Dew Point Calculator
Find the dew point — the temperature at which air becomes saturated so moisture can condense as dew, frost, or fog.
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Measurements convert when you switch units
Result
The formula
Dew point is computed from air temperature and relative humidity using the Magnus–Tetens approximation, a standard engineering fit for vapor pressure over water. When RH approaches 100%, dew point approaches the air temperature.
Worked example
- Air temperature 20 °C
- Relative humidity 60%
- Dew point ≈ 12 °C
Result: ≈ 12 °C
How dew point is calculated
Dew point tracks absolute moisture in the air better than relative humidity alone.
Magnus approximation
We invert a Magnus-type saturation vapor-pressure formula. Constants a and b are empirical; results are accurate enough for everyday weather and HVAC use, not for laboratory humidity metrology.
RH vs dew point
Relative humidity rises when air cools even if moisture content is unchanged. Dew point stays roughly steady when moisture is steady — which is why meteorologists prefer it for “how muggy is it?”
Comfort and fog
Many people find dew points above ~18 °C (64 °F) muggy and above ~21 °C (70 °F) oppressive. When temperature nears dew point, fog or dew becomes more likely.
Interesting facts
Comfort cue
Many people find dew points above ~18 °C (64 °F) muggy, and above ~21 °C (70 °F) oppressive.
Fog clue
When air temperature nears the dew point, relative humidity approaches 100% and fog or dew becomes more likely.
Not the same as RH
Relative humidity depends on temperature. A cool morning can show high RH while the dew point (absolute moisture) stays modest.
Heat-index cousin
High dew points make hot days feel worse because sweat evaporates slowly — the same moisture that raises heat index.
Aviation and frost
Pilots watch temperature–dew-point spreads for fog risk; gardeners watch them for overnight frost potential.
Frequently asked questions
Relative humidity depends on temperature; dew point is an absolute moisture measure expressed as a temperature. High dew point means more moisture in the air.
No. At 100% RH they are equal; otherwise dew point is lower than air temperature for moist air over liquid water.
From temperature and RH using a Magnus vapor-pressure approximation, then solving for the temperature that would saturate the air at that moisture content.
Many people find dew points above about 18 °C (64 °F) muggy and above about 21 °C (70 °F) oppressive — personal comfort still varies.
When the dew point is at or below 0 °C (32 °F) and air temperature approaches it, moisture condenses as frost instead of liquid dew, and freezing fog becomes possible. Gardeners and pilots watch this spread closely overnight.
Heat index combines air temperature with humidity to estimate perceived heat, and high dew points are a major driver of it — moist air slows sweat evaporation, making the same temperature feel hotter.
Yes. In cold, dry air the dew point is often well below 0 °C — for example, a winter day at −5 °C air temperature might have a dew point near −15 °C, indicating very dry conditions.
References
- Dew point vs humidity Official explanation of dew point as a moisture measure.
- Magnus formula background Worked meteorological relationships among T, RH, and dew point.