How to use the dew point calculator
- Choose °F or °C.
- Choose what to calculate: the dew point, or the relative humidity.
- Enter the air temperature.
- Enter the relative humidity to get the dew point, or the dew point to get the humidity.
- Read the result, the vapor pressure and absolute humidity, the working, and how the air is likely to feel.
How the dew point is calculated
The dew point is the temperature to which air must cool, at constant pressure, for its water vapor to start condensing. The calculator uses the Magnus formula for saturation vapor pressure over water, with the constants recommended in the World Meteorological Organization's guide to instruments (WMO-No. 8, from Sonntag 1990):
Relative humidity is the actual vapor pressure divided by the saturation vapor pressure at the air temperature. The dew point is the temperature at which the actual vapor pressure would be the saturation value. These constants hold from −45 °C to 60 °C (−49 °F to 140 °F). The calculator also reports absolute humidity, the grams of water vapor per cubic meter of air, from 216.7 × e ÷ (T + 273.15).
Worked example
The defaults are an air temperature of 80 °F with 60% relative humidity:
- Convert to Celsius: (80 − 32) × 5 ÷ 9 = 26.667 °C
- ln(60 ÷ 100) = −0.5108
- 17.62 × 26.667 ÷ (243.12 + 26.667) = 1.7416
- γ = −0.5108 + 1.7416 = 1.2308
- Td = 243.12 × 1.2308 ÷ (17.62 − 1.2308) = 18.26 °C
- In Fahrenheit: 18.26 × 9 ÷ 5 + 32 = 64.9 °F, on the edge of muggy
Going the other way, 80 °F air with a 65 °F dew point has a relative humidity of 60.3%. In Celsius, 27 °C at 60% humidity has a dew point of 18.6 °C, and 25 °C at 50% has a dew point of 13.9 °C.
How dew point feels
Dew point is a better guide to comfort than relative humidity, because it measures the actual amount of moisture in the air. The scale below is a general guide used by many forecasters; people used to humid climates may rate it differently.
| Dew point | How it feels |
|---|---|
| Below 50 °F (10 °C) | Dry |
| 50–59 °F (10–15 °C) | Comfortable |
| 60–64 °F (16–17 °C) | Slightly humid |
| 65–69 °F (18–20 °C) | Humid and muggy |
| 70–74 °F (21–23 °C) | Very humid, oppressive |
| 75 °F (24 °C) or more | Extremely humid, miserable |
Using the dew point
When the air temperature falls to the dew point, fog, dew or frost forms, so a small gap between the two on a calm evening is a sign of fog by morning. Indoors, any surface colder than the dew point, such as a cold-water pipe or a single-pane window, collects condensation; keeping indoor humidity lower prevents mold. Pilots use the temperature-dew point spread to estimate cloud base: roughly 1,000 feet for every 4.4 °F (2.5 °C) of spread near the ground. In hot weather, a high dew point is what makes the heat index climb.
Frequently asked questions
What is a comfortable dew point?
Most people find a dew point between 50 °F and 59 °F (10–15 °C) comfortable. Above 65 °F (18 °C) the air starts to feel sticky, and above 70 °F (21 °C) most people find it oppressive.
Can the dew point be higher than the temperature?
No. The dew point equals the air temperature when the air is saturated at 100% humidity, and it is lower at any other humidity. If you enter a dew point above the temperature, the calculator asks you to check it.
What is the difference between dew point and relative humidity?
Relative humidity is a percentage of how much moisture the air could hold at its current temperature, so it changes as the air warms or cools. The dew point measures the moisture itself and stays steady when the temperature changes during the day.
What happens below freezing?
The formula gives the dew point over liquid water. Over ice, vapor condenses as frost at a slightly higher temperature, called the frost point. The difference is about 1 °C near −10 °C.
Related calculators
- Heat Index Calculator — Heat index (feels-like temperature) from air temperature and humidity with the NWS method, in °F or °C.
- Wind Chill Calculator — Wind chill (feels-like) temperature with the NWS formula, in °F or °C, with the frostbite risk.
- Density Calculator — Solve ρ = m ÷ V for density, mass or volume in metric or US units, with common material densities.
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