What barometric pressure conversion means
Barometric pressure is the force the atmosphere exerts at a given point, and meteorologists, pilots, divers, and engineers all report it in different units depending on their instruments and country. A weather station might read 1013 hPa, a US aviation altimeter setting might read 29.92 inHg, and an old mercury barometer might read 760 mmHg — but these three numbers describe the exact same atmospheric condition. This calculator converts a pressure value from one unit to hectopascals/millibars, inches of mercury, millimeters of mercury (torr), kilopascals, atmospheres, and psi at the same time, so you can see how a single reading maps across every common scale.
The reference point: standard atmosphere
Every conversion here is anchored to one internationally agreed constant: 1 standard atmosphere (atm) is defined as exactly 101,325 pascals, i.e. 1013.25 hectopascals. This is the ICAO/ISO standard atmosphere value used worldwide for altimeter calibration and weather reporting, and it is also treated as standard sea-level pressure. From that definition, the calculator derives every other unit:
- 1 atm = 1013.25 hPa (millibar) — hPa and mbar are numerically identical, just different names for the same unit.
- 1 atm = 29.9212 inHg (inches of mercury), so 1 inHg = 33.8639 hPa.
- 1 atm = 760 mmHg (millimeters of mercury, also called torr), so 1 mmHg = 1.33322 hPa.
- 1 atm = 101.325 kPa, so 1 kPa = 10 hPa exactly.
- 1 atm = 14.6959 psi (pounds per square inch), so 1 psi = 68.9476 hPa.
The calculator converts your input to hPa first (multiplying by the appropriate factor above), then divides that hPa value by each unit's own hPa-per-unit factor to produce the other five readings. Because every factor traces back to the same 1013.25 hPa reference, all six output numbers always describe one identical physical pressure.
Typical readings you can check the tool against
Standard sea-level pressure (1013.25 hPa) is the most useful sanity check: entering 1013.25 hPa should return 29.9212 inHg, 760.00 mmHg, 101.33 kPa, 1.0000 atm, and 14.696 psi. Real-world barometric pressure at sea level typically ranges from about 980 hPa in a strong low-pressure storm system to around 1040 hPa in a strong high-pressure system. US aviation weather reports ("METAR") commonly quote altimeter settings between 29.50 and 30.50 inHg for the same range. Car and bicycle tire gauges usually read in psi (a typical car tire is 30-35 psi), which is a *gauge* pressure above atmospheric, not the same kind of absolute barometric reading — worth keeping in mind if you're converting a tire spec rather than a weather reading.
Why the units differ by field
Meteorologists in most of the world use hectopascals (identical in size to the older "millibar," so weather services simply renamed the unit without changing any numbers). US aviation and much of US broadcast weather use inches of mercury because altimeters were historically calibrated with mercury columns. Many European and Asian weather services, along with medical blood-pressure devices, use millimeters of mercury. Engineers and industrial gauge specifications often use kilopascals or psi because those tie directly into the SI and imperial force/area systems used in mechanical design. This tool exists because none of those communities normally need to convert into each other's units — until a pilot reads a European METAR, or an engineer needs to sanity-check a weather feed against a gauge datasheet.
Frequently asked questions
Is a millibar the same as a hectopascal? Yes — 1 mbar = 1 hPa exactly. Weather services switched from "millibar" to "hectopascal" for SI consistency, but the numeric value never changed, so old millibar charts can be read directly as hPa.
What is "standard atmosphere" and why 1013.25? It is an internationally defined reference pressure (101,325 Pa) approximating average sea-level air pressure, standardized so instruments, altimeters, and scientific calculations have one common baseline.
Is mmHg the same as torr? They are effectively identical for all practical purposes (1 torr is defined as 1/760 atm, which equals 1.00000014 mmHg) — this calculator treats them as equal.
Why does 30 inHg not equal exactly 1 atm? Standard atmospheric pressure is 29.9212 inHg, not 30 — 30 inHg is a common rounded reference in aviation but is not the formal standard-atmosphere value; it converts to roughly 1015.9 hPa.
Can I convert tire pressure or blood pressure with this tool? The unit math is identical, but remember tire-pressure gauges and most blood-pressure cuffs report gauge pressure (pressure above the surrounding atmosphere), not absolute barometric pressure, so treat those conversions as unit conversions only, not as equivalent physical readings to a weather barometer.