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Dew Point Calculator: Temperature, Humidity and Condensation
Enter the air temperature with the relative humidity to get the dew point, or enter the air temperature with a known dew point to get the relative humidity. Results are in both Celsius and Fahrenheit.
What the dew point tells you
The dew point is the temperature to which air must be cooled, at constant pressure and moisture content, before it becomes saturated and water begins to condense. Relative humidity depends on the air temperature, so it falls when the air warms even though the amount of moisture is unchanged. The dew point does not change that way, which makes it a steadier measure of how much water vapor the air actually holds.
When a surface is colder than the dew point of the surrounding air, water condenses on it. This explains drips on a cold drink, fogged windows, damp on cold walls, and fog or frost outdoors when the night air cools to the dew point.
The Magnus formula
The calculator uses the Magnus approximation to the saturation vapor pressure curve with the parameters b = 17.625 and c = 243.04 °C. First compute an intermediate value, then solve for the dew point:
γ = ln(RH ÷ 100) + b × T ÷ (c + T)
Td = c × γ ÷ (b − γ)
For example, at 25 °C and 60 percent humidity, γ is about 1.1330 and the dew point is about 16.7 °C. The reverse calculation uses RH = 100 × exp(b·Td ÷ (c + Td) − b·T ÷ (c + T)). When the dew point equals the air temperature, the humidity is exactly 100 percent.
Comfort and practical guide
People often judge comfort by dew point rather than humidity. The scale below is an informal rule of thumb. It varies with person, activity and climate.
| Dew point | Typical feel |
|---|---|
| Below 10 °C (50 °F) | Dry and comfortable |
| 10 to 16 °C (50 to 60 °F) | Comfortable |
| 16 to 21 °C (60 to 70 °F) | Humid, getting sticky |
| Above 21 °C (70 °F) | Oppressive |
Indoors, keeping surface temperatures above the dew point helps prevent condensation and mold growth. For hot weather risk, see the heat index calculator.
Accuracy and limits
The Magnus formula with these constants is accurate to a small fraction of a degree over the normal range of weather, roughly −45 °C to 60 °C. It treats humidity over liquid water, so below freezing the value is the dew point rather than the frost point. Humidity sensors have their own errors, often a few percent, which feeds through to the result. This calculator is for weather, home and learning use and is not suitable for calibrating instruments or designing industrial drying and refrigeration systems, where a qualified engineer should use certified psychrometric data.
Worked example
Room air at 25 °C and 60% relative humidity.
Inputs
| What do you want to find? | Dew point (from temperature and humidity) |
|---|---|
| Temperature unit | Celsius (°C) |
| Air temperature (°C or °F) | 25 |
| Relative humidity | 60 % |
Result
Dew point 16.7 °C / 62.1 °F at 60.0% relative humidity. The air feels humid or sticky by a common informal scale.
| Air temperature | 25.0 °C / 77.0 °F |
|---|---|
| Relative humidity | 60.0% |
| Dew point | 16.7 °C / 62.1 °F |
| Dew point depression (air minus dew point) | 8.3 °C / 14.9 °F |
- A surface colder than the dew point collects condensation. Magnus constants used: b = 17.625, c = 243.04 °C.
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
What is the dew point?
It is the temperature at which air becomes saturated with water vapor when cooled. Below that temperature, moisture condenses into dew, fog or frost on surfaces.
How do I calculate dew point from humidity?
Use the Magnus formula: compute gamma as the natural log of humidity over 100 plus 17.625 times temperature over 243.04 plus temperature, then dew point is 243.04 times gamma over 17.625 minus gamma.
What dew point is uncomfortable?
Many people find dew points above about 21 °C (70 °F) oppressive, while values below 10 °C (50 °F) feel dry. The scale is subjective and depends on climate and activity.
Can the dew point be higher than the air temperature?
Not in normal conditions. At 100 percent relative humidity the dew point equals the air temperature, and values above that indicate fog or a measurement problem.
Why does condensation form on cold windows?
The glass surface is colder than the dew point of the room air. Air touching it cools to saturation and releases water, which collects as droplets on the pane.
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References and sources
- Alduchov, O. A. and Eskridge, R. E. (1996). Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology.
- WMO Guide to Meteorological Instruments and Methods of Observation: humidity measurement. Link