Barometric Pressure Guide: hPa, inHg, mbar & Weather Interpretation
The standard atmosphere is 1013.25 hPa = 29.92 inHg = 760 mmHg. Learn all pressure unit conversions and how barometric readings relate to weather conditions.
Last updated: 2026-04-28
What Is Barometric Pressure?
Barometric pressure (atmospheric pressure) is the force per unit area exerted by the weight of the air column above any point. It is measured by a barometer and is a key variable in weather forecasting. Pressure decreases with altitude — at 5,500 m (about 18,000 ft) it is roughly half of sea-level pressure.
The SI unit of pressure is the pascal (Pa). Weather services use hectopascals (hPa) — 1 hPa = 100 Pa. Importantly, 1 hPa = 1 millibar (mbar) exactly, so hPa and mbar are interchangeable in all contexts.
Standard Atmosphere in All Pressure Units
The international standard atmosphere (ISA) at sea level is defined as 101,325 Pa exactly. All values below are exact or precisely calculated from this definition.
| Unit | Symbol | Value (1 atm) | Notes |
|---|---|---|---|
| Pascals | Pa | 101,325 | SI base unit |
| Kilopascals | kPa | 101.325 | Common in science |
| Hectopascals | hPa | 1,013.25 | Used by meteorologists |
| Millibars | mbar | 1,013.25 | 1 mbar = 1 hPa exactly |
| Inches of mercury | inHg | 29.9213 | Used in US aviation/weather |
| Millimeters of mercury | mmHg | 760.000 | Used in medicine (blood pressure) |
| Atmospheres | atm | 1.000 | Definition reference |
| Bar | bar | 1.01325 | Engineering; 1 bar ≈ 1 atm |
| Pounds per square inch | psi | 14.696 | US engineering and tires |
Quick Unit Conversion Reference
| From | To hPa | To inHg | To mmHg | To kPa |
|---|---|---|---|---|
| 1 atm | 1013.25 | 29.921 | 760.00 | 101.325 |
| 1 hPa (= 1 mbar) | 1.000 | 0.02953 | 0.7501 | 0.100 |
| 1 inHg | 33.864 | 1.000 | 25.400 | 3.386 |
| 1 mmHg (1 torr) | 1.3332 | 0.03937 | 1.000 | 0.1333 |
| 1 kPa | 10.000 | 0.2953 | 7.501 | 1.000 |
| 1 bar | 1000.00 | 29.530 | 750.06 | 100.00 |
Weather Interpretation by Pressure Level
Meteorologists analyze both the absolute pressure value and the rate of change (tendency) to forecast weather. The following guidelines apply at sea level; stations at elevation must correct readings to sea-level equivalent pressure for meaningful comparison.
| Pressure (hPa) | Pressure (inHg) | Weather Indication |
|---|---|---|
| >1030 | >30.42 | Strong high pressure; very clear and dry |
| 1020–1030 | 30.12–30.42 | High pressure; fair weather, low wind |
| 1010–1020 | 29.83–30.12 | Normal; settled conditions typical |
| 1000–1010 | 29.53–29.83 | Slightly below normal; clouds possible |
| 990–1000 | 29.24–29.53 | Low pressure; unsettled, rain likely |
| 980–990 | 28.94–29.24 | Deep low; significant storm risk |
| 960–980 | 28.35–28.94 | Severe storm or hurricane conditions |
| <960 | <28.35 | Intense hurricane or typhoon center |
Pressure Tendency (Rate of Change)
The rate of pressure change often matters as much as the absolute reading. A drop of more than 3 hPa per hour indicates a fast-moving weather system and is called a "pressure fall." A rise of 3+ hPa per hour signals clearing conditions. Drops exceeding 6 hPa per hour are associated with rapidly intensifying storms ("bomb cyclogenesis").
Altitude and Pressure
Pressure decreases by roughly 1.2 hPa for every 10 m of altitude gain at sea level, following the barometric formula. Practical reference points:
- Denver, CO (1,609 m / 5,280 ft): ~833 hPa average
- Mount Everest summit (8,849 m / 29,032 ft): ~317 hPa
- Commercial aircraft cabin (equivalent to ~2,400 m): ~750 hPa
Our temperature converter pairs with pressure when calculating altitude density or standard atmosphere conditions.