Dew Point and Condensation Calculator

What Is the Dew Point?

The dew point is the temperature at which air becomes saturated with water vapor.
That is the point where condensation starts.
When air cools to its dew point, water droplets form on any surface at or below that temperature, which is what dew and fog are.

Below freezing the same calculation gives a frost point instead, since vapour turns straight to ice without passing through liquid. The numbers here use the coefficients for liquid water, so below about 0°C read the answer as approximate, roughly half a degree low against the proper ice values.

This page is about surfaces, not weather

There are two useful questions you can ask a dew point, and they want different answers.

This one is the indoor, building-science question: what is the coldest thing in the room allowed to be before it starts sweating? Type the temperature of that surface into the last box, a window pane, a cold-water pipe, the inside face of an outside wall, and the calculator tells you whether it will condense, by how much you are over or under the line, and what you would have to drop the humidity to in order to stop it.

If you want the outdoor question instead, how muggy the air feels, whether fog is likely overnight, and how high the cloud base sits, that is the Dew Point Calculator. Both pages use the same comfort scale and return the same dew point to the tenth of a degree, so you can move between them freely.

The Magnus Formula (Simplified)

This calculator uses the August-Roche-Magnus approximation:

γ = (a × T) ÷ (b + T) + ln(RH/100) Dew Point = (b × γ) ÷ (a − γ)

Where:

• T = Air temperature (°C)
• RH = Relative humidity (%), and ln is the natural logarithm
• a = 17.625, b = 243.04°C, the Alduchov-Eskridge coefficients

This is accurate to within 0.4°C for temperatures between −40°C and 60°C. You will also see 17.67 and 243.5 quoted, the Bolton fit, which the weather page linked above uses. At room temperature the two land within a hundredth of a degree of each other, so the choice never shows up in an answer.

Running it backwards gives the humidity that would put the dew point exactly on a chosen surface, which is where the “you would need to hold humidity below X%” line in the result comes from.

Dew Point Comfort Scale

The boundaries are round numbers in Fahrenheit and untidy ones in Celsius because the 50/60/65/70/75 scale is the one American forecasters have used for decades. Rounding them to 10/15/18/21/24°C looks neater and moves three of the five band edges, which is enough to put the same afternoon in two different rows on two different pages.

Where the surface number actually matters

Mould. Mould does not need liquid water, only a surface sitting near the dew point for long enough. The usual target is to keep wall surfaces two to three degrees clear of it. A room at 20°C and 60% has a dew point of 12.0°C, so a poorly insulated corner at 11°C is already wet.

Single glazing and cold pipes. These are almost always the coldest surface in the room, which is why they run with water first. They are the early warning, not the problem.

HVAC: Air conditioning lowers the dew point by pulling moisture out, which is why a cold room also feels less clammy.

Aviation and agriculture: Pilots watch the dew point for fog and carburettor icing, and growers watch it for overnight frost and dew.

Example

Air temperature 25°C, relative humidity 70%, dew point 19.1°C.

Any surface cooler than 19.1°C in that air will collect condensation, which is why a glass of iced water sweats and a single-glazed window in winter runs with water. Put a 17°C window into the last box and the page will tell you it is 2.1 degrees under the line, and that holding humidity below about 61% would keep it dry.