Relative Humidity Calculator

Relative humidity (RH) is the ratio of the air's actual water-vapor pressure to the saturation vapor pressure at the same temperature, times 100. Absolute humidity is the actual mass of water vapor per unit volume (g/m³) — it does not depend on temperature. Dew point is the temperature at which the air would become saturated (RH = 100%). So RH describes how close air is to saturation at its current temperature, absolute humidity describes how much water is actually in the air, and dew point describes how close the air is to condensing.

This calculator uses the Magnus correlation of Alduchov and Eskridge (1996): the saturation vapor pressure is e_s(T) = 610.94 × exp(17.625 × T / (T + 243.04)) in pascals with T in °C. The actual vapor pressure e equals the saturation pressure at the dew point, e = e_s(Td). Then RH = 100 × e / e_s(T). For example, with air temperature 25°C and dew point 15°C, e_s(25) ≈ 3161.7 Pa and e_s(15) ≈ 1702.0 Pa, giving RH ≈ 53.8%.

Invert the same Magnus correlation: with α = ln(RH/100) + 17.625 × T / (T + 243.04), the dew point is Td = 243.04 × α / (17.625 − α), in °C. For example, at 30°C and 33% RH the dew point is about 12.0°C. Feeding that dew point back into the RH calculation returns 33%, which is how you can sanity-check the result.

A common guideline is 30–60% RH for indoor comfort: below 30% the air may feel dry and irritate skin and airways, above 60% it can feel muggy and promote condensation. But RH does not measure 'drying power' directly — evaporation rates are driven by the vapor-pressure deficit (the gap between saturation and actual pressure, in kPa), not by RH alone.

In real unsaturated air, no — the dew point can never exceed the air temperature; when they are equal, the air is saturated and RH is 100%. If you enter a dew point above the air temperature, this calculator clamps the result to 100% and notes that this indicates saturation (or measurement error).

This is a dedicated relative-humidity calculator: RH is the output, computed from several input routes (air temperature + dew point, air temperature + absolute humidity, inverse dew-point solve, and a wet-bulb inversion via Stull's method). The psychrometric calculator takes RH as an input and solves the full moist-air state, while the wet-bulb calculator is focused on wet-bulb temperature. For mixing ratio as the headline quantity, see the mixing-ratio-of-air calculator.