Dew point calculator (humidity, condensation and mold risk)
Temperature + humidity → dew point, absolute humidity and condensation risk on a window or wall.
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Calculate the dew point and air humidity
The dew point tells you at what temperature the moisture in the air turns into water. It is the key figure behind window condensation, moldy walls, sweating cold water pipes and floor coatings that fail to bond. This calculator gives the dew point (or the relative humidity if you know the dew point), the frost point below freezing, the absolute humidity in g/m³, the mixing ratio in g/kg and, if you measure a surface, the risk of condensation and mold.
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Measure the air
Read the temperature and relative humidity with a thermo-hygrometer, away from heat sources. If you already know the dew point, switch modes to get the relative humidity instead.
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Add a surface (optional)
Measure the temperature of the window pane, wall, pipe or substrate with an infrared or contact thermometer.
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Read the results
Dew point in large type, dew point spread, vapor pressure, absolute humidity and mixing ratio. The surface check flags condensation, mold risk and whether you can paint.
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Compare scenarios
The table under the result lists the dew point from 20% to 100% humidity at the temperature entered, so you see how much drier the air needs to be.
The Magnus formula
The tool uses the Magnus formula with the DIN 4108-3 constants (a = 17.62 and b = 243.12 °C over water; a = 22.46 and b = 272.62 °C over ice for the frost point). Compared with Hardy’s ITS-90 reference formulation, it stays within 0.2 °C between −40 and 60 °C (−40 and 140 °F).
- Saturation vapor pressure:
Es(T) = 6.112 × exp(17.62 × T / (243.12 + T))in hPa, i.e. 23.33 hPa at 20 °C. - Vapor pressure:
e = RH / 100 × Es(T), i.e. 14.00 hPa at 20 °C and 60%. - Dew point:
Td = 243.12 × ln(e / 6.112) / (17.62 − ln(e / 6.112))= 12.0 °C (53.6 °F). - Absolute humidity:
216.68 × e / (T + 273.15)= 10.34 g of vapor per m³ of air. - Mixing ratio:
621.98 × e / (p − e)= 8.71 g of water per kg of dry air at 1013.25 hPa. - Humidity at a surface:
e / Es(Tsurface); at 15 °C (59 °F) it reaches 82.2%, above the mold threshold.
Dew point chart
| Air temperature | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|
| 10 °C (50 °F) | −6.8 | −3.0 | 0.0 | 2.6 | 4.8 | 6.7 | 8.4 |
| 15 °C (59 °F) | −2.5 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 |
| 18 °C (64.4 °F) | 0.1 | 4.2 | 7.4 | 10.1 | 12.4 | 14.5 | 16.3 |
| 20 °C (68 °F) | 1.9 | 6.0 | 9.3 | 12.0 | 14.4 | 16.4 | 18.3 |
| 22 °C (71.6 °F) | 3.6 | 7.8 | 11.1 | 13.9 | 16.3 | 18.4 | 20.3 |
| 25 °C (77 °F) | 6.2 | 10.5 | 13.9 | 16.7 | 19.1 | 21.3 | 23.2 |
| 30 °C (86 °F) | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 |
Dew point in °C by air temperature and relative humidity (Magnus formula, DIN 4108-3 constants). Switch the tool to °F for Fahrenheit values.
Condensation, mold and painting: three thresholds
- Condensation: as soon as a surface is at or below the dew point. With air at 20 °C and 60%, any surface at 12 °C (53.6 °F) or colder gets wet.
- Mold: once humidity at the surface stays at 80% (DIN 4108-2). Still at 20 °C, a surface at 15.4 °C (59.7 °F) is enough with 60% air, or 12.6 °C (54.7 °F) with 50% air: a thermal bridge can grow mold without ever dripping.
- Paint and coatings: the substrate must be at least 3 °C (about 5 °F) above the dew point (ISO 8502-4). To work out how much paint you need, use our paint calculator.
In summer the dew point is also the best measure of mugginess: above about 16 °C (60 °F) the air feels sticky, above 21 °C (70 °F) oppressive. For the feels-like temperature in hot weather, see our heat index calculator.
Below freezing the tool also shows the frost point, computed over ice: the temperature at which frost forms. For 0 °C and 80%, the dew point is −3.0 °C and the frost point −2.7 °C.
Frequently asked questions
What is the dew point?
It is the temperature to which air must be cooled, without changing its water vapor content, to become saturated (100% relative humidity). Below it, vapor condenses into droplets: fog on a window, dew on grass, beads on a cold can. For air at 20 °C (68 °F) and 50% humidity, the dew point is 9.3 °C (48.7 °F).
How do you calculate the dew point?
With the Magnus formula: first γ = ln(RH/100) + 17.62 × T / (243.12 + T), then Td = 243.12 × γ / (17.62 − γ), with T in °C. For 20 °C and 60%: γ = ln(0.6) + 1.3393 = 0.8285, so Td = 243.12 × 0.8285 / 16.79 = 12.0 °C (53.6 °F). The calculator does this instantly in °C or °F.
Why do my windows have condensation?
Because the glass is colder than the dew point of the indoor air. At 20 °C and 60% humidity the dew point is 12 °C (53.6 °F): a pane whose inner face drops to 10 °C (50 °F) fogs up. Lower the humidity (ventilation, extractor fans, drying laundry outside, a dehumidifier) or warm the surface (double or triple glazing, lighter curtains at night).
At what humidity does mold start to grow?
Mold can grow once the relative humidity at a surface stays around 80% for several days, even without visible condensation. That is the criterion used by the German standard DIN 4108-2. With indoor air at 20 °C and 60%, it is reached on any surface at 15.4 °C (59.7 °F) or colder, or 12.6 °C (54.7 °F) if the air is at 50%.
How far above the dew point must a surface be for painting?
ISO 8502-4, used for paints, epoxy and floor coatings, requires the substrate to be at least 3 °C (about 5 °F) above the dew point. With a 12 °C dew point the surface must be 15 °C or warmer; otherwise an invisible film of water ruins adhesion.