Dew Point Calculator
Find the dew point, relative humidity, or air temperature when you know the other two. The dew point is the temperature at which air becomes saturated and moisture can condense.
| Air temperature | |
| Relative humidity | % |
| Dew point |
Result
Your result will appear here.
Formula (Magnus approximation)
Using constants a = 17.62 and b = 243.12 °C (over water):
α = ln(RH/100) + (a × T) / (b + T)
Dew point Td = (b × α) / (a − α)
Where T and Td are in °C and RH is in percent. The same relationship can be rearranged to solve for RH or T when the other two are known.
Notes and assumptions
Relative humidity is between 0% and 100%. At 100% RH the air is saturated and the dew point equals the air temperature. If the dew point is below 0°C (32°F), frost can form instead of dew. This uses a common Magnus-form approximation for saturation vapor pressure over liquid water; results are very close for typical weather conditions but are estimates.
How it works
The dew point is the temperature air must cool to (at constant pressure) for moisture to condense. Higher dew point usually feels more humid.
Formula (Magnus approximation)
- With a=17.62, b=243.12°C and γ = (a·T/(b+T)) + ln(RH/100), Td = (b·γ)/(a − γ).
Worked example
25°C at 60% RH → dew point about 16.7°C. At 100% RH, dew point equals air temperature.
Estimate for general information only. This is a common approximation for typical conditions.
How to use the Dew Point Calculator
It takes a few seconds, and every figure is calculated in your browser.
- Enter the air temperature.
- Enter the relative humidity.
- Enter the dew point.
- Press Calculate — the result appears in the dark panel beside the form.
- Use Copy, CSV, Share or Print under the result if you want to keep it.
Dew Point Calculator — frequently asked questions
What formula does the Dew Point Calculator use?
Using constants a = 17.62 and b = 243.12 °C (over water): α = ln(RH/100) + (a × T) / (b + T) Dew point T d = (b × α) / (a − α) Where T and T d are in °C and RH is in percent. The same relationship can be rearranged to solve for RH or T when the other two are known.
How does the Dew Point Calculator work?
The dew point is the temperature air must cool to (at constant pressure) for moisture to condense. Higher dew point usually feels more humid.
What assumptions and limitations should I know about?
Relative humidity is between 0% and 100%. At 100% RH the air is saturated and the dew point equals the air temperature. If the dew point is below 0°C (32°F), frost can form instead of dew. This uses a common Magnus-form approximation for saturation vapor pressure over liquid water; results are very close for typical weather conditions but are estimates.
Can you walk through a worked example?
25°C at 60% RH → dew point about 16.7°C. At 100% RH, dew point equals air temperature.
Last updated 10 November 2026 · Formula, worked example and assumptions are documented above, and every result is calculated in your browser.