporting standard. Keep that convention when communicating the result.
- MedicinemmHg
Blood-pressure readings are reported as systolic and diastolic values in millimetres of mercury. Converting the unit does not interpret the reading or replace clinical advice.
NHLBI blood-pressure diagnosis guide- WeatherhPa and mbar
One hectopascal equals one millibar, so the numbers are identical. Modern meteorological practice prefers hPa, while many reports and instruments still show mbar.
WMO Guide to Meteorological Instruments- AviationinHg and hPa
US altimeter settings commonly use inches of mercury. International operations often use hectopascals, so pilots must preserve both the value and the reference setting.
FAA barometric altimeter procedures- VacuumPa, Torr, and mbar
Vacuum work spans many orders of magnitude. State the unit explicitly and retain significant figures appropriate to the gauge and calibration range.
NIST vacuum gauge standards- EngineeringPa, kPa, MPa, bar, and psi
Engineering specifications may mix SI and customary units. Convert through pascals, keep the original reference basis, and round only after the final calculation.
NIST pressure conversion table
Torr and mmHg: equal in this calculator
One torr is defined as exactly 1/760 of a standard atmosphere. UnifyUnits uses conventional mmHg with the same factor, 101,325 ÷ 760 pascals. This makes 1 Torr equal to 1 mmHg for calculations on this site.
A physical mercury column depends on specified temperature and gravity conditions. The WMO reference distinguishes conventional mercury units from direct physical readings, so calibration work should use the instrument's documented convention.
Standards and primary references
Conversion factors and application notes were checked against the following standards bodies and public authorities on 1 September 2026.
Units and factors
- BIPM: The International System of Units, 9th edition
SI definitions and the pascal as the coherent unit of pressure.
- NIST: Pressure and Gas Flow Unit Conversions
Conversion factors for Pa, mbar, Torr, psi, atm, and inHg.
Applied measurement
- WMO via NOAA: Guide to Meteorological Instruments and Methods of Observation
Atmospheric-pressure units, standard conditions, hPa, mbar, mmHg, and inHg.
- FAA: Barometric Altimeter Errors and Setting Procedures
Aviation altimeter settings and the 29.92 inHg reference.
- NHLBI: High Blood Pressure Diagnosis
Medical use of mmHg for systolic and diastolic blood pressure.
- NIST: Vacuum Gauges
Examples of Pa and Torr across high-vacuum calibration ranges.