Dew Point Calculator

Written byOleh Soloninupdated 25 September 2026

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.

How to use

Enter the air temperature and relative humidity in the room.
Give the temperature of the surface you are worried about: a window pane, a wall corner, a reveal.
Read the margin to the dew point and the humidity limit — the level the air has to come down to for that surface to stay dry.

The dew point at 20 degrees

The dew point is the temperature at which air of a given humidity starts giving up water. Any surface colder than that collects condensation. At 20 degrees indoors, every ten points of humidity moves the dew point by two and a half to four degrees — more in dry air, less in humid air.

The consequence is simple: at 40 per cent humidity glass has to cool to 6 degrees before it mists, while at 60 per cent 12 is enough. For a single-pane or basic double-glazed window in frost, that difference is decisive.

Every value comes from the calculator on this page, which uses the Magnus formula. For glass at 10 degrees the line falls between 50 and 60 per cent — exactly where a dry window turns into a wet one.
Humidity at 20 °C	Dew point	Glass at 10 °C
30 %	1.9 °C	dry
40 %	6.0 °C	dry
50 %	9.3 °C	dry
60 %	12.0 °C	mists up
70 %	14.4 °C	mists up

How far the humidity has to come down

The calculator's most useful output is the humidity limit: how dry the room must be for a surface at a given temperature to stay dry. That is the answer to "how much should I air the room".

The colder the surface, the drier the air has to be. Glass at 15 degrees tolerates 73 per cent, glass at 10 only 52.6, and glass at 5 just 37. Holding that in winter with poor glazing is hard without a dehumidifier, which is why warmer windows solve the problem more reliably than airing.

Every value comes from the calculator on this page. Above the stated humidity, a surface at that temperature gets wet; below it, it stays dry.
Surface temperature	Humidity limit with the room at 20 °C
15 °C	73.0 %
10 °C	52.6 %
5 °C	37.4 %

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 poorly insulated glazing in a frost sits below that.

What humidity is normal?

Russia's indoor-climate standard, GOST 30494, sets 30 to 45 per cent as the winter optimum and 30 to 60 per cent in summer. Higher risks condensation and mould on cold surfaces; lower dries out sinuses and shrinks furniture.

How dry must the air be to stop the windows misting?

Down to the humidity limit the calculator gives for the glass temperature. For glass at 10 degrees with the room at 20 that is 52.6 per cent; for glass at 5 degrees it is 37.4.

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. Warm air rising from a radiator to the glass raises its temperature by itself.

What is absolute humidity?

How many grams of water a cubic metre of air holds. At 20 degrees and 60 per cent it is 10.35 grams. Relative humidity is the share of the maximum at that temperature, which is why heating air lowers it even though no water has gone anywhere.

What is the dew point inside a wall?

The plane where the temperature within the structure equals the dew point. Where it lies depends on the wall's layers, and its position in the insulation does not by itself prove that moisture will not build up: that takes a hygrothermal assessment of the structure. This calculator only checks a surface against the room air.

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 below 52.6 percent humidity, with some margin for the hygrometer's error. Airing out works exactly this way: cold outdoor air becomes noticeably drier once it warms up indoors.

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Result

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