Dew Point Calculator

Calculate the dew point temperature. Enter the current air temperature and relative humidity (%) and this calculator will determine the dew point (in °C, °F, or K).

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Dew Point Formula

The following equation (Magnus formula) is commonly used to approximate dew point. In this form, the temperature inputs are in °C. If you start with °F or K, convert to °C first, then convert the result back if needed.

 T_d = (b * \alpha) / (a - \alpha) 

• Where T_d is the dew point temperature (°C)
• T is the current air temperature (°C)
• RH is the relative humidity (%)
• a = 17.62
• b = 243.12
• α = ln(RH/100) + (a*T)/(b+T)

Dew Point Definition

Dew point is the temperature to which air must be cooled (at roughly constant pressure and water vapor content) for it to become saturated (100% relative humidity) and for condensation (dew) to begin forming. (The water you see on the grass in the mornings)

How to calculate dew point

We will now look at an example of how to calculate the dew point on any given day. Let’s say it’s a nice hot summer morning at 80 degrees Fahrenheit. You look outside and see no dew on the grass but you’re curious what temperature it would need to be for dew to form.

Next, you must go on either an app or website that tells you the current relative humidity. Let’s assume it’s 25%.

Finally, plug these values into the formula above (remembering that the formula uses °C internally) and you get a dew point of about 41.0°F (about 5.0°C). If you look at that temperature and think it’s very low, you are right. That’s because the relative humidity is extremely low at 25%. The more water vapor you have in the air, the easier it is for dew to form, so if the humidity was closer to 75% (at the same 80°F air temperature) the dew point would be about 71.3°F, meaning the air would only need to cool to around 71°F for dew to form.

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