Barometric Pressure Calculator — Convert Units and Understand Weather Trends
Convert barometric pressure between hPa, mb, inHg, mmHg, atm, and kPa. Understand how pressure changes predict weather and altitude effects on pressure readings.
Your weather app shows 1013 hPa. Your old barometer reads 29.92 inHg. A European aviation chart lists QNH in millibars. They're all saying the same thing — standard sea-level pressure — just in different units. And the direction pressure is moving matters at least as much as the number itself.
Convert and interpret barometric pressure with the barometric pressure calculator on CalcHub.
Pressure Unit Conversions
| Unit | Standard Sea Level | Common Use |
|---|---|---|
| Hectopascal (hPa) | 1013.25 hPa | International meteorology, aviation |
| Millibar (mb) | 1013.25 mb | Same as hPa (1 hPa = 1 mb) |
| Inches of mercury (inHg) | 29.921 inHg | US weather stations, older barometers |
| Millimeters of mercury (mmHg) | 760.0 mmHg | Medical, legacy meteorology |
| Atmosphere (atm) | 1.0 atm | Science, reference |
| Kilopascal (kPa) | 101.325 kPa | SI unit, engineering |
| PSI | 14.696 psi | Industrial equipment |
How to Use the Calculator
- Enter pressure value in any unit
- Get instant conversions to all other pressure units
Pressure and Weather
Barometric pressure trends are more useful for weather prediction than absolute values. A rapidly falling barometer typically precedes deteriorating weather; a rising barometer suggests clearing and improving conditions.
| Pressure Change (3 hours) | Weather Implication |
|---|---|
| Rising > 1 hPa/hour | Rapid clearing, strong high pressure building |
| Rising 0.5–1 hPa/hour | Gradual improvement |
| Steady (< 0.5 hPa change) | Stable conditions likely to continue |
| Falling 0.5–1 hPa/hour | Deteriorating, rain or clouds approaching |
| Falling > 1 hPa/hour | Rapid deterioration, possible storm |
| Falling > 2 hPa/hour | Severe storm, very strong low pressure incoming |
Pressure vs. Altitude
Pressure decreases with altitude roughly exponentially. The rule of thumb: pressure drops about 1 hPa per 8 meters near sea level. At higher altitudes the rate changes, but here's a reference:
| Altitude | Approximate Pressure | O₂ as % of Sea Level |
|---|---|---|
| Sea level | 1013 hPa | 100% |
| 1,500 m (Denver) | ~845 hPa | 83% |
| 3,000 m | ~701 hPa | 69% |
| 5,500 m (Everest BC) | ~500 hPa | 50% |
| 8,849 m (Everest summit) | ~333 hPa | 33% |
Why do altimeters use barometric pressure?
GPS gives horizontal position accurately, but altitude is harder — GPS altitude has more error than horizontal position. Barometric altimeters, calibrated to current local pressure, give more reliable altitude readings for aviation, especially for approach and landing. Modern aircraft use both: GPS for primary navigation, barometric for altitude.
Does weather affect altimeter readings?
Yes, which is why pilots update their altimeter setting (QNH) throughout flights. In a low-pressure system, the actual altitude is lower than the altimeter indicates (setting not corrected). The aviation memory phrase: "From high to low, look out below." A false high-altitude reading can result in an aircraft being lower than indicated.
How do barometric pressure changes affect human health?
Some people experience joint pain, headaches, and sinus pressure with significant pressure changes — the mechanism is related to changes in fluid pressure in sinuses, joints, and the inner ear. The evidence is mixed but real for some individuals. Large pressure drops (approaching storms) are more commonly reported as symptomatic than pressure rises.
Related Calculators
- Wind Chill Calculator — complete the weather picture
- Dew Point Calculator — atmospheric moisture alongside pressure
- Cloud Base Calculator — altitude of cloud formation