Dew Point Calculator

What the dew point calculator does

The dew point is the temperature at which air becomes so full of moisture that water starts to condense out of it. This calculator finds the dew point from the air temperature and the relative humidity.

Below is what the dew point is, why it is a better guide to mugginess than humidity alone, the equation behind it, and a worked example.

How to use it

1. Enter the air temperature, in Celsius, Fahrenheit, or kelvin.
2. Enter the relative humidity as a percentage.
3. Press Calculate for the dew point, which you can read in your chosen temperature unit, or Reset to clear it.

What the dew point is

The dew point is the temperature to which air must be cooled, with its moisture held constant, for it to reach saturation. At that temperature the air is holding all the water vapour it can, and cooling even slightly further forces some of that vapour to condense into liquid water. It is, in effect, the temperature at which dew begins to form, which is where the name comes from.

Unlike relative humidity, which is a percentage that shifts as the temperature changes, the dew point is an actual temperature tied directly to the amount of moisture in the air. A higher dew point means more water vapour is present, full stop. This makes it a steady, honest indicator of how moist the air really is, which is why meteorologists often watch the dew point more closely than the relative humidity.

Why dew point measures muggy air

The dew point turns out to be the best simple guide to how humid the air feels, better than relative humidity for that purpose. The reason is that it tracks the actual moisture content, which is what governs whether your sweat can evaporate and cool you. When the dew point is high, the air is already laden with moisture, evaporation slows, and the day feels sticky and oppressive no matter what the thermometer says.

As a rough guide, a dew point below about 10 degrees Celsius feels comfortably dry, the middle teens feel noticeably humid, and a dew point above about 20 degrees feels muggy and uncomfortable, with the air close to tropical. Because the dew point reflects real moisture rather than a percentage of capacity, two days with the same relative humidity can feel completely different if their dew points differ, and the dew point is the figure that tells you which will be the sticky one.

The equation it uses

The calculator uses the August-Roche-Magnus approximation, the standard formula linking temperature, humidity, and dew point. It combines the air temperature with the relative humidity through a term that captures how saturated the air is, then converts that into the dew point temperature:

T_d = λ · γ ÷ (β − γ), where γ = ln(RH ÷ 100) + β · T ÷ (λ + T)

Here T is the air temperature, RH is the relative humidity, T_d is the dew point, and β and λ are the Magnus constants, about 17.625 and 243.04. The intermediate term γ folds together the temperature and humidity, and the dew point follows from it. The formula is an approximation, but an accurate one throughout the normal range of weather.

Dew, fog, and condensation

The dew point explains a whole family of everyday sights. Dew on the grass appears when the ground cools overnight until the air touching it drops to its dew point, and moisture condenses onto the cool surfaces. Fog is the same process happening in the open air, when a whole layer cools to its dew point and water condenses into a cloud at ground level.

The same effect fogs up a cold drink on a warm day: the glass chills the air around it below its dew point, and water condenses on the outside. It mists bathroom mirrors, forms condensation on cold windows in winter, and dictates the altitude at which clouds form as rising air cools. In every case the rule is the same: when a surface or a parcel of air falls to the dew point, the moisture it can no longer hold turns to liquid. Knowing the dew point tells you exactly when that will happen, which is what this calculator provides.

Working range and precision

The calculator works across the range where the Magnus approximation is reliable, roughly −45 to 60 degrees Celsius, covering all ordinary weather and indoor conditions, and it accepts and reports temperatures in Celsius, Fahrenheit, or kelvin. Relative humidity must lie between 0 and 100 percent. The formula is a close empirical fit rather than an exact law, with very small error across normal conditions. Results are shown to a couple of decimal places.

A worked example

Suppose the air temperature is 25 degrees Celsius and the relative humidity is 60 percent.

Feeding these into the Magnus relationship gives a dew point of about 16.7 degrees Celsius. So this air would need to cool to about 16.7 degrees for moisture to start condensing. With the dew point in the middle teens, the air would feel noticeably humid but not oppressive. If the dew point had come out above 20 degrees, the day would feel distinctly muggy.

Questions people ask

What does the dew point tell you?

The temperature at which air becomes saturated and moisture begins to condense. A higher dew point means more water vapour in the air, and it is a direct measure of moisture rather than a percentage.

Why is dew point better than humidity for comfort?

Because it reflects the actual moisture content, which governs how well sweat evaporates. A high dew point feels muggy regardless of the relative humidity figure, making it a more reliable comfort guide.

Why does dew form on cool mornings?

Because the ground and the air touching it cool overnight until they reach the dew point. At that temperature the air can no longer hold its moisture, so water condenses onto the cool surfaces as dew.

Can the dew point be higher than the air temperature?

No. The dew point is at most equal to the air temperature, which happens when the air is saturated at 100 percent humidity. Below saturation, the dew point is always lower than the temperature.

References

A quick note on where the science comes from. The dew point and its calculation through the August-Roche-Magnus approximation are standard meteorology, described by the US National Weather Service and in the Wikipedia article on dew point. The HyperPhysics overview from Georgia State University covers the underlying humidity and saturation concepts.

1. National Weather Service (NOAA), Dewpoint and humidity calculations. https://www.weather.gov/epz/wxcalc_rh
2. Wikipedia, Dew point. https://en.wikipedia.org/wiki/Dew_point
3. HyperPhysics, Relative Humidity and Dewpoint. http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html