Dew Point Calculator
Free dew point calculator finds the dew point from temperature and relative humidity using the Magnus-Tetens formula, with a comfort scale and the math shown.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Dew Point Calculator Works
The dew point is the temperature at which the air becomes saturated and water vapor begins to condense. It is the single best indicator of how “muggy” the air feels. Enter the air temperature and relative humidity, and this calculator returns the dew point along with a comfort rating.
The Formula
This tool uses the widely accepted Magnus-Tetens approximation:
γ = (a·T)/(b + T) + ln(RH ÷ 100) Dew point = (b·γ)/(a − γ)
with constants a = 17.27 and b = 237.7°C, where T is air temperature in °C and RH is relative humidity in percent. Fahrenheit inputs are converted to Celsius first, then the result is converted back. This approximation is accurate to within about ±0.4°C for temperatures from 0°C to 60°C.
Worked Example
For 25°C air at 60% relative humidity:
- γ = (17.27 × 25)/(237.7 + 25) + ln(60 ÷ 100)
- γ = (431.75 ÷ 262.7) + (−0.5108) = 1.6435 − 0.5108 = 1.1327
- Dew point = (237.7 × 1.1327) ÷ (17.27 − 1.1327) ≈ 269.25 ÷ 16.137 ≈ 16.7°C (about 62°F)
So the air will start to form dew if it cools to roughly 16.7°C.
Dew Point Comfort Scale
| Dew point (°F) | How it feels |
|---|---|
| Below 50 | Dry, very comfortable |
| 50-55 | Comfortable |
| 56-60 | Slightly humid |
| 61-65 | Humid, noticeable |
| 66-70 | Very humid, uncomfortable |
| 71-75 | Oppressive |
| 76+ | Miserable, heat-stress risk |
Why Dew Point Matters
- It predicts fog and frost: when night temperatures fall to the dew point, dew or fog forms.
- It’s a better comfort gauge than relative humidity because it doesn’t swing with temperature.
- HVAC and greenhouse systems use it to prevent condensation on surfaces and equipment.
Frequently asked questions
What is the dew point?+
The dew point is the temperature to which air must be cooled, at constant pressure, for water vapor to start condensing into dew, fog or clouds. It is a direct measure of how much moisture is in the air: a higher dew point means more water vapor and a muggier feel.
How do you calculate dew point from humidity?+
The common Magnus-Tetens approximation uses γ = (a·T)/(b+T) + ln(RH/100), then dew point = (b·γ)/(a − γ), with constants a = 17.27 and b = 237.7°C, where T is air temperature in °C and RH is relative humidity in percent.
What dew point feels comfortable?+
A dew point below 55°F (about 13°C) feels comfortable and dry. From 55-65°F it becomes noticeably humid, 65-70°F feels muggy, and above 70°F the air is oppressive. Above about 75°F dew point, heat stress risk rises sharply.
What is the difference between dew point and relative humidity?+
Relative humidity is the percent of the maximum moisture the air can hold at its current temperature, so it changes as temperature changes even with the same moisture. Dew point is an absolute measure of moisture content and does not change unless the actual water vapor changes, making it a better comfort indicator.
Can the dew point be higher than the air temperature?+
No. The dew point can equal the air temperature when relative humidity is 100% (saturated air, often fog), but it can never exceed it. If a calculation suggested a higher dew point, the relative humidity input would be physically impossible.