Relative Humidity Calculator

Relative humidity describes how much water vapor is present compared with the maximum amount the air could hold at its current temperature. It is expressed as a percentage. A reading of 100 percent means the air is saturated, while a lower reading means additional evaporation is possible. Because warm air has a higher saturation vapor pressure than cool air, relative humidity can change when temperature changes even if the actual amount of water vapor stays constant. This calculator uses air temperature and dew point, two common weather and indoor-air measurements. Dew point is the temperature at which the existing water vapor would saturate the air. The calculator applies the Magnus equation to both temperatures. Saturation vapor pressure is 6.1094 times the exponential of 17.625 times temperature divided by temperature plus 243.04. Actual vapor pressure is the saturation pressure evaluated at the dew point. Dividing actual pressure by saturation pressure at the air temperature and multiplying by one hundred gives relative humidity. The Magnus coefficients used here are well suited to ordinary atmospheric conditions from about minus 40°C to 60°C. Within that range the formula offers excellent practical accuracy for weather observations, building comfort checks, greenhouse monitoring, drying processes, and many educational calculations. It remains an approximation, and specialized high-accuracy or extreme-temperature work should use an appropriate reference formulation and calibrated instruments. Relative humidity and dew point answer different questions. Relative humidity indicates proximity to saturation at the current temperature, while dew point more directly represents absolute moisture content. If air cools toward its dew point, relative humidity rises. Once the temperature reaches the dew point, condensation can form on surfaces. That relationship matters when evaluating windows, ducts, cold pipes, insulation, storage areas, and any material vulnerable to moisture. For indoor comfort, values around 30 to 60 percent are often considered reasonable, although climate, activity, and building guidance vary. Persistently high humidity can encourage mold and condensation. Very low humidity can contribute to dry skin, irritated airways, and static electricity. Measurements should be taken away from direct sunlight, drafts, humidifiers, and cold surfaces, and sensors should be allowed to stabilize before their readings are used.