Relative humidity (RH) tells you how close the air is to being saturated with water vapor at its current temperature — it's the number on every weather app and thermostat. This calculator computes RH three ways depending on what data you have (temperature and dew point, direct vapor pressures, or temperature and RH), and also converts to absolute humidity, the actual mass of water vapor per cubic meter of air.

How the Relative Humidity Calculator works

All three tabs rely on the same building block: the Magnus-Tetens approximation for saturation vapor pressure, e_s(T) = 6.112 × e^(17.62T / (243.12 + T)), using the WMO/CIMO Guide coefficients. On the dew-point tab, the calculator evaluates this formula at both the air temperature and the dew point — the dew point is, by definition, the temperature at which the current actual vapor pressure would be the saturation vapor pressure, so e_s(dew point) gives the actual vapor pressure directly. Relative humidity is then the ratio of actual to saturation vapor pressure, RH = 100 × e_actual / e_saturation. Absolute humidity converts a vapor pressure into a mass concentration via the ideal gas law for water vapor: AH = 216.7 × e (hPa) / T (Kelvin), in grams per cubic meter.

Inputs and what they mean

Air temperature and dew point are entered in °C — dew point is always less than or equal to air temperature (they're equal only at 100% RH). Vapor pressures are entered in hectopascals (hPa); at sea level, saturation vapor pressure ranges from under 1 hPa at very cold temperatures to over 100 hPa on a hot, humid day. Relative humidity is a percentage from 0% (bone dry) to 100% (saturated) — values above 100% are physically possible in supersaturated conditions like fog formation, and the calculator flags this rather than treating it as an error.

Limits and edge cases

The Magnus-Tetens approximation is accurate to within about 0.1% over its typical validity range, roughly −45°C to 60°C, and assumes standard atmospheric pressure — it does not apply a separate correction for high-altitude locations where total air pressure is lower. Dew point can never exceed air temperature in real conditions; if you enter a dew point above the air temperature, the calculator still computes a result (RH above 100%) but flags it as a supersaturated reading rather than a normal atmospheric state. This calculator is a meteorological approximation, not a substitute for a calibrated hygrometer when precise measurement matters (e.g. for lab or industrial humidity control).