Enter any two of air temperature, relative humidity, and dew point, then leave the third blank. The calculator finds the missing value along with water vapor pressure, absolute humidity, and moisture concentration.
The dew point calculator on this page links the three quantities that describe moisture in the air - air temperature, relative humidity, and dew point temperature. Provide any two of them and the calculator finds the third, then goes further to report the water vapor pressure, the saturation vapor pressure, the absolute humidity, and the moisture concentration by volume and by weight. Whether you are a meteorology student, an HVAC technician, a grower, a pilot, or simply someone trying to understand why a summer day feels so oppressive, this tool turns a couple of familiar numbers into a complete picture of the moisture in the air.
You can enter temperatures in Celsius, Fahrenheit, or Kelvin, and mix units freely between the air temperature and dew point fields. Results are displayed in all three temperature scales so you never have to convert by hand.
Humidity is the amount of water vapor present in the air. Because air can hold different amounts of water vapor depending on its temperature, humidity is described in several different ways, and the distinctions matter.
Relative humidity (RH) is the measure most people know from weather reports. It expresses the current water vapor content as a percentage of the maximum the air could hold at that temperature. At 100% relative humidity the air is saturated and can hold no more moisture, so dew, fog, or rain becomes likely. The catch is that relative humidity depends heavily on temperature: as air warms, its capacity to hold moisture rises, so the same amount of vapor produces a lower relative humidity. This is why relative humidity falls during the warmth of the afternoon and climbs again at night even when no moisture has been added or removed.
Absolute humidity is the actual mass of water vapor in a given volume of air, expressed in grams per cubic meter (g/m³). Unlike relative humidity, it does not depend on temperature - it tells you exactly how much water is in the air. Warm air can carry far more water vapor than cold air, which is why a humid tropical day has a high absolute humidity while a freezing winter day, even at 100% relative humidity, has very little.
The dew point is the temperature to which air must be cooled, at constant pressure and constant water vapor content, for it to become saturated - that is, to reach 100% relative humidity. Cool the air below its dew point and water vapor begins to condense into liquid water, forming dew on grass, fog in the air, or droplets on a cold glass. When the dew point is below freezing, water vapor deposits directly as frost instead of dew.
What makes dew point so valuable is that it is an absolute measure of moisture. Unlike relative humidity, it does not swing up and down with the daily temperature cycle. A dew point of 20°C means the same amount of moisture in the air whether the thermometer reads 25°C or 35°C. The closer the dew point is to the current air temperature, the more saturated - and the more humid and uncomfortable - the air feels. When dew point equals air temperature, relative humidity is 100%.
This calculator uses the Arden Buck equation, a highly accurate relationship between temperature and saturation vapor pressure that is widely used in meteorology. The saturation vapor pressure (the pressure of water vapor when the air is fully saturated) at temperature T in degrees Celsius is:
es(T) = 6.1121 × exp[ (18.678 − T / 234.5) × (T / (257.14 + T)) ] (hPa)
The actual water vapor pressure is then the saturation pressure scaled by the relative humidity:
ea = es(T) × RH / 100
Finally, the dew point is the temperature at which the saturation vapor pressure equals this actual vapor pressure. In other words, the calculator finds the temperature Td for which es(Td) = ea. The Arden Buck equation is accurate to a small fraction of a percent across the full range of temperatures encountered in the atmosphere, which is why it is preferred over simpler approximations for precise work.
Working in the other directions uses the same relationships. If you know the air temperature and dew point, the relative humidity is RH = 100 × es(Td) / es(T). If you know the dew point and relative humidity, the air temperature is the value of T that satisfies the same saturation equation.
The saturation water vapor pressure is the partial pressure water vapor would exert if the air were completely saturated at the current temperature. The water vapor pressure (actual partial pressure) is how much pressure the water vapor currently exerts. Their ratio, multiplied by 100, is the relative humidity. Both are reported in pascals (Pa).
The calculator reports absolute humidity in grams of water vapor per cubic meter of air. It is derived directly from the actual vapor pressure and the air temperature. At 20°C and 65% relative humidity, for example, the absolute humidity is about 11.2 g/m³. At 30°C and 50% relative humidity it rises to roughly 15.2 g/m³, even though the relative humidity is lower - a vivid demonstration of how much more moisture warm air can carry.
The moisture volume concentration expresses water vapor as a fraction of the total air, in parts per million (ppm) and percent, based on the ratio of vapor pressure to total atmospheric pressure. The moisture weight concentration does the same on a mass basis, accounting for the fact that a water molecule is lighter than the average air molecule. These figures are widely used in industrial drying, compressed air systems, and gas analysis.
Relative humidity is familiar, but dew point is the better comfort indicator in warm weather, and the reason is its stability. Consider a dew point of 20°C. At an air temperature of 25°C that corresponds to about 74% relative humidity; at 35°C the same dew point is only about 41% relative humidity. The relative humidity numbers look very different, yet the air holds the same amount of moisture and feels similarly muggy because sweat cannot evaporate efficiently. Meteorologists therefore lean on dew point to convey how oppressive the air really is.
While comfort is subjective and people acclimatize, the following general guide is widely used:
Because dew point remains stable as temperature changes, forecasters use it to gauge the true moisture content of an air mass. The gap between air temperature and dew point - the temperature-dewpoint spread - shows how close the air is to saturation. A small spread of a few degrees signals likely fog, low clouds, or precipitation, while a large spread indicates dry air.
In heating, ventilation, and air conditioning, dew point determines where condensation will form. When a surface drops below the surrounding air's dew point, moisture condenses on it. This is why cold pipes sweat in summer, why windows fog in winter, and why poorly insulated walls can grow mold. Engineers size dehumidifiers, choose insulation, and set humidity targets using dew point.
The temperature-dewpoint spread is a standard part of aviation weather reports. A narrow spread warns pilots of probable fog, and dew point near or below freezing flags the risk of icing on wings, carburetors, and runways.
Growers track dew point because prolonged leaf wetness from dew encourages fungal diseases such as mildew and blight. Dew point forecasts inform irrigation timing, spraying schedules, and frost protection on cold, clear nights.
Electronics, pharmaceutical, painting, and food industries control dew point tightly to prevent condensation, corrosion, static discharge, and spoilage. Compressed-air systems monitor dew point to keep moisture out of tools and pipelines.
On clear, calm nights, surfaces such as grass and car roofs lose heat by radiating it to the sky and can cool below the surrounding air. When a surface cools to the dew point, water vapor in the adjacent air condenses onto it as dew. If the dew point is below 0°C, the vapor deposits directly as ice crystals, producing frost. The temperature at which this happens below freezing is sometimes called the frost point, though in everyday use it is treated as the dew point.
No. The dew point can never exceed the air temperature. If a calculation seems to produce one, the inputs are physically impossible - for instance a relative humidity above 100%. When dew point equals air temperature, the air is fully saturated at 100% relative humidity.
Most people find dew points below about 13°C (55°F) comfortable, with 10-15°C considered ideal. Discomfort grows above 16°C (60°F), and dew points above 21°C (70°F) feel oppressive to most people.
Relative humidity is a percentage of the air's moisture capacity, and that capacity rises as the air warms. So when the temperature climbs during the day, the same amount of moisture represents a smaller percentage and relative humidity falls, even though the dew point - the absolute moisture measure - stays nearly constant.
Both describe atmospheric moisture, but the wet-bulb temperature is the lowest temperature achievable by evaporative cooling and always sits between the air temperature and the dew point. The dew point is the temperature at which the air itself becomes saturated. At 100% relative humidity all three are equal.
It uses the Arden Buck equation, which is accurate to a small fraction of a percent across normal atmospheric temperatures. For everyday weather, HVAC, agriculture, and general science it is more than precise enough.
Any of them. The calculator accepts Celsius, Fahrenheit, or Kelvin for both air temperature and dew point, converts internally, and shows results in all three scales, so use whichever you prefer.
This Dew Point Calculator is provided for educational and general informational purposes. It uses standard meteorological formulas and is suitable for weather observation, HVAC estimates, agriculture, and general science. For safety-critical or regulated applications, confirm results with calibrated instruments and professional guidance.