Dew Point Calculator
Find the dew point from air temperature and relative humidity, with a comfort rating.
Results update as you type.
About this calculator
The dew point is the temperature to which air must be cooled for its water vapour to start condensing into dew, and it is the most honest single measure of how much moisture the air actually holds. A higher dew point means more water in the air and a muggier, stickier feel; it is also the temperature at which fog forms, grass gets damp overnight and a cold drink beads with condensation. This dew point calculator derives it from the air temperature and the relative humidity you enter.
It uses the Magnus-Tetens approximation with the Alduchov-Eskridge coefficients (a = 17.625, b = 243.04 °C): first γ = ln(RH / 100) + aT / (b + T), then the dew point = bγ / (a − γ), with T in °C. As a worked example, air at 25 °C and 60% relative humidity has a dew point of about 16.7 °C — comfortable edging toward sticky. Drop the humidity to 30% at the same 25 °C and the dew point falls to roughly 6 °C, distinctly dry. The method is accurate to within a few tenths of a degree across ordinary weather, and the tool works in either °C or °F.
Dew point predicts comfort far better than relative humidity, because relative humidity depends on temperature: 90% humidity on a cold morning feels dry, while 60% on a hot afternoon feels oppressive. As an absolute measure, dew point cuts through that — below about 13 °C (55 °F) feels comfortable, 16–18 °C (60–65 °F) starts to feel sticky, and above 21 °C (70 °F) is oppressive and hard to cool down in. The same value tells you when condensation will form on windows, pipes and aircraft surfaces.
Frequently asked questions
What formula is used for the dew point?
The Magnus-Tetens approximation with Alduchov-Eskridge constants: γ = ln(RH/100) + 17.625·T/(243.04+T), and dew point = 243.04·γ/(17.625−γ), with T in °C. It is accurate to within a few tenths of a degree over normal weather conditions.
Why does dew point matter more than humidity?
Relative humidity depends on temperature, so 90% humidity at 10 °C feels dry while 60% at 30 °C feels muggy. Dew point is an absolute measure of moisture, so it tracks how sticky the air actually feels far better.
What dew point feels comfortable?
Below about 13 °C (55 °F) feels comfortable and dry, 16–18 °C (60–65 °F) starts to feel sticky, and above 21 °C (70 °F) is oppressive and hard to cool down in.
Can the dew point ever be higher than the air temperature?
No. At most the dew point equals the air temperature, which happens at 100% relative humidity when the air is fully saturated. Any cooler and vapour condenses, so the dew point is always at or below the current temperature.
How is dew point related to fog and dew forming?
When surfaces or the air cool to the dew point, vapour condenses — that is when dew settles on grass, fog forms and condensation beads on a cold glass. A dew point close to the overnight low is a good sign fog or dew is likely.
Does the dew point change when the temperature changes?
Not on its own — the dew point reflects the actual moisture content, which stays put as the air warms or cools. What changes with temperature is the relative humidity, which is why dew point is the steadier measure of muggy air.
How accurate is the Magnus-Tetens formula this uses?
With the Alduchov-Eskridge coefficients it is accurate to within a few tenths of a degree Celsius across the normal range of weather temperatures and humidities, which is well within the precision of everyday humidity readings.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/dew-point/curl
curl "https://calculator.free/api/v1/dew-point/?units=c&temp=25&humidity=60"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/dew-point/?" + new URLSearchParams({
"units": "c",
"temp": "25",
"humidity": "60"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.