Dew Point Calculator
Calculate the dew point from air temperature and relative humidity.
Try an example
55.1°F
About this calculator
What Is the Dew Point?
The dew point is the temperature to which air must be cooled, at constant pressure, before it becomes saturated with water vapor and moisture starts to condense into dew, fog or frost. It is a direct measure of how much moisture is in the air: the higher the dew point, the muggier it feels. Give this dew point calculator any two of air temperature, relative humidity and dew point, and it finds the third.
Humidity Basics
Humidity is the amount of water vapor in the air. Absolute humidity is the mass of water vapor per volume of air, usually in grams per cubic meter. Relative humidity compares the amount of vapor present with the most the air could hold at its temperature, expressed as a percentage — and that maximum rises with temperature. That is why relative humidity alone can be misleading: 70% on a cold morning is a very different amount of moisture than 70% on a hot afternoon. The dew point, being an actual temperature, avoids that confusion.
Worked Example
At 75°F with 50% relative humidity, the dew point is about 55.1°F — a moderate, generally comfortable humidity level.
How Comfortable Is a Given Dew Point?
- Below 50°F (10°C): dry and comfortable.
- 50–60°F (10–16°C): pleasant.
- 60–65°F (16–18°C): getting humid; some stickiness.
- 65–70°F (18–21°C): muggy and uncomfortable.
- Above 70°F (21°C): oppressive; sweat evaporates poorly.
When the air temperature falls to the dew point, relative humidity is 100% and fog or dew can form. Frost forms when the dew point is below freezing.
The Calculation
A widely used approximation is the Magnus formula: with γ = ln(RH/100) + (b·T)/(c + T), the dew point is Td = (c·γ)/(b − γ), where T is in °C, b ≈ 17.62 and c ≈ 243.12 °C. It is accurate to within a fraction of a degree in normal weather conditions.
Where It Is Used
- Forecasting: predicting fog, dew, frost and cloud base heights.
- Home and building: avoiding condensation and mold on windows and walls.
- Comfort and health: deciding when to run a dehumidifier or air conditioning.
- Agriculture and aviation, where frost and fog risk matter.
To see how heat and humidity combine, use the Heat Index Calculator, and for cold weather see the Wind Chill Calculator.
Worked example
At 75°F with 50% relative humidity, the dew point is about 55.1°F — a moderate, generally comfortable level of humidity.
Frequently asked questions
Why is dew point a better humidity indicator than relative humidity?
Relative humidity depends on temperature (the same absolute moisture reads as a higher percentage on a cooler day), while dew point is a more direct measure of how much moisture is actually in the air — meteorologists generally consider it more useful for gauging how humid it will actually feel.
What dew point levels feel comfortable versus muggy?
Roughly: below 55°F feels dry and comfortable, 55-65°F feels noticeable but acceptable, and above 65°F starts to feel muggy or oppressive, with 70°F+ feeling quite uncomfortable to most people.
