Dew Point Calculator
The dew point from air temperature and relative humidity, or the relative humidity from a dew point, by the Magnus equation, in °F or °C.
Result
Dew point: 62.06 °FEstimate
The Magnus approximation, accurate to within about 0.1 °C over ordinary weather conditions.
- Relative humidity
- 60%Estimate
- Air temperature minus dew point
- 14.94 °FEstimate
https://calcgallery.com/weather/dew-point-calculator/
Results come from the published equations applied to the values you enter. They are not electrical design, conductor sizing, laboratory safety or medical guidance. Weather indices describe a standard person in standard conditions and are not a forecast. Where codes apply, local rules vary and must be checked. Read the full disclaimer
How to use the Dew Point Calculator
- Choose whether to find the dew point from the humidity, or the humidity from a dew point.
- Enter the air temperature in °F or °C, and the relative humidity or the dew point.
- Read the dew point or the relative humidity, and the gap between the air and dew point temperatures.
The formula
With the air temperature T in °C and the relative humidity RH as a fraction:
γ = ln(RH) + 17.625 T ÷ (243.04 + T)dew point = 243.04 γ ÷ (17.625 − γ)Run backwards, the relative humidity from a dew point Td is exp(17.625 Td ÷ (243.04 + Td) − 17.625 T ÷ (243.04 + T)). The equation is published in Celsius; Fahrenheit entries are converted exactly on the way in and out.
Why 0% is not allowed
The equation takes the logarithm of the humidity, and the logarithm of zero is undefined. Air with no water vapour at all has no dew point, so the humidity field starts at 1%.
Condensation
The gap between the air temperature and the dew point is how far a surface can cool before it gets wet. That is what matters for condensation on windows, inside walls and on cold-water pipes, which is why the insulation calculator links here.
With your numbers
- T = 25 °C, dew point = 16.7 °C
- Relative humidity = 60%
Quick reference
The Magnus approximation, accurate to within about 0.1 °C over ordinary weather conditions.
| Example | Result |
|---|---|
| 25 °C, 60% | Dew point: 16.7 °C |
| 30 °C, 80% | Dew point: 26.17 °C |
| 20 °C, 100% | Dew point: 20 °C |
| 10 °C, 30% | Dew point: -6.8 °C |
| 77 °F, 60% | Dew point: 62.06 °F |
| Humidity at 25 °C with a 15 °C dew point | Relative humidity: 53.83% |
Common questions about the Dew Point Calculator
- What is the dew point?
- The temperature to which air must cool, at the same pressure and moisture content, for water vapour to start condensing. Any surface colder than the dew point gets wet: a cold drink, a window, a pipe, the grass at night.
- Why is dew point a better measure of mugginess than humidity?
- Relative humidity depends on temperature as well as moisture: the same air shows a higher percentage in the cool morning than in the warm afternoon. The dew point tracks the moisture itself. 25 °C at 60% humidity has a dew point of about 16.7 °C; the same moisture at 30 °C would read as only about 45%.
- Can the dew point be higher than the air temperature?
- No. At 100% humidity the two are equal, which is when fog or dew forms, and the page shows them equal exactly. If a dew point above the air temperature is entered, the page says the air would be more than saturated instead of computing a humidity above 100%.
- How accurate is the formula?
- The Magnus form with Alduchov and Eskridge's coefficients is accurate to within about a tenth of a degree over the range of ordinary weather. It is an approximation to the full thermodynamic relation, not a measurement.