Dew Point Calculator
This calculator estimates the temperature to which air must be cooled to become saturated with water vapor and form dew.
Please provide any two of the three variables below to calculate the third.
What is humidity?
Humidity measures how much water vapor—water in its gaseous state—is present in the atmosphere. It signals the likelihood of dew, frost, fog, and rain. The capacity of air to hold water vapor rises with temperature; warmer air can contain more vapor before it becomes saturated.
When discussing humidity we usually refer to absolute humidity and relative humidity, which this tool also uses. The calculation returns the absolute humidity value, but it is the relative humidity that people encounter daily and that feeds into the dew‑point computation.
Absolute humidity quantifies the mass of water vapor per unit volume of air, typically expressed in grams per cubic metre. It is obtained by dividing the total water‑vapor mass by the air’s volume. If the volume stays constant, absolute humidity remains unchanged even if temperature varies. In the open atmosphere, where volume shifts with temperature and pressure, absolute humidity will change accordingly.
Relative humidity expresses the current absolute humidity as a percentage of the maximum amount the air could hold at that temperature. A larger percentage means the air is closer to saturation. Both temperature and pressure influence this figure; the same water‑vapor content yields a higher relative humidity in cooler air than in warmer air.
Relative humidity is a staple metric in weather bulletins and forecasts, serving as a useful cue for rain, dew, frost, fog, and the feel of temperature. The term “apparent temperature” describes how hot or cold it feels to people. In hot weather, a high relative humidity raises the apparent temperature because moisture slows sweat evaporation, making us feel hotter.
When relative humidity reaches 100 %, the air is fully saturated, meaning no additional water vapor can be held under the present conditions. This is also the threshold at which moisture begins to condense into dew.
What is dew point?
The dew point is the temperature at which a parcel of air, at a specific pressure, becomes saturated with water vapor, leading to condensation and the appearance of dew. Dew appears as the tiny droplets on grass or flowers in the early morning. The more water vapor present, the higher the dew point; consequently, higher relative humidity pulls the dew point nearer to the ambient temperature. At 100 % relative humidity, the dew point equals the current temperature. If the dew point falls below the freezing point (0 °C or 32 °F), the vapor crystallizes directly into frost.
People’s comfort levels differ, and some can adjust to higher dew points, but generally a high dew point feels muggy because excess moisture hampers sweat evaporation, making it harder for the body to cool. Conversely, very low dew points can dry skin, cause cracks, and irritate the respiratory tract. OSHA advises maintaining indoor temperatures between 68 °F and 76 °F with relative humidity in the 20 %–60 % range.
In aviation, dew point information helps pilots assess risks such as carburetor icing and reduced visibility from fog. Specialized instruments called dew‑point hygrometers determine this value across a broad temperature span. They operate by cooling a polished metal mirror while air flows over it; the temperature at which a moisture film first appears on the mirror is recorded as the dew point.