Last updated October 10, 2026 · Report an error
Dew point is the temperature at which air becomes saturated with moisture and water starts to condense — a more reliable measure of how muggy the air actually feels than relative humidity alone.
This calculator finds the dew point from air temperature and relative humidity using the Magnus-Tetens approximation.
How it works
Enter the air temperature in °F and relative humidity as a percentage. The calculator converts temperature to Celsius, applies the Magnus formula \(\gamma = \ln(RH/100) + \dfrac{17.625T}{243.04+T}\), solves for dew point in Celsius, then converts back to Fahrenheit.
- Enter air temperature (°F).
- Enter relative humidity (%).
- Click Calculate to see your results.
Examples
75°F at 50% humidity
At 75°F with 50% relative humidity, the dew point is about 55°F — comfortable conditions for most people.
Who should use it
- Judging how muggy or comfortable outdoor conditions will feel.
- Estimating fog or frost risk when dew point approaches the air temperature.
Industry applications
- Meteorology and weather forecasting
- Agriculture and outdoor event planning
Advantages
- Uses the widely accepted Magnus-Tetens approximation for accurate results.
- Converts between Fahrenheit and Celsius automatically.
Limitations
- The Magnus approximation is slightly less precise at extreme temperatures far outside typical weather conditions.
Common mistakes to avoid
- Confusing dew point with relative humidity — a "50% humidity" day can feel very different depending on the actual temperature.
- Entering relative humidity as a decimal fraction instead of a percentage.
Best practices
- Use dew point rather than relative humidity percentage to judge how muggy conditions will actually feel outdoors.
Tips
- When the forecasted temperature and dew point are close together, expect fog, dew, or (below freezing) frost to form overnight.