Free relative humidity calculator. Find RH from air temperature and a dew point or wet-bulb reading, with an indoor comfort scale and dew point.
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Relative humidity indicates how much moisture is in the air compared to the maximum it can hold at that temperature. Our calculator uses the Magnus formula to accurately compute relative humidity from either the dew point or wet bulb temperature.
Relative humidity (RH) is the ratio of the water vapor actually in the air to the most it could hold at that temperature, as a percentage. At 100% RH the air is saturated and fog or dew forms. Because warm air holds far more moisture than cold air, RH is RELATIVE — the very same air can read 50% in the afternoon and 90% overnight without any moisture being added, simply because it cooled. That is why forecasters prefer the dew point, which measures the absolute moisture and barely changes. Indoors, 30–50% RH is the healthy comfort range. To convert the other direction, use our dew point calculator.
Relative Humidity Formula
Monitor and control indoor humidity for building comfort and health.
Weather stations calculate humidity for forecasting precipitation.
Greenhouses and storage facilities monitor humidity for optimal conditions.
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Maintain optimal 30-50% humidity for health and comfort indoors.
Predict fog, dew, and precipitation likelihood from humidity levels.
Control humidity for manufacturing, storage, and agriculture.
For most people, 30-50% relative humidity is ideal indoors. Below 30% can cause dry skin and respiratory issues. Above 60% feels sticky and promotes mold growth.
Warmer air can hold more water vapor. When temperature rises but moisture stays constant, relative humidity drops. This is why heated indoor air in winter often feels very dry.
Dew point is the temperature at which air becomes saturated (100% RH). Wet bulb is the lowest temperature achievable through evaporative cooling. Both can be used to calculate relative humidity.
In theory, air can become supersaturated above 100% briefly before condensation occurs. In practice, 100% is the maximum as excess moisture condenses into fog or dew.