Dew Point Calculator
Estimate dew point from air temperature in °C and relative humidity. Uses the Magnus-Tetens approximation with a = 17.625 and b = 243.04. Not a weather forecast or HVAC design.
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How to estimate dew point
- Enter air temperature in degrees Celsius between -45 °C and 60 °C.
- Enter relative humidity as a percent greater than 0 and at most 100.
- Read dew point in °C and °F with the air temperature used in the formula.
- Treat the result as an estimate for comfort and condensation awareness, not a forecast product.
Dew point
Temperature at which air becomes saturated for the given moisture content
| Relative humidity | Water vapor pressure over saturation vapor pressure |
|---|---|
| Air temperature | Dry-bulb temperature of the air parcel |
| Magnus coefficients | a = 17.625, b = 243.04 (°C form) |
| Formula | Td = b α / (a − α) with α = aT/(b+T) + ln(RH/100) |
| Condensation | Surfaces colder than Td can collect dew |
| Range | Approximation intended about -45 °C to 60 °C |
What dew point tells you
Dew point is the temperature where the current moisture content would saturate the air. Higher dew point means more absolute moisture. At 25 °C and 50% RH the Magnus estimate is about 13.9 °C.
Temperature, humidity, and saturation
Dry-bulb temperature is what a thermometer reads in the air. Relative humidity compares actual vapor pressure to saturation at that temperature. Dew point collapses those two into one moisture temperature.
How Magnus-Tetens is applied
The page builds α from temperature and RH, then Td = b α / (a − α). Results print in Celsius and Fahrenheit. Relative humidity of 0% is rejected because the logarithm is undefined.
Limits
Approximation for moist air in the stated range. Not a forecast, psychrometric chart replacement, or HVAC Manual design. See the disclaimer.