Dew Point Calculator

The dew point is the temperature to which air must be cooled, at constant pressure and moisture content, for its water vapor to start condensing into dew, fog or cloud. Enter the air temperature and relative humidity to get the dew point, or enter the temperature and dew point to get the relative humidity. Temperatures can be in Celsius or Fahrenheit. A chart shows how the dew point changes with humidity at your temperature.

γ = ln(RH ÷ 100) + a × T ÷ (b + T)    dew point = b × γ ÷ (a − γ)

RH = 100 × exp(a × Td ÷ (b + Td) − a × T ÷ (b + T))

T is the air temperature and Td the dew point, both in degrees Celsius. The constants a = 17.625 and b = 243.04 °C come from Alduchov and Eskridge (1996), who fitted them to measured saturation vapor pressures; the formula is accurate to a fraction of a degree between about −40 and 50 °C. Fahrenheit inputs are converted to Celsius first.

Warm and humid. At 25 °C (77 °F) and 60% relative humidity, the dew point is 16.7 °C (62.1 °F).

Humidity from dew point. If the air is 30 °C and the dew point is 20 °C, the relative humidity is 55.1%.

Relative humidity depends on temperature: the same amount of moisture gives a high relative humidity on a cool morning and a low one in the afternoon heat. The dew point depends only on how much water vapor is in the air, so it is a steadier guide to how muggy it feels. Meteorologists often describe dew points in the 50s °F as comfortable for most people, the 60s °F as increasingly sticky, and 70 °F or above as oppressive, though perceptions vary.

• Fog and frost: when the evening temperature is expected to fall to the dew point, fog or dew is likely; below freezing, frost forms instead.
• Buildings: condensation forms on any surface colder than the dew point of the indoor air, such as windows and cold pipes, which can lead to mold.
• Painting and coatings: many coating specifications require surfaces to be several degrees above the dew point while work is done.
• Compressed air and HVAC: dew point indicates how dry the air is and how much cooling is needed to remove moisture.

1. Choose whether you want the dew point or the relative humidity.
2. Enter the air temperature, in Celsius or Fahrenheit, from a thermometer or a weather report.
3. Enter the relative humidity from a hygrometer or forecast, or the dew point if you are working the other way.
4. Read the result in both units. The spread between the temperature and the dew point tells you how close the air is to saturation.

Indoors, many guides suggest keeping relative humidity roughly between 30 and 50 percent to limit mold and dust mites while avoiding very dry air. In winter, check the dew point of your indoor air against the temperature of your windows: if the glass is colder than the dew point, condensation will form. Lowering indoor humidity or improving window insulation reduces it.

High humidity makes hot weather feel hotter, which the heat index calculator estimates using the National Weather Service equation. Wind makes cold weather feel colder, which the wind chill calculator covers. For gas behavior more generally, the ideal gas law calculator relates pressure, volume and temperature.