Dew Point Calculator
Calculate the exact dew point temperature from air temperature and relative humidity using the Magnus-Tetens formula.
Calculator Inputs
Ambient air temperature, in degrees Fahrenheit.
Relative humidity, greater than 0 and up to 100 percent.
- Dew Point Temperature
- 62.03
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
How It's Calculated
Formula
This calculator finds the dew point — the temperature air would need to cool to for water vapor to condense at the current humidity — using the classic Magnus-Tetens approximation with coefficients a=17.27 and b=237.7°C. Air temperature is converted to Celsius for the formula and the result is converted back to Fahrenheit. At 100% relative humidity, the dew point exactly equals the air temperature, since the air is already fully saturated. This calculator reports only the dew-point temperature — it does not provide a separate comfort-level classification.
Worked Examples
77°F (25°C), 60% relative humidity
- Convert to Celsius: T = 25°C
- gamma = (17.27 x 25) / (237.7 + 25) + ln(0.60) ≈ 1.1329
- Td = (237.7 x 1.1329) / (17.27 - 1.1329) ≈ 16.68°C
- Convert back to Fahrenheit: ≈ 62°F
100% relative humidity
- At RH=100%, ln(1) = 0
- Dew point equals the air temperature exactly
Frequently Asked Questions
Why does 100% humidity give a dew point equal to the air temperature?
At 100% relative humidity, the air is already fully saturated with water vapor, so condensation begins immediately at the current temperature — the dew point and air temperature are the same.
Why convert to Celsius internally?
The Magnus-Tetens formula's coefficients (a=17.27, b=237.7) are calibrated for temperature in degrees Celsius. Air temperature is entered and displayed in Fahrenheit for convenience, but converted to Celsius for the calculation and converted back for the result.
Does this show a comfort-level rating?
No. This version reports only the numeric dew-point temperature. A comfort classification is a separate concern kept out of this calculator's current scope.