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Dew Point & Humidity Calculator

Calculate the dew point and absolute humidity of your indoor air, check the mould risk on your exterior wall and find out whether airing the room pays off right now.

100% freeNo data storedMagnus formula & DIN 4108-2

Indoor air

°C
%

Outdoor air

°C
%

Exterior wall

Wall surface temperature

The estimate applies to the undisturbed wall area. Room corners, window reveals and walls behind wardrobes are often much colder — measure there if in doubt.

Dew point of indoor air

9.3 °C

20.0 °C · 50 % — If a surface cools down to this temperature, water condenses on it.

Absolute humidity

8.6 g/m³

Max. humidity at room temperature

17.2 g/m³

Humidity is within the recommended range of 40–60%. Recommendation of the German Environment Agency (UBA): 40–60% relative humidity.

Mould risk on the exterior wall

Elevated risk

Surface humidity is between 70 and 80%. Still below the mould criterion but with little margin — corners and spots behind furniture may already be critical.

Surface temperature

14.0 °C

Humidity at the surface

73 %

Mould-critical surface temperature

12.6 °C

80% humidity at the surface

Max. indoor humidity for this wall

55 %

Up to this relative humidity the wall surface stays below 80%.

Temperature factor fRsi

0.70

min. 0.70 per DIN 4108-2

Needed for your values

≥ 0.63

Ventilation: does it pay off right now?

Yes, airing dries the room

Every cubic metre of exchanged air carries about 4.8 g of water outside. Short, intensive airing with windows wide open is effective now.

Indoors

8.6 g/m³

20.0 °C · 50 %

Outdoors

4.1 g/m³

Outdoor dew point: -2.2 °C

Outdoor air warmed to room temperature

3.8 g/m³

rel. humidity after warming: 22 %

Dew point and mould limit curve

At 20.0 °C: if the wall temperature falls below the orange line, the surface becomes mould-critical; below the blue line, condensation forms.

Dew point table

Dew point in °C by room temperature and relative humidity (Magnus formula).

Room temperature40 %50 %60 %70 %
16 °C2.4 °C5.6 °C8.2 °C10.5 °C
18 °C4.2 °C7.4 °C10.1 °C12.4 °C
20 °C6.0 °C9.3 °C12.0 °C14.4 °C
22 °C7.8 °C11.1 °C13.9 °C16.3 °C
24 °C9.6 °C12.9 °C15.8 °C18.2 °C

Basis of calculation & notes

  • Saturation vapour pressure via the Magnus formula over water with the coefficients by Sonntag (1990): E = 6.112 hPa · exp(17.62 · T / (243.12 + T)), as used in the WMO Guide (WMO-No. 8) and by Germany's weather service DWD. Valid from about −45 to +60 °C. Absolute humidity via the gas law with R = 461.5 J/(kg·K).
  • Mould criterion per DIN 4108-2: relative humidity at the component surface should stay below 80%. For this, the standard requires a temperature factor fRsi ≥ 0.70 at 20 °C / 50% indoors and −5 °C outdoors, i.e. a surface temperature of at least 12.6 °C.
  • The surface temperature derived from the U-value is a one-dimensional steady-state estimate with Rsi = 0.25 m²·K/W. Thermal bridges, corners, furniture against the wall and fluctuating temperatures are not taken into account.
  • This calculator does not replace an assessment by experts. If you see mould, notice a musty smell or have health complaints, consult specialists or your doctor.

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Guide: Humidity, Ventilation & Mould

Understand the dew point, ventilate correctly and prevent mould on walls and windows

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Frequently Asked Questions

With the Magnus formula. First determine the saturation vapour pressure E = 6.112 hPa · exp(17.62 · T / (243.12 + T)) and multiply it by the relative humidity. You get the dew point by solving the formula for temperature: dew point = 243.12 · γ / (17.62 − γ) with γ = ln(RH/100) + 17.62 · T / (243.12 + T). At 20 °C and 50% this gives about 9.3 °C, at 20 °C and 60% about 12.0 °C.

What matters is not the room humidity but the humidity directly at the wall surface. If it stays at 80% or more for several days, mould can grow — that is the criterion of the German standard DIN 4108-2. At 20 °C and 50% indoor humidity this limit is reached once the wall is colder than about 12.6 °C. At 60% indoor humidity the limit is already around 15.5 °C.

Germany's Environment Agency (UBA) recommends a relative humidity between 40 and 60%. During the heating season and with poorly insulated exterior walls the lower part of this range is safer, because wall surfaces are colder. Whether your wall becomes critical at your humidity is shown by the calculator's maximum indoor humidity for your wall.

Absolute humidity states how much water vapour is actually in the air, usually in grams per cubic metre. Relative humidity relates this amount to the maximum possible amount at the given temperature. Air at 20 °C can hold about 17.2 g/m³, air at 0 °C only about 4.8 g/m³. That is why relative humidity rises when air cools, even though the amount of water stays the same.

In winter, usually yes. Cold outdoor air holds little water even at 90% relative humidity. When warmed to 20 °C indoors its relative humidity drops sharply — from 85% at 0 °C outside to only about 22%. In summer it can be the other way round: warm, humid air can deposit moisture in cooler rooms such as cellars. The calculator's ventilation check compares exactly that.

The temperature factor fRsi describes where the inner surface temperature lies between outdoor and room temperature: fRsi = (θsi − θe) / (θi − θe). The German standard DIN 4108-2 requires at least 0.70 for thermal bridges. At 20 °C inside and −5 °C outside this corresponds to a surface temperature of 12.5 °C, This matches the mould-critical temperature of 12.6 °C at 50% indoor humidity; the limit of 0.70 is rounded from it.

Because the glass surface is colder than the dew point of the indoor air. Older glazing and especially the glass edge get so cold in winter that water condenses. If the panes fog up regularly, the humidity is too high for these windows: air the room after showering, cooking and in the bedroom in the morning, and wipe water off the window sill.

It applies to the undisturbed wall area at steady temperatures. In room corners, window reveals, ceiling junctions and behind wardrobes the surface is often several degrees colder. For a reliable result, measure the wall temperature at the coldest spot, for example with an infrared thermometer, and use the calculator's “Measured myself” mode.