Dew Point
This free dew point calculator finds the dew point from air temperature and relative humidity, using the Magnus formula meteorologists rely on, plus a plain-language comfort rating from Dry to Oppressive. Nothing you enter is sent anywhere or stored.
How to Use It
- Enter the air temperature.
- Enter the relative humidity.
- The dew point and comfort level update instantly, no submit button.
How It Works
Dew point is the temperature air would need to cool to before it becomes fully saturated with the moisture already inside it. The Magnus formula estimates that temperature directly from the current air temperature and relative humidity, using constants fitted to real measurements of how water vapor behaves in air, the same approach the National Weather Service uses.
Formula
alpha = ln(RH / 100) + (17.625 × T) / (243.04 + T), with T in Celsius
dew point = (243.04 × alpha) / (17.625 − alpha), converted back to Fahrenheit
A worked example: at 75°F with 50% relative humidity, the formula gives a dew point of about 55.1°F, comfortable by most people's standards. At 85°F with 70% relative humidity, it rises to about 74.1°F, solidly muggy even though the air temperature only went up 10 degrees. That gap is the whole reason dew point, not relative humidity, is the number that actually predicts how sticky the air feels.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
Why do meteorologists use dew point instead of relative humidity?
Relative humidity alone does not say much without also knowing the temperature, since it is a ratio, not an amount: 50% humidity at 40°F holds far less actual moisture than 50% humidity at 90°F. Dew point measures how much moisture is actually in the air, independent of temperature, so a dew point of 70°F feels muggy whether the air temperature is 75°F or 95°F, while the same relative humidity number would not tell you that.
What counts as a comfortable dew point, and what counts as muggy?
Dew points below 55°F generally feel dry or comfortable, 55°F to 65°F is a range most people notice but tolerate, and above 65°F starts to feel humid, with anything above 70°F widely considered muggy or oppressive. This calculator labels the result on that same scale.
Can the dew point be higher than the air temperature?
No. The dew point is the temperature air would need to cool to before it becomes fully saturated with the moisture already in it, so it can equal the air temperature (100% relative humidity) but never exceed it. If a calculation ever produced a dew point above the entered air temperature, something in the inputs would be wrong.
Why does dew point change more slowly than relative humidity through the day?
Relative humidity swings with temperature even when the actual moisture in the air stays the same, since warmer air can hold more moisture before it is saturated, so the same moisture level reads as a lower percentage in the afternoon heat and a higher percentage overnight. Dew point tracks the moisture itself, so it stays comparatively steady over a day unless a genuinely different air mass moves in.
How does this differ from the Heat Index Calculator?
Dew point measures the actual moisture content of the air on its own. Heat index combines that humidity with the air temperature to estimate how hot it feels to a person. A muggy dew point contributes to a high heat index, but the two numbers answer different questions: use Dew Point for "how much moisture is in the air," and Heat Index for "how hot does it feel."
Can you give a real-world example?
At 75°F and 50% relative humidity, the dew point works out to about 55°F, a comfortable spring or fall day. At 85°F and 70% relative humidity, it climbs to about 74°F, solidly in muggy, oppressive territory even though the air temperature is only 10 degrees higher.