What this calculator does
Dew point is the temperature air would need to cool to before the moisture in it starts condensing into liquid water, whether as dew on grass, fog, or condensation on a cold glass. Unlike relative humidity, which changes with air temperature even if the actual moisture content stays the same, dew point tracks the real amount of water vapour in the air, which is why meteorologists treat it as the more honest measure of how humid a day actually feels.
A dew point below about 10°C feels comfortable and dry; above 20°C it starts to feel muggy, and above 24°C it is oppressive to most people regardless of the air temperature itself. This calculator uses the Magnus formula, a well-established approximation, from air temperature and relative humidity.
The formula
The Magnus formula first computes an intermediate value γ = (17.62 × T) ÷ (243.12 + T) + ln(RH ÷ 100), then solves for dew point = (243.12 × γ) ÷ (17.62 − γ). The constants 17.62 and 243.12°C are empirical values fitted to water vapour pressure over the typical weather range.
| Term | Meaning |
|---|---|
| Dew point | The temperature at which air, cooled at constant pressure and moisture content, would become saturated and start condensing. |
| Relative humidity (RH) | The current water vapour content as a percentage of the maximum the air could hold at its current temperature. |
| Dew point spread | Air temperature minus dew point; a small spread means the air is close to saturation, and fog or dew is more likely. |
The inputs explained
| Field | What to enter |
|---|---|
| Air temperature (°C) | The current air temperature. |
| Relative humidity (%) | The current relative humidity, as a percentage from just above 0% to 100%. |
When to use it
Judging how muggy a forecast will actually feel
Two days with the same air temperature but different dew points feel very different: a high dew point means sticky, humid air even if the thermometer reading looks mild, which is why dew point is often a better comfort indicator than relative humidity alone.
Anticipating fog, dew or frost
When overnight air temperature is forecast to drop close to the dew point, condensation (dew, fog, or frost if the dew point is below freezing) becomes likely, which matters for aviation, agriculture and simply knowing whether the car will need de-icing.
Preventing condensation on cold surfaces
A cold pipe, window or drink container will start to sweat once its surface temperature drops to the surrounding air's dew point, useful for planning insulation or simply expecting condensation indoors.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How dew point rises with relative humidity at a fixed air temperature
Dew point at a fixed 20°C air temperature, across a range of relative humidity levels.
Questions
What is dew point in simple terms?
It is the temperature at which the air currently holds as much moisture as it possibly can. Cool the air further than that and the excess moisture starts condensing out as liquid water, whether as dew, fog or condensation on a cold surface.
How is dew point different from relative humidity?
Relative humidity is relative to the current air temperature and rises automatically as air cools even if no water vapour is added or removed, which makes it a moving target. Dew point tracks the actual amount of moisture in the air directly, so it stays roughly constant through the day even as temperature and relative humidity swing.
What dew point feels comfortable?
As a rough guide, below 10°C feels dry, 10 to 16°C is comfortable for most people, 16 to 20°C starts to feel humid, and above 20°C feels muggy, with anything above 24°C considered oppressive by most people regardless of the air temperature.
Can dew point be higher than air temperature?
No. Dew point can equal air temperature (at 100% relative humidity, when the air is fully saturated) but cannot exceed it, since dew point represents the temperature the air would need to cool to in order to reach saturation.
For the fluid density behind related weather and water calculations, see the water density calculator.