Dew Point Calculator
Find the dew point from air temperature and relative humidity.
About the Dew Point Calculator
The dew point is the temperature at which air becomes saturated and water condenses. Unlike relative humidity, it doesn't change with the day's temperature swing, so it's a better measure of how humid air feels: dew points above about 65 °F (18 °C) feel muggy. This uses the Magnus formula with Alduchov–Eskridge constants (accurate to about 0.4 °C for −40 to 50 °C).
The formula
Td = b γ ÷ (a − γ), γ = ln(RH ÷ 100) + a T ÷ (b + T), a = 17.625, b = 243.04 °C
Worked example
Example: Dew point = 22.4 °C (72.4 °F)
- Inputs: Temperature in: °F; Air temperature: 85; Relative humidity: 66%.
- Formula: Td = b γ ÷ (a − γ), γ = ln(RH ÷ 100) + a T ÷ (b + T), a = 17.625, b = 243.04 °C
- Result: Dew point = 22.4 °C (72.4 °F)
Questions people ask
Why does dew form at night?
Surfaces cool below the dew point.
Can dew point exceed temperature?
No; at 100% humidity they're equal.