Relative Humidity Calculator
Calculate relative humidity from two thermometer readings — dry-bulb (air) temperature and the dew point. The calculator also gives saturation vapor pressure, actual vapor pressure, and absolute humidity.
°C
°C
Comfortable
- 1
Saturation VP at air temperature (Magnus)
6.1121 × exp((18.729 − 25/227.3) × 25 ÷ (257.87 + 25)) = 31.684 hPa - 2
Actual VP = saturation VP at dew point
6.1121 × exp((18.729 − 15/227.3) × 15 ÷ (257.87 + 15)) = 17.051 hPa - 3
Relative humidity
17.051 ÷ 31.684 × 100 = 53.8Ratio of actual vapor pressure to saturation vapor pressure, expressed as a percentage.
How does this calculator work?
Relative humidity is the actual vapor pressure divided by the saturation vapor pressure, expressed as a percentage. The saturation pressure follows the Magnus formula e(T) = 6.1121 × exp[(18.729 − T/227.3) × T/(257.87 + T)]. Enter air temperature and dew point to get RH, absolute humidity, and comfort category.
Formula
How this is calculated
Relative humidity (RH) is the ratio of how much water vapor is actually in the air to the maximum amount the air could hold at the same temperature. Air's maximum capacity is the saturation vapor pressure e_s(T), which rises steeply with temperature. The actual amount of water vapor present equals e_s at the dew point Td — the temperature to which air must be cooled (at constant pressure) before condensation begins.
This calculator uses the Magnus formula with the Alduchov & Eskridge (1996) coefficients — e(T) = 6.1121 × exp[(18.729 − T/227.3) × T/(257.87 + T)] in hectopascals — to compute saturation vapor pressure at both the dry-bulb temperature and the dew point. Relative humidity then equals e(Td)/e(T) × 100 %.
Absolute humidity (g/m³) is derived from the ideal-gas approximation: ρ_w ≈ 216.7 × e_act / (T_K), where T_K is air temperature in kelvin. This formula is valid for temperatures roughly −40 °C to +60 °C and all typical atmospheric pressures at sea level; it may diverge slightly at very high altitudes.
Frequently asked questions
The dew point is the temperature at which air becomes saturated and condensation begins. A dew point equal to the air temperature means 100 % RH (fog or rain). The wider the gap between air temperature and dew point, the lower the relative humidity.
Most people find 40–60 % RH comfortable. Below 30 % the air feels dry and may irritate the nose and throat; above 70 % it feels muggy and promotes mold growth. The calculator labels the range as "comfortable" for 40–60 %.
Yes — wet-bulb temperature can be converted to dew point using the psychrometric equation, but that conversion depends on atmospheric pressure. The dew point method used here is simpler and independent of pressure, making it more portable.
Also known as
TG we-Calculate Editorial Team. (2026). Relative Humidity Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/relative-humidity-calculator
TG we-Calculate Editorial Team. "Relative Humidity Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/relative-humidity-calculator.
TG we-Calculate Editorial Team, "Relative Humidity Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/relative-humidity-calculator
@misc{wecalculate_relative_humidity_calculator, title = {Relative Humidity Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/relative-humidity-calculator}}, year = {2026}, note = {TG we-Calculate} }
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