Dew Point Calculator

How Dew Point Is Calculated

Dew point is the temperature air would need to cool to, at constant pressure and moisture content, for it to become fully saturated with water vapor. The Magnus formula (also called the August-Roche-Magnus approximation) estimates it from air temperature and relative humidity using a simple two-step calculation:

$$\alpha = \ln\left(\frac{RH}{100}\right) + \frac{17.27\,T}{237.3+T}$$

$$T_{dp} = \frac{237.3\,\alpha}{17.27-\alpha}$$

Because the constants in the Magnus formula (17.27 and 237.3) are calibrated in Celsius, this calculator converts a Fahrenheit input to Celsius first, runs the formula, then converts the result back to whichever unit is selected for display.

Worked Example

At an air temperature of 77 degrees Fahrenheit (25 degrees Celsius) and 50 percent relative humidity: alpha = ln(0.5) + (17.27 × 25) / (237.3 + 25) = -0.693 + 1.646 = 0.953. Dew point in Celsius = (237.3 × 0.953) / (17.27 - 0.953) ≈ 13.9 degrees Celsius, which converts to about 56.9 degrees Fahrenheit — landing in the Comfortable range.

A Brief History of the Magnus Formula

The formula's roots trace to 19th-century work on saturation vapor pressure, often credited to German physicist Gustav Magnus around 1844, though the specific coefficients in common use today were refined over time by later researchers (Sonntag, and Alduchov and Eskridge among them), which is why different published versions of "the" Magnus formula use slightly different constants. The version above is one of the most widely used approximations and is accurate to within about half a degree Celsius across ordinary atmospheric conditions.

Dew Point vs. Relative Humidity

Relative humidity is a ratio — how much moisture the air is holding compared to how much it could hold at its current temperature — so it changes throughout the day even when the actual moisture content stays constant, since warmer air can hold more water vapor before saturating. Dew point reflects the actual moisture content directly, independent of the current temperature, which is why meteorologists consider it the more reliable comfort indicator: a 60-degree dew point feels muggy whether the air temperature is 70 or 95, while a relative humidity reading of, say, 55% could mean very different things depending on temperature alone.

Common Dew Point Calculation Mistakes

Forgetting to convert Fahrenheit to Celsius before applying the Magnus formula's constants is the most common error when doing this by hand — the 17.27 and 237.3 coefficients only work correctly with a Celsius input, so plugging in a Fahrenheit value directly produces a meaningless result. Confusing dew point with relative humidity is another: a "70% humidity" reading tells you little about actual comfort without knowing the temperature, while dew point alone is a direct, temperature-independent moisture reading. Assuming dew point can exceed air temperature is a third mistake to avoid — physically, dew point is always at or below the current air temperature, since air can't be more saturated than 100% relative humidity.

Dew Point Terms You Should Know

Dew Point — the temperature at which air becomes fully saturated with water vapor and condensation (dew) begins to form.

Relative Humidity — the ratio of the air's current moisture content to the maximum it could hold at that temperature, expressed as a percentage.

Saturation — the point at which air is holding the maximum water vapor possible at a given temperature (100% relative humidity).

Magnus Formula — the standard approximation relating temperature, relative humidity, and dew point, using empirically fitted constants.