What Is the Dew Point Calculator?
The dew point is the temperature to which air must be cooled, at constant pressure, for water vapor to begin condensing into liquid (dew, fog, or clouds). It is an absolute measure of moisture in the air — unlike relative humidity, the dew point doesn't change as the air warms or cools, making it a far better indicator of how "muggy" it actually feels. This calculator converts an air temperature and relative humidity into a precise dew point using the widely used Magnus approximation.
How to Use It
Enter the current air temperature in degrees Celsius and the relative humidity as a percentage (0–100). Press calculate and the tool returns the dew point in °C. A dew point below 10°C feels dry and comfortable; 13–16°C feels noticeably humid; above 21°C feels oppressive and sticky.
The Formula Explained
The Magnus formula gives the dew point as:
$$\gamma = \ln\!\left(\frac{\text{RH}}{100}\right) + \frac{a \cdot T}{b + T}, \quad \text{then} \quad T_d = \frac{b \cdot \gamma}{a - \gamma}$$
with constants \(a = 17.62\) and \(b = 243.12\)°C. A quick mental estimate is the simpler rule \(T_d \approx T - \frac{100 - \text{RH}}{5}\), which is accurate to about ±1°C when humidity is above 50%.
Worked Example
Suppose \(T = 25\)°C and \(\text{RH} = 60\%\). First $$\gamma = \ln(0.60) + \frac{17.62 \times 25}{243.12 + 25} = -0.5108 + \frac{440.5}{268.12} = -0.5108 + 1.6429 = 1.1321.$$ Then $$T_d = \frac{243.12 \times 1.1321}{17.62 - 1.1321} = \frac{275.24}{16.488} = 16.69\ \text{°C}.$$ So dew is about 16.7°C.
FAQ
Is dew point the same as humidity? No. Relative humidity is a ratio that depends on temperature; dew point is an absolute temperature representing actual moisture content.
Why can the dew point be negative? In cold, dry air the dew point can fall well below freezing — that is called the frost point.
How accurate is this calculator? The Magnus formula is accurate to within about 0.4°C for temperatures from −45°C to 60°C and humidities above roughly 1%.
