Humidity and Dew Point Calculator
Calculate relative humidity, dew point temperature, and absolute humidity from dry-bulb temperature and either a wet-bulb reading or a known relative humidity value.
Input Method
Temperatures
Formulas Used
1. Saturation Vapour Pressure — Buck Equation (1981)
es(T) = 6.1121 · exp[(18.678 − T/234.5) · T / (257.14 + T)]
T in °C, es in hPa. Valid from −40 °C to +60 °C.
2. Actual Vapour Pressure from RH
e = (RH / 100) · es(Tdb)
3. Actual Vapour Pressure from Wet-Bulb — Sprung Formula
e = es(Twb) − γ · (Tdb − Twb)
where γ = 0.000665 · P (psychrometric constant, hPa/°C) and P is atmospheric pressure.
4. Atmospheric Pressure from Elevation — Barometric Formula
P = 1013.25 · (1 − 2.25577×10⁻⁵ · h)5.25588
5. Dew Point — Magnus Inversion
Td = 243.04 · ln(e / 6.1121) / [17.368 − ln(e / 6.1121)]
6. Absolute Humidity
AH = 216.7 · e / (273.15 + Tdb) [g/m³]
Assumptions & References
- Buck equation coefficients from Buck (1981) — accurate to ±0.05 % over −40 °C to 60 °C.
- Psychrometric constant γ = 0.000665 · P assumes a standard Assmann psychrometer (ventilated).
- Barometric formula assumes the International Standard Atmosphere (ISA) lapse rate.
- Magnus inversion uses Lawrence (2005) coefficients (Bull. Amer. Meteor. Soc.).
- Absolute humidity formula derived from the ideal gas law for water vapour (Mw = 18.015 g/mol).
- Wet-bulb temperature must be ≤ dry-bulb temperature; both must be in the range −60 °C to 60 °C.
- Relative humidity is capped at 100 % (supersaturation not modelled).
- ASHRAE comfort zone: 30 %–60 % RH at 20 °C–26 °C (ASHRAE Standard 55-2020).