With our dew point calculator, you can easily calculate the dew point for a given temperature and relative humidity.
Enter the temperature and relative humidity — the air-Q dew point calculator calculates the dew point from this, i.e. the temperature at which water vapor in the air begins to condense.
The dew point is the temperature at which air is saturated with water vapour, i.e. reaches 100 % relative humidity. When air or a surface cools below this temperature, water condenses: on window panes, cold exterior walls or behind furniture.
The calculator uses the Magnus formula with the parameters a = 17.62 and b = 243.12 °C, valid for air temperatures from −45 °C to 60 °C:
γ = ln(RH / 100) + (17.62 · T) / (243.12 + T)
Td = (243.12 · γ) / (17.62 − γ)
T is the air temperature in °C, RH the relative humidity in percent and Td the dew point in °C.
Example: At 20 °C and 60 % relative humidity, γ = ln(0.6) + (17.62 · 20) / 263.12 = −0.511 + 1.339 = 0.829. That gives Td = (243.12 · 0.829) / (17.62 − 0.829) ≈ 12.0 °C. Any surface in the room colder than 12 °C fogs up.
Air temperature | Relative humidity | Dew point |
|---|---|---|
20 °C | 50 % | 9.3 °C |
20 °C | 60 % | 12.0 °C |
20 °C | 70 % | 14.4 °C |
Rule of thumb: a wall whose surface temperature is at least 2 to 3 °C above the dew point of the room air stays dry.
The dew point describes the temperature at which relative humidity reaches 100% and the water vapor present in the air would immediately begin to condense into liquid water and form droplets.
We continue to improve the air-Q Dew Point Calculator and welcome your feature requests and suggestions.
For living spaces the dew point is therefore the decisive figure against mould: when a wall or window reveal cools below the dew point of the room air, moisture settles there, and from about 80 % humidity at the surface mould spores find ideal conditions. An air monitor such as the air-Q continuously shows temperature, humidity and the dew point calculated from them, and warns before it gets critical.
From air temperature and relative humidity with the Magnus formula: γ = ln(RH/100) + 17.62 · T / (243.12 + T), then Td = 243.12 · γ / (17.62 − γ). At 20 °C and 60 % this gives 12.0 °C. The calculator above does the maths; an air monitor such as the air-Q calculates the dew point continuously from its own readings.
The dew point is the temperature to which air must be cooled for the water vapour it contains to start condensing. It describes the absolute moisture of the air: the more water vapour it holds, the higher the dew point. On any surface colder than the dew point, dew or fogging forms.
It is not the dew point alone that matters but its distance to the coldest surface in the room. If the wall temperature is below the dew point, water condenses; mould already grows from about 80 % humidity at the surface. At 20 °C and 60 % room humidity (dew point 12 °C), cold corners at 13 to 14 °C are already at risk. Keep surfaces at least 2 to 3 °C above the dew point.
Relative humidity states what percentage of the maximum amount of water vapour possible at the current temperature the air contains; it changes with every change in temperature. The dew point is independent of temperature and describes the actual water content of the air. That makes it the better figure for predicting condensation.
It is rarely measured directly; it is calculated from temperature and relative humidity. A hygrometer or an air monitor such as the air-Q provides both values and displays the dew point continuously. With an infrared thermometer you can also check the surface temperature of critical walls and compare it with the dew point.
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