Dew Point Calculator

The dew point is the temperature to which air must cool, at constant pressure, to become saturated. Once the air reaches it, water vapor begins to condense into dew, fog or cloud droplets. Unlike relative humidity, the dew point does not change as the air warms or cools during the day, so it is a steadier measure of how much moisture the air actually holds.

It uses the Magnus formula. First it finds gamma = ln(RH/100) + (b*T)/(c+T), then the dew point is (c*gamma)/(b-gamma), with b = 17.62, c = 243.12 and T in degrees Celsius. These Sonntag 1990 coefficients are accurate to within about 0.1 C over the range -45 C to 60 C.

For everyday air temperatures the Magnus approximation is within a few tenths of a degree of the exact thermodynamic value. Accuracy is best between roughly -40 C and 60 C. The main real-world error comes from the relative humidity reading itself, since consumer hygrometers are often only accurate to plus or minus 3 to 5 percent.

At 100 percent relative humidity the air is already saturated, so no further cooling is needed to reach the dew point. In that case ln(RH/100) is ln(1), which is zero, and the formula collapses so the dew point comes out equal to the air temperature.

The formula takes the natural log of RH/100, and the log of zero is undefined (it tends to negative infinity). Physically a dew point only exists when some water vapor is present, so the tool requires a relative humidity greater than 0 and at most 100.

A dew point below about 16 C (60 F) feels comfortable to most people. Between 16 and 21 C the air starts to feel sticky, and above roughly 21 C (70 F) it feels oppressive because sweat evaporates slowly. These thresholds are about human comfort and are independent of the calculation itself.

Yes. In cold or very dry air the dew point can fall well below 0 C. When it is below freezing it is often called the frost point, because water vapor then deposits directly as frost or ice crystals rather than settling as liquid dew.

This calculator assumes standard sea-level pressure and depends only on temperature and relative humidity. The true dew point does shift slightly with pressure, so at high altitude or inside pressurized systems the result is an approximation rather than an exact value.

Yes. The Magnus formula runs locally in your browser using JavaScript, and no temperature or humidity value is sent to a server. You can even use the tool offline once the page has loaded.