Calculate the relative humidity based on dry bulb temperature and dew point.
Last updated: March 2026 | By ForgeCalc Engineering
Relative humidity (RH) is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain.
The dew point is the temperature to which air must be cooled to become saturated with water vapor. When the air temperature and the dew point are the same, the relative humidity is 100%.
Where:
• e is the actual vapor pressure
• e_s is the saturation vapor pressure
Warm air can hold more water vapor than cold air. Even if the absolute amount of water is the same, the relative humidity will be lower in warmer air, but the 'muggy' feeling comes from high dew points.
Most people find relative humidity between 30% and 50% to be comfortable. Below 30% can cause dry skin and respiratory irritation, while above 60% can feel sticky and promote mold growth.
In very clean air without condensation nuclei (like dust), air can become supersaturated (RH > 100%), but in nature, it almost always condenses into fog or rain once it hits 100%.
Dew point is often a better measure of comfort than RH. A dew point above 18°C (65°F) is considered sticky, and above 21°C (70°F) is very uncomfortable.
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