Dew Point Calculator

Air temperature, °C

Relative humidity, %

Surface temperature, °C

The glass, reveal or wall you are checking.

Result

Dew point, °C12
Will it condenseYes, this surface will run with condensation
Margin to the dew point, °C-2
Humidity limit without condensation, %52.6%
Absolute humidity, g/m³10.35
Air temperature, °C20

The temperature at which air gives up

Works out the dew point from air temperature and relative humidity, checks whether a given surface will run with condensation, and gives the humidity limit that keeps it dry.

The dew point is the temperature air has to be cooled to before the vapour in it starts turning into water. Anything colder than that gets condensation on it: glass, a window reveal, a cold corner, a cold-water pipe.

That answers the question of why windows mist up. Not because the windows are bad, but because their surface fell below the dew point. There are two cures — warm the surface or take moisture out of the air — and the second is usually easier.

What this calculator shows

The dew point for the temperature and humidity you enter
Whether a surface at the given temperature will condense, and the margin left
The humidity limit that would keep that surface dry

What to keep in mind

The Magnus formula is used, the version building physics relies on; accuracy is around 0.1 degrees in the everyday range.
Absolute humidity is given in grams of water per cubic metre — the quantity a dehumidifier actually removes.

FAQs

Why do windows mist up indoors?

Because the glass is colder than the dew point. At 20 degrees and 60 percent humidity the dew point is 12, and single-chamber glazing in a frost sits well below that.

What humidity is normal?

30 to 45 percent in winter, up to 60 in summer. Higher risks condensation and mould; lower dries out sinuses and shrinks furniture.

How do I get rid of condensation?

Lower the humidity by airing out or with a dehumidifier, and raise the surface temperature: pull heavy curtains away from the glass, clear the sill, check the ventilation works.

What is the dew point inside a wall?

The plane where the temperature within the structure equals the dew point. Correctly insulated walls push it out into the external insulation, where the moisture can evaporate outward instead of collecting in the masonry.

Worked example

An ordinary room in winter

Input: 20 °C, 60% humidity, glass at 10 °C

Output: Dew point 12 °C — the glass will mist up

Note: To keep glass at ten degrees dry, the room needs to come down to 53 percent humidity. Airing out works exactly this way: cold outdoor air becomes noticeably drier once it warms up indoors.

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Dew Point Calculator

Result

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