Dew Point Calculator — from temperature and humidity

How it works

The dew point is the temperature at which air becomes saturated and water vapour begins to condense into dew, fog or cloud. Unlike relative humidity — which changes as the air warms and cools — the dew point is an absolute measure of how much moisture the air actually holds, which is why meteorologists prefer it.

This calculator uses the National Weather Service method. It first finds the saturation vapour pressure at the current temperature, scales it by the relative humidity to get the actual vapour pressure, then inverts the relationship to find the temperature at which that vapour pressure would mean saturation:

T_d = 243.5 · ln(e / 6.112) / (17.67 − ln(e / 6.112))

Temperature is entered and returned in degrees Celsius, and humidity in percent. At 100% humidity the air is already saturated, so the dew point equals the air temperature.

Worked example

At 30 °C with 50% relative humidity, the dew point is about 18.5 °C — the air would need to cool to 18.5 °C for dew to form. Muggier air has a higher dew point relative to how it feels: 20 °C at 80% gives 16.4 °C. And at 30 °C with 100% humidity the air is saturated, so the dew point is 30 °C, exactly the air temperature.

Frequently asked questions

What is the dew point?

The dew point is the temperature to which air must be cooled, at constant pressure, for its water vapour to reach saturation and start condensing. It is a direct measure of the amount of moisture in the air.

What is the difference between dew point and relative humidity?

Relative humidity is the percentage of the maximum moisture the air *could* hold at its current temperature, so it changes as the temperature changes even if the actual moisture does not. The dew point is the absolute moisture content — it stays the same as the air warms or cools, making it a better comfort indicator.

What dew point feels humid?

As a rule of thumb, a dew point below about 10 °C feels dry, 10–16 °C feels comfortable, 16–21 °C starts to feel humid, and above 21 °C feels oppressive. The higher the dew point, the muggier and more uncomfortable the air.

Why does the dew point equal the air temperature at 100% humidity?

At 100% relative humidity the air is already saturated — it cannot hold any more water vapour — so it is already at its dew point. No further cooling is needed for condensation, which is why fog and dew form when the temperature drops to the dew point.

Can the dew point be higher than the air temperature?

No. The dew point is always at or below the air temperature, because air cannot hold more than 100% of its saturation moisture. When the temperature falls to the dew point, the excess vapour condenses out as dew, fog or clouds.

How is the dew point related to comfort and the heat index?

A high dew point means humid air, which slows sweat evaporation and makes it harder to cool off — the same effect the [heat index](heat-index-calculator) captures. Dew point and relative humidity are two ways of describing that moisture; the heat index uses humidity together with temperature to give a feels-like value.