Dew Point Calculator
Find the dew point from temperature and humidity.
Results update as you type.
How it works
The dew point is the temperature to which air must cool for water vapour to start condensing into dew, fog, or clouds. This dew point calculator uses the widely trusted Magnus formula to derive it from the air temperature and relative humidity you enter. Unlike relative humidity, which changes as the air warms or cools, the dew point is an absolute measure of how much moisture the air actually contains, which makes it the meteorologist's favourite gauge of muggy or dry conditions. This matters for comfort, weather, and building science. A high dew point above roughly 20°C feels sticky and oppressive because your sweat cannot evaporate, while a low dew point feels crisp and dry. Forecasters use it to predict fog, frost, and thunderstorms, and engineers use it to prevent condensation that causes mould, corrosion, and window fogging. A dew point calculator turns a temperature and humidity reading into this single, meaningful comfort and moisture indicator. The formula works by finding the temperature at which the air's current moisture content would saturate it completely. Because the dew point can never exceed the air temperature, cooling air toward its dew point raises relative humidity until condensation begins. Weather watchers, HVAC designers, farmers, and pilots all rely on a dew point calculator to anticipate moisture behaviour and judge how the air will really feel.
γ = (a·T)/(b+T) + ln(RH/100); dew point = b·γ/(a−γ), a = 17.27, b = 237.7
The dew point is the temperature at which air becomes fully saturated and water vapour begins to condense, so it depends on both how warm the air is and how much moisture it holds. The Magnus formula works by relating saturation vapour pressure to temperature through an exponential fit, then using relative humidity to find the temperature at which the current moisture content would saturate the air. The intermediate term gamma combines the temperature effect and the logarithm of humidity, and solving for the saturation temperature yields the dew point directly in Celsius.
Source:Magnus-Tetens dew point approximation (NWS)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter air temperature and relative humidity.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Dew point updates instantly as you type — no button to press.
Frequently asked questions
How is dew point different from relative humidity?
Relative humidity is the percentage of moisture the air holds compared with the most it could hold at that temperature, so it changes as the air warms or cools even when the actual moisture stays fixed. Dew point is an absolute temperature that reflects the real amount of water vapour present. Because it does not swing with temperature, dew point is a more reliable indicator of how humid or muggy the air genuinely feels.
What dew point feels comfortable?
A dew point below about 10 degrees Celsius feels dry and comfortable, while 10 to 16 degrees feels pleasant to slightly humid. From 16 to 20 degrees the air starts to feel sticky, and above 20 degrees it becomes oppressive and uncomfortable because sweat evaporates poorly. Dew points above 24 degrees are rare and feel oppressively tropical. These bands make dew point a handy shorthand for describing summer comfort.
Can the dew point be higher than the air temperature?
No. The dew point can equal the air temperature, which happens at 100 percent relative humidity when the air is fully saturated and fog or dew forms, but it can never exceed it. If it could, the air would hold more moisture than physically possible. As air cools toward its dew point, relative humidity rises to 100 percent, and any further cooling forces water vapour to condense out.
Why does dew form on grass overnight?
On clear, calm nights the ground radiates heat away and cools faster than the air above it. When surfaces like grass or car windows drop below the dew point of the surrounding air, the water vapour touching them condenses into liquid droplets we call dew. If the dew point is below freezing, that same process produces frost instead, as the vapour deposits directly as ice crystals.
What units does this calculator use?
This calculator takes air temperature in degrees Celsius and relative humidity as a percentage from 0 to 100, and it returns the dew point in degrees Celsius. If your temperature is in Fahrenheit, convert it by subtracting 32 and dividing by 1.8 before entering it. The Magnus coefficients used here, 17.27 and 237.7, are the standard values that keep the result accurate across typical everyday weather conditions.