Barometric Pressure Conversion
Barometric pressure is the weight of the air column above a place, written in the units used by weather stations, pilots, ship reports, laboratory instruments, and forecast models. This calculator converts one atmospheric pressure reading into another without changing what the reading means. The default example starts at 1013.25 hPa and converts to inHg, matching the familiar standard-atmosphere value near 29.9213 inHg.
The page is intentionally focused on weather and atmosphere units. Hectopascals and millibars dominate meteorology; inches of mercury appear in U.S. aviation altimeter settings and many home barometers; millimeters of mercury connect to older mercury-column instruments; atmospheres are convenient when comparing a pressure with standard sea-level air. If you are working with shop air, materials stress, or a pressure vessel, the same unit math applies, but the context may be different. Gauge readings measure pressure above local air pressure, while barometric readings are normally absolute pressure. For broader mechanical conversions, compare this page with the pressure calculator, the bar to psi converter, and the psi to atm conversion.
Units this weather converter understands
The converter supports pascals, hectopascals, kilopascals, millibars, bars, atmospheres, millimeters of mercury, inches of mercury, and pounds per square inch. The weather-facing units are not interchangeable labels; each has a history and a domain.
| Unit | Meaning in this calculator | Common domain |
|---|---|---|
| Pa | SI pressure unit, one newton per square meter | science, standards |
| hPa | 100 Pa | weather maps and station pressure |
| mbar | 100 Pa | weather maps, legacy meteorology |
| kPa | 1000 Pa | engineering and metric instruments |
| bar | 100000 Pa | gauges, compressors, some forecasts |
| atm | 101325 Pa | standard atmosphere comparisons |
| mmHg | 133.322387415 Pa (conventional mercury at 0 °C) | mercury-column pressure |
| inHg | 3386.388640341 Pa (conventional mercury at 0 °C) | barometers and U.S. altimeters |
| psi | about 6894.757293168 Pa | mechanical pressure references |
Because hPa and mbar both equal 100 Pa, the conversion between them is one-to-one. A forecast pressure of 997 hPa is exactly 997 mbar in this calculator. That equivalence is a frequent source of confusion because the names look unrelated.
Formula used by the converter
Every supported unit has a pascal factor. The calculator first converts the entered pressure to pascals, then converts pascals to the selected output unit:
Here, the two factors are the number of pascals in one source unit and one destination unit. This is why a single form can convert hPa to inHg, mmHg to bar, psi to kPa, or any other pair in the list. The result panel also reports the intermediate pascals, atmospheres, bars, and the direct conversion factor, matching the displayed values.
Barometric Pressure Conversion example
For example:
- Pressure: 1013.25
- From unit: hPa
- To unit: inHg
The calculation uses 100 Pa per hPa and 3386.388640341 Pa (conventional mercury at 0 °C) per inHg:
The primary result is therefore 29.9213 inHg. The supporting rows show 101325 Pa, 1 atm, 1.01325 bar, and a conversion factor of about 0.02952998 from hPa to inHg. If you reverse the direction and enter 29.9213 inHg, the displayed result returns close to 1013.25 hPa, with the small difference caused by rounding the visible input.
Atmosphere, bar, pascal, and psi presets
One standard atmosphere is exactly 101,325 Pa, while one bar is exactly 100,000 Pa. Therefore 1 bar is about 0.9869232667 atm, and reversing that unrounded result returns 1 bar. The psi factor is derived from the exact pound-force, standard-gravity, and inch definitions rather than a shortened decimal: 1 psi is about 6,894.757293168 Pa, so 1 bar is about 14.503773773 psi.
Pressure-unit conversion does not change the reference of a reading. A gauge-pressure value and an absolute-pressure value can have the same unit but describe different physical references. Keep the unrounded result for a reverse check and round only the final reported value.
Weather, aviation, and instrument context
Weather reports often reduce station pressure to sea level so readings from mountain stations and coastal stations can be compared on the same map. A low-pressure center might be labeled near 980 hPa; a strong high-pressure system might exceed 1030 hPa. The conversion does not perform any sea-level reduction or altitude correction. It only changes the unit attached to the pressure you already have.
Aviation altimeter settings are another barometric use case. A U.S. pilot may hear an altimeter setting in inches of mercury, while an international forecast discussion may give the same environment in hectopascals. Converting between inHg and hPa lets the same pressure be read in either convention, but it does not replace official aviation procedures or instrument calibration.
Laboratory and data-logger records can mix mmHg, kPa, and atm. Millimeters of mercury are tied to column height, while atmospheres are a reference scale. Under the calculator’s constants, 760 mmHg is approximately 1 atm: 760 × 133.322387415 Pa (conventional mercury at 0 °C) ÷ 101325 Pa is about 1.0000001425 atm.
Common pitfalls
- Confusing hPa and kPa. One kPa is 10 hPa, so a weather pressure of 101.3 kPa is the same as 1013 hPa, not 101.3 hPa.
- Comparing absolute barometric pressure with gauge pressure from a tire or compressor. The conversion is mathematical, but the physical reference is different.
- Treating local station pressure as sea-level pressure. Unit conversion does not adjust for elevation.
- Rounding an altimeter-style inHg value too early. A few hundredths of an inch of mercury correspond to several tenths of a hectopascal.
- Assuming every mercury unit is the same. This calculator labels mmHg and inHg separately because the column height unit is different.
Related calculators
For a compact hPa, bar, psi, and pascal workflow, use the pressure calculator. If a gauge is labeled in bar and the manual wants psi, use the bar to psi converter. For a standard-atmosphere comparison from an imperial pressure, use the psi to atm conversion. For mercury-column pressure specifically, the mmHg to atm conversion is a narrower companion.
Sources
- NIST, Definitions of the SI base units and SI prefixes — SI unit and prefix reference used for pascal-based conversions.
- BIPM, The International System of Units, 9th edition — official SI brochure for pascal and coherent derived units.
- NIST, Guide for the Use of the International System of Units, SP 811 — Appendix B tables for conventional mercury-column and pressure conversion factors.
- National Weather Service, Pressure conversion chart — meteorological hPa/inHg conversion reference.