Dew Point Calculator

How This Calculator Works

Dew point is the temperature air would need to cool to for it to become fully saturated with the moisture it already holds. It’s a more reliable “how humid does it actually feel” indicator than relative humidity alone, since relative humidity changes with temperature even when the actual amount of moisture in the air doesn’t — the same air reads as a much higher relative humidity percentage on a cold morning than on a warm afternoon, while its dew point stays roughly constant.

Enter the air temperature and relative humidity, and this calculator estimates the dew point using the Magnus formula, a widely-used meteorological approximation.

The Formula

The Magnus formula first computes an intermediate value $\alpha$ from temperature and relative humidity, then solves for dew point from it:

$$\vC{\alpha} = \ln\left(\frac{\vB{RH}}{100}\right) + \frac{17.625 \times \vA{T}}{243.04 + \vA{T}}$$ $$\text{Dew Point} = \frac{243.04 \times \vC{\alpha}}{17.625 - \vC{\alpha}}$$

where $\vA{T}$ is air temperature in Celsius and $\vB{RH}$ is relative humidity as a percent (0–100]. The constants 17.625 and 243.04 are the Magnus formula’s standard coefficients, accurate to within about 0.35°C across the -45°C to 60°C range this calculator supports.

Worked Example

At 25°C with 60% relative humidity:

1. $\vC{\alpha} = \ln(0.6) + {\displaystyle\frac{17.625 \times \vA{25}}{243.04 + \vA{25}}} = -0.510826 + 1.643895 = \vC{1.133069}$.
2. $\text{Dew Point} = {\displaystyle\frac{243.04 \times \vC{1.133069}}{17.625 - \vC{1.133069}}} \approx \mathbf{16.70°C}$ (about 62.1°F) — a widely-cited reference value for this exact temperature/humidity pairing.