Find relative humidity from air temperature and dew point — the reverse of the usual dew-point calculation.
How it works
Using the Magnus-Tetens approximation, air at 25°C with a dew point of 16.7°C works out to about 60% relative humidity. The closer the dew point sits to the air temperature, the higher the humidity.
What this does not include
This is the reverse of this site’s dew point calculator, which goes from temperature and humidity to dew point rather than back the other way. Both use the same approximation, accurate to within about 0.4°C-equivalent over typical ambient conditions.
How to use this calculator
- Enter the air temperature.
- Enter the dew point.
A worked example
A temperature of 25°C with a dew point of 16.6931°C: relative humidity ≈ 60%.
Temperature 20°C, dew point 9.2552°C: relative humidity ≈ 50%.
What the variables mean
| Variable | Meaning |
|---|---|
| Temperature | Current air temperature |
| Dew point | Temperature at which the air becomes saturated with moisture |
Edge cases worth knowing
The closer the dew point is to the actual temperature, the higher the relative humidity. At 100% humidity, dew point equals temperature exactly — the air is fully saturated.
A dew point higher than the actual temperature is physically impossible — dew point can never exceed air temperature, so the calculator declines to show a result for that combination.
Why must dew point always be lower than air temperature?
Dew point represents the temperature air would need to cool to before becoming saturated — since warmer air can hold more moisture, the dew point can never exceed the actual air temperature.
What does 100% relative humidity mean?
The air is fully saturated — its dew point equals the current air temperature exactly, which is why fog and condensation form at that point.
Is this the same calculation weather apps use?
Weather services often use more refined psychrometric formulas with additional correction terms, but the Magnus-Tetens approximation used here is accurate enough for everyday humidity estimates.
