Relative humidity calculator.

Worked example

Air at 25°C has a saturation vapour pressure of 31.67 hPa. A dew point of 15°C corresponds to a vapour pressure of 17.04 hPa. Relative humidity = 17.04 ÷ 31.67 × 100 = 53.8%. If the dew point stays at 15°C and the air cools to 20°C, the saturation pressure falls to 23.37 hPa and the relative humidity rises to 72.9%.

How it works

The calculator evaluates the saturation vapour pressure curve e_s(T) = 6.112 × exp[17.67 × T / (T + 243.5)] hPa (T in °C) at the dew point, to get the actual vapour pressure, and at the air temperature, to get the saturation value. Relative humidity is 100 × e(T_d) / e_s(T).

The curve is an approximation used by the National Weather Service. Other published fits differ slightly, so the third digit should not be treated as exact.

Sources and basis (all opened 2026-10-08): US National Weather Service El Paso, “Relative Humidity and Wet-bulb from Dewpoint” calculator (relative humidity = vapour pressure at the dew point ÷ saturation vapour pressure at the air temperature).

FAQ

How do you calculate relative humidity from dew point?

Divide the saturation vapour pressure at the dew point by the saturation vapour pressure at the air temperature and multiply by 100. This calculator does that with the National Weather Service form of the curve.

What is the relative humidity at 25 degrees C with a 15 degrees C dew point?

About 53.8%.

What happens to relative humidity when the air cools?

It rises, because the saturation vapour pressure falls while the actual vapour pressure stays the same. With a 15 degrees C dew point, air at 25 degrees C is 53.8% and air at 20 degrees C is 72.9%.

Why is the dew point not allowed above the air temperature?

That would mean more than 100% relative humidity, which this method does not treat as ordinary air.

Is this a measurement?

No. It is a calculation from the two temperatures you enter. A hygrometer measures humidity directly.