How Barometric Pressure Conversion Works
All barometric pressure units measure the same physical quantity: the force exerted by a column of air (or fluid) per unit area. They differ only in the size of the unit. Because the relationships between units are fixed constants, conversion is exact multiplication.
The calculator converts your input to pascals (Pa) first, then from pascals to every other unit using the following exact factors:
| Unit | Pascals per unit | Formula (from Pa) |
|---|
| 1 hPa (mbar) | 100 Pa | Pa / 100 |
| 1 inHg (at 0 degC) | 3386.389 Pa | Pa / 3386.389 |
| 1 mmHg (Torr) | 133.322 Pa | Pa / 133.322 |
| 1 kPa | 1000 Pa | Pa / 1000 |
| 1 Pa | 1 Pa | Pa |
| 1 bar | 100,000 Pa | Pa / 100000 |
| 1 atm | 101,325 Pa | Pa / 101325 |
| 1 psi | 6894.757 Pa | Pa / 6894.757 |
Worked Example
Using the default value of 1013.25 hPa (standard sea-level pressure):
- 1013.25 hPa x 100 = 101,325 Pa
- 101,325 / 3386.389 = 29.9213 inHg
- 101,325 / 133.322 = 760.00 mmHg
- 101,325 / 1000 = 101.325 kPa
- 101,325 / 100,000 = 1.01325 bar
- 101,325 / 101,325 = 1.00000 atm
- 101,325 / 6894.757 = 14.6959 psi
These results match exactly what the calculator shows for the default input, confirming the conversions are correct.
Which Units Are Used Where?
hPa and mbar are numerically identical (1 hPa = 1 mbar exactly) and are the standard for meteorology worldwide, including New Zealand's MetService, Australian Bureau of Meteorology, and the World Meteorological Organization (WMO).
inHg (inches of mercury) is used in aviation in the United States and some other countries for altimeter settings. New Zealand aviation uses hPa.
mmHg and Torr are used in medicine (blood pressure, vacuum systems) and chemistry. One Torr equals exactly 1 mmHg by definition since 2019 (they were previously slightly different).
Pa and kPa are the SI units. Scientific papers and engineering standards typically use Pa or kPa. Tyre pressures in New Zealand are often given in kPa.
bar is widely used in engineering, industrial gas, and scuba diving. 1 bar is close to but not exactly 1 atm (1 bar = 0.98692 atm).
psi is common in the United States and some industrial applications. Tyre pressures in older manuals may be given in psi.
Barometric Pressure and the Weather
Atmospheric pressure at any location is the weight of the air column above that point. It naturally decreases with altitude (roughly 1 hPa for every 8 m gain near sea level). Meteorologists correct station pressure to sea-level pressure (QNH) so that readings from different altitudes can be compared.
In New Zealand, pressure typically ranges from around 950 hPa in a severe tropical cyclone to about 1040 hPa during a strong winter anticyclone. A rapid pressure drop (more than 5 hPa in 3 hours) is a sign of rapidly deteriorating weather. A steady rise indicates improving conditions.
Related Calculators
Sources and method: Conversion factors from NIST (National Institute of Standards and Technology) and ISO 80000-4. Standard atmosphere definition: 101,325 Pa exactly (Resolution 4 of the 10th CGPM, 1954). inHg factor based on mercury density at 0 degC and standard gravity (9.80665 m/s2). mmHg/Torr redefinition: BIPM 2019.
This calculator converts barometric pressure between common units using exact SI-defined conversion factors. The inHg factor uses mercury density at 0 degrees Celsius; some aviation and weather references use a slightly different factor based on 15 degrees Celsius (3376.85 Pa/inHg). For precision work, check the specific definition used by your instrument or standard.
