Dew point and humidity calculator
Enter the air temperature and relative humidity to get the dew point, or the temperature and dew point to get the relative humidity. Absolute humidity and vapour pressure are shown too.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose Celsius or Fahrenheit and what you want to find.
- To find the dew point, enter the air temperature and the relative humidity.
- To find the relative humidity, enter the air temperature and the dew point instead.
- Read the result and the figures below it.
Saturation vapour pressure over water, in hPa, with temperature T in °C:
e_s = 6.112 · exp(17.62 · T / (243.12 + T))
With relative humidity RH in percent, let γ = ln(RH / 100) + 17.62 · T / (243.12 + T). The dew point is Td = 243.12 · γ / (17.62 − γ). In the other direction, RH = 100 · e_s(Td) / e_s(T).
Vapour pressure is e = e_s(T) · RH / 100. Absolute humidity in g/m³ is 216.7 · e / (T + 273.15), with e in hPa.
Worked example
Air at 20 °C and 50 percent humidity: e_s is about 23.33 hPa, so e is 11.66 hPa. γ = ln 0.5 + 1.3393 = 0.6462, so the dew point is 243.12 × 0.6462 / (17.62 − 0.6462) = 9.26 °C. The absolute humidity is 216.7 × 11.66 / 293.15 = 8.6 g/m³.
Notes
At 100 percent humidity the dew point equals the air temperature. Below freezing the formula keeps using the value over water, so the result is a dew point with respect to water, not a frost point.
Frequently asked questions
What is the dew point?
It is the temperature to which air must cool, at constant pressure and moisture, for water vapour to start condensing as dew. The closer it is to the air temperature, the more humid the air is.
Which formula does the calculator use?
The Magnus formula with the Sonntag 1990 coefficients, a = 17.62 and b = 243.12 °C, for saturation vapour pressure over water. It is valid from −45 °C to 60 °C.
Why can another calculator show a slightly different dew point?
Several coefficient sets of the Magnus formula are in use, for example 17.27 and 237.7 or 17.625 and 243.04. They give results that differ a little, so this page states the one it uses. Read the result to a tenth of a degree.
What is absolute humidity?
It is the mass of water vapour in one cubic metre of air, in grams. It is calculated from the vapour pressure and the temperature with the ideal-gas law for water vapour.