Dew point calculator.

Worked example

Air at 20°C and 50% relative humidity has a saturation vapour pressure of 23.37 hPa, so the actual vapour pressure is 11.68 hPa. The dew point is the temperature at which 11.68 hPa would be the saturation pressure: 9.3°C. The spread between air and dew point is 10.7°C. If that same air warms to 30°C with no moisture added, the dew point stays 9.3°C but the relative humidity falls to about 27.5%.

How it works

The calculator finds the saturation vapour pressure at the air temperature, e_s = 6.112 × exp[17.67 × T / (T + 243.5)] hPa (T in °C), multiplies by the relative humidity to get the actual vapour pressure e, then inverts the same curve: T_d = 243.5 × ln(e / 6.112) / (17.67 − ln(e / 6.112)). At 100% humidity the dew point equals the air temperature.

This is an approximation of the saturation curve, accurate enough for weather and home use but not a laboratory standard. Other published fits differ by a fraction of a degree. Valid here for air temperatures from -60 to 60°C and humidity above 0%.

Sources and basis (all opened 2026-10-08): US National Weather Service El Paso, “Dewpoint and Wet-bulb from Relative Humidity” calculator (page script: es = 6.112 × exp[17.67T / (T + 243.5)], T_d = 243.5 ln(e/6.112) / (17.67 − ln(e/6.112))).

FAQ

What is the dew point?

It is the temperature to which air must be cooled, with its moisture content unchanged, for the relative humidity to reach 100%. This calculator computes it from air temperature and relative humidity.

What is the dew point at 25 degrees C and 60% humidity?

About 16.7 degrees C (62.1 degrees F), with a spread of 8.3 degrees C between the air temperature and the dew point.

Why does relative humidity change during the day but the dew point does not?

Relative humidity depends on the air temperature. If air at 20 degrees C and 50% warms to 30 degrees C without gaining moisture, its dew point stays 9.3 degrees C but its relative humidity falls to about 27.5%.

Can the dew point be higher than the air temperature?

Not in ordinary air. A dew point equal to the air temperature means 100% relative humidity, and this calculator does not accept humidity above 100%.

How is this different from the humidex?

Humidex combines temperature and dew point into one feels-like number. This tool only gives the dew point. Use the humidex calculator on this site for the feels-like value.