Dew point calculator

What is the dew point?

The temperature to which air must cool for water vapor to condense into dew. At 80°F and 60% relative humidity, the dew point is about 65°F.

How is the dew point calculated?

With the Magnus formula: γ = ln(RH ÷ 100) + 17.625T ÷ (243.04 + T), then dew point = 243.04γ ÷ (17.625 − γ), with T in °C. The constants are from Alduchov and Eskridge (1996).

What is the difference between dew point and relative humidity?

Relative humidity is how full the air is of water vapor compared with the most it can hold at that temperature, so it changes as the air warms or cools. The dew point measures the actual moisture and does not.

Can I find the humidity from the dew point?

Yes — pick "Humidity from dew point". At 80°F with a 65°F dew point, the relative humidity is about 60%.

Can the dew point be higher than the temperature?

No. When the air cools to its dew point, the humidity is 100% and water starts to condense.

Why does a cold glass or window get wet?

Because the surface is colder than the dew point of the air touching it. Air at 70°F and 40% relative humidity has a dew point of about 45°F, so any surface colder than that, such as a single-pane window on a freezing day, collects condensation. At 60% humidity the dew point rises to about 55°F, so the surface does not have to be as cold.

If the temperature falls overnight, what happens to the humidity?

Relative humidity climbs while the dew point stays the same, as long as no moisture is added or removed. Air at 80°F and 60% (dew point about 65°F) cooled to 75°F reads about 71%. Only when the temperature reaches the dew point does the humidity hit 100%, and below it dew or fog forms.

The Magnus formula and its range

The calculator uses the Magnus approximation to the saturation vapor pressure, with the constants of Alduchov and Eskridge: a = 17.625 and b = 243.04 °C. It takes the temperature T in °C and the relative humidity RH in percent, forms γ = ln(RH ÷ 100) + aT ÷ (b + T), and returns the dew point as bγ ÷ (a − γ). The formula is an empirical fit to a range of ordinary air temperatures, and it computes the dew point over liquid water; below freezing the frost point differs slightly. Relative humidity must be above 0%, because the logarithm of zero is not defined.

Worked example: a summer afternoon and a living room

At 90°F and 50% relative humidity the dew point is about 68.9°F. At 80°F and 60% it is about 64.9°F, and the same air at 70°F would have a relative humidity of 84%. Indoors, 70°F and 40% corresponds to a dew point of 44.6°F. The dew point is always at or below the air temperature, and equals it only at 100% relative humidity.

Common mistakes

Putting °F into the formula is the most common error, since T must be in °C; the calculator converts for you if you pick the unit. Comparing the relative humidity at two different temperatures as if it measured the same amount of moisture is another: 60% at 90°F contains far more water than 60% at 50°F, and the dew point shows the real moisture content. A third is using the dew point of the room air for a cold outside wall; condensation depends on the temperature of the surface, not of the room.

Sources: Alduchov, O. A. and Eskridge, R. E. (1996), “Improved Magnus form approximation of saturation vapor pressure,” Journal of Applied Meteorology 35, 601–609 (constants 17.625 and 243.04). Related: heat index calculator, dehumidifier size calculator, humidifier size calculator and growing degree days calculator.