Barometric Pressure Converter
Convert barometric pressure between inHg, hPa, millibar, atm, and psi.
Includes a weather forecast guide for high, normal, and low pressure readings.
Barometric pressure (atmospheric pressure) is the weight of the air column above a given point. It is measured in hectopascals (hPa), millibars (mb), millimeters of mercury (mmHg), or inches of mercury (inHg). Pressure changes predict weather; altitude affects pressure at a known rate.
Key formulas:
Pressure change rate for weather prediction: Trend = (Current Pressure − Pressure 3 Hours Ago) ÷ 3 hours
Barometric Altitude Formula: Altitude = 44,330 × (1 − (P ÷ P₀)^0.1903) meters where P₀ = sea level pressure (1013.25 hPa standard)
Pressure at altitude (simplified): P = P₀ × (1 − 0.0000226 × h)^5.256 where h = altitude in meters
Unit conversions:
- 1 hPa = 1 mb = 0.02953 inHg = 0.75006 mmHg
- Standard sea level: 1013.25 hPa = 29.92 inHg = 760 mmHg
Weather interpretation of pressure trends (per 3 hours):
- Rising > 1 hPa: clearing weather likely
- Steady (±1 hPa): stable conditions
- Falling 1–2 hPa: rain possible
- Falling > 2 hPa: storm approaching
- Falling > 5 hPa: severe storm warning
Worked example — altitude from pressure: Your barometer reads 900 hPa. What is your approximate altitude?
Altitude = 44,330 × (1 − (900 ÷ 1013.25)^0.1903) = 44,330 × (1 − 0.9879) = 44,330 × 0.0121 ≈ 988 meters (about 3,240 feet)
Reference pressures:
- Dead Sea (lowest land): ~1,065 hPa | Sea level: 1,013.25 hPa | Denver (5,280 ft): ~843 hPa | Everest summit: ~337 hPa
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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