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