Dew Point Calculator

Calculate the total dew point in Fahrenheit. Enter the current temperature and relative humidity ($) and this calculator will determine the dew point.

Dew point is the highest possible temperature at which water can condense to form dew. (The water you see on grass in the mornings)

Dew Point Formula

The following equation is used to calculate the dew point on any given day.

Ts = (b * α) / (a – α)

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

As you can see the formula for the dew point is quite complicated. That’s because the values are all dependent on both current temperature and relative humidity. Furthermore, relative humidity and be calculated using the following equation.

relative_humidity = 100% * Pw / Pws

  • Where Pw is the water vapor pressure
  • Pws is the water vapor pressure at saturation

This is a little more complicated, and we won’t go into here on this article.

How to calculate dew point

We will now look at an example on 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. Lets assume it’s 25%.

Finally, plug these values into the formula above, and you get a value of 32.53 degrees Fahrenheit. 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% you would only need to be at 61 F for dew to form.

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