How it works
The dew point is the temperature air must be cooled to, at constant pressure, for its water vapor to start condensing. Unlike relative humidity, it does not change as the air warms and cools during the day, so it is a better measure of how much moisture is really in the air.
This calculator uses the Magnus formula with the WMO-recommended coefficients (a = 17.62, b = 243.12 °C), accurate to about ±0.35 °C between −45 °C and 60 °C.
Dew points above about 65 °F (18 °C) feel sticky and muggy for most people; above 70 °F (21 °C) feels oppressive. Surfaces colder than the dew point, like a cold drink or a window, collect condensation.
Formula
γ = ln(RH/100) + aT ÷ (b + T) Td = bγ ÷ (a − γ), a = 17.62, b = 243.12 °C
Questions people ask
Why is dew point better than humidity for comfort?
Relative humidity depends on temperature: 50% at 95 °F is very muggy, while 50% at 50 °F is dry. Dew point tracks the actual water content, so the same dew point feels the same at any temperature.
Can the dew point be higher than the temperature?
No. When the temperature falls to the dew point the air is saturated (100% RH) and fog, dew or clouds form.
How do I prevent condensation on windows or pipes?
Keep surfaces above the dew point, by insulating them, or lower the dew point indoors with ventilation or a dehumidifier.