Dew Point Calculator Online & Excel File

As chemical engineers, we need to calculate the dew point in processes like gas drying, air compression, and HVAC control. For this, we have an online Dew Point Calculator which will help you just by entering the temperature and relative humidity.

Dew Point Calculator

🌡️ Dew Point Calculator

Temperature

°C

°F

Relative Humidity (%)

—

Dew Point Temperature

°C

°F

How to Use the Dew Point Calculator

Enter Temperature – Type in the air temperature in °C, °F, or Kelvin.
Select Unit – Choose the correct unit for the temperature you entered.
Enter Relative Humidity (RH) – Input the humidity value between 0–100%.
Click “Calculate” – You will get the answer.
Output Units – View results in °C, °F, or Kelvin by selecting the unit buttons below the result.

What is Dew Point?

The dew point is the temperature at which air becomes fully saturated with water vapor. When air cools down to this temperature, condensation begins, forming dew, fog, or even frost.

Higher dew point = more moisture in the air.
Lower dew point = dry air

Another term that we need to learn to find the dew point is relative humidity.

What is Relative Humidity?

Relative Humidity (RH) is the ratio between the actual water vapor in the air and the maximum amount of water vapor the air can hold at that temperature.

$RH = \frac{e}{e_s} \times 100\%$

$\text{where: } e \text{ = actual vapor pressure, } e_s \text{ = saturation vapor pressure}$

Magnus–Tetens equation

Step 1: Calculate helper function α

$\alpha(T, RH) = \frac{a \cdot T}{b + T} + \ln\left(\frac{RH}{100}\right)$

Step 2: Calculate dew point

$T_{dew} = \frac{b \cdot \alpha(T, RH)}{a - \alpha(T, RH)}$

Where: – T = air temperature (°C)

RH = relative humidity (%)

a = 17.27, b = 237.7 (empirical constants)

There are several equations available to calculate the dew point, but the Magnus–Tetens formula is considered the most accurate for practical applications, with an uncertainty of about ±0.35 °C in the range of –40 °C to +50 °C.

If you’re interested in related vapor–liquid properties, check out our Antoine Equation Calculator, which estimates vapor pressure at different temperatures.”

Example :

Calculate the dew point if the air temperature is 30 °C and the Relative Humidity is 70%?

Step 1:

$\alpha = \frac{17.27 \times 30}{237.7 + 30} + \ln(0.70) = 1.62$

Step 2:

$T_{dew} = \frac{237.7 \times 1.62}{17.27 - 1.62} = 24.0 \, °C$

Result: Dew point is 24 °C.

We have created an Excel file which you can download from the link below

FAQ (calculate dew point temperature)

1. How do you calculate dew point temperature?

Dew point can be calculated using air temperature and relative humidity. It’s the temperature where air becomes saturated and condensation begins.

2. How to find the relative humidity (RH)?

Relative humidity can be found using air temperature and dew point, with the Magnus equation for vapor pressure. It expresses how much moisture is in the air compared to the maximum it can hold at that temperature.

3. What inputs do I need for the Dew Point Calculator?

You just need to enter the air temperature and relative humidity. The calculator will instantly give you the dew point temperature.

4. Are there different equations to calculate dew point?

Yes. Methods include the Magnus–Tetens, WMO, and Goff–Gratch equations, plus simple approximations. Among them, Magnus–Tetens is the most common because it balances accuracy with ease of use.