Dew Point Calculator
Dew point from temperature and relative humidity, with the vapour pressure and absolute humidity.
Results
What this tool does
The dew point is the temperature air has to be cooled to before its water starts condensing, and unlike relative humidity it does not change as the air warms or cools — which makes it the honest measure of how much moisture is actually present. Uses the Magnus formula with Sonntag coefficients, accurate to about 0.4 °C between −45 and 60 °C.
Formula
γ = ln(HR÷100) + 17.62T ÷ (243.12+T) ; Td = 243.12γ ÷ (17.62−γ)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
t | Temperature | °C |
rh | Relative humidity | % |
TD | Dew point | °C |
GA | Difference | K |
VP | Vapour pressure | hPa |
SV | Vapour pressure | hPa |
AH | Absolute humidity | g/m³ |
Worked example
- Temperature22 °C
- Relative humidity65 %
- Dew point15.115 °C
- Difference6.885 K
- Vapour pressure17.1432 hPa
- Vapour pressure26.3742 hPa
- Absolute humidity12.5854 g/m³
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
How does this tell me if I will get mould?
Any surface colder than the dew point will grow condensation, and anything that stays wet will eventually grow mould. Measure the temperature of the coldest spot in the room — usually a window reveal, an external corner or behind a wardrobe on an outside wall — and compare it with the dew point here. Mould growth generally needs surfaces held above about 80 % relative humidity for extended periods, so the margin matters as much as the crossing point.