Dew Point Calculator
Enter the air temperature and the relative humidity to get the dew point in °C — the temperature at which the air saturates and dew, fog, or condensation begins to form.
Temperature plus humidity
Enter the air temperature in °C and the relative humidity in percent and the calculator returns the dew point with the Magnus formula.
Use Celsius
Temperature is in degrees Celsius and humidity is a percentage from 1 to 100. At 100 % humidity the dew point equals the air temperature.
What is the dew point?
When air saturates
This dew point calculator turns the air temperature and the relative humidity into the dew point — the temperature to which the air must cool, at constant pressure, before water vapour starts to condense into dew, fog, or droplets. It tells you how much moisture the air actually holds, which is why meteorologists often trust the dew point more than the humidity percentage for describing how the air feels. A higher dew point means a wetter, muggier atmosphere; a low dew point means dry air. The calculation uses the Magnus formula, a well-established approximation that is accurate across everyday weather conditions.
Enter a temperature in °C and a relative humidity in percent to get the dew point instantly.
The Magnus formula uses two empirical constants, a = 17.625 and b = 243.04 °C. First an intermediate term γ is built from the humidity and temperature, then the dew point is derived from it.
dewPoint = b × γ / (a − γ), γ = ln(RH/100) + a × T / (b + T)The term γ combines the natural logarithm of the humidity fraction with a temperature term. Because the relative humidity enters through a logarithm and the temperature through a ratio, the dew point always sits at or below the air temperature, meeting it exactly when the humidity reaches 100 %.
Suppose the air temperature is 25 °C and the relative humidity is 60 %.
Take the humidity log term
ln(60 / 100) = ln(0.6) ≈ −0.5108 — the contribution from the humidity.
Add the temperature term
17.625 × 25 / (243.04 + 25) ≈ 1.6440, so γ ≈ −0.5108 + 1.6440 = 1.1332.
Solve for the dew point
243.04 × 1.1332 / (17.625 − 1.1332) ≈ 16.70 °C — the dew point. Below this temperature the air would start to condense.
The dew point is the temperature where the air becomes saturated and water vapour begins to condense into dew, fog, or droplets. For the example above, air at 25 °C with 60 % humidity has a dew point near 16.7 °C: cool a surface below that and condensation forms on it. The dew point is also a direct measure of comfort, because it tracks the absolute moisture in the air rather than the relative figure. As a rough guide, a dew point below about 13 °C feels comfortable and dry, the high teens feel noticeably humid, and 20 °C or more feels oppressive and muggy. This is why two days with the same relative humidity can feel completely different — the warmer day holds far more water and has the higher dew point. When the air cools all the way to its dew point, the relative humidity reaches 100 % and the dew point equals the temperature.
The Magnus formula is an approximation, so a couple of practical points are worth keeping in mind.
An approximation over a sensible range
The Magnus formula is an empirical fit, not an exact physical law. It is accurate to within a few hundredths of a degree across roughly −40 °C to +60 °C, which covers essentially all weather conditions, but it drifts at very extreme temperatures. Relative humidity must be between 1 and 100 % — values outside that range have no physical meaning here. The result assumes constant pressure and standard conditions over a liquid-water surface.