Dew point calculator
The dew point is the temperature air must be cooled to before its water vapour starts condensing. It does not move when the air temperature does, so any surface below it gets wet: a window, a cold-water pipe, the inner face of an uninsulated wall.
How to use the dew point calculator
The figure comes from saturation vapour pressure through the Magnus approximation, which holds to about ±0.4 °C between −40 and 50 °C. That accuracy is why the answer is given to a tenth of a degree and no further. The spread — air temperature minus dew point — is the second number to watch: a spread of zero is fog, and the same figure gives cloud base, roughly 125 metres of altitude for every degree of spread.
For comfort the dew point beats relative humidity because it is absolute rather than a percentage of what the air could hold at that moment. Sixty percent at 10 °C and sixty percent at 30 °C are different worlds; their dew points, 2 °C and 21 °C, say so immediately. Below about 10 °C reads dry, 13 to 16 comfortable, 18 to 21 sticky, above 21 oppressive, and those bands hold regardless of the thermometer.
For buildings the number is a threshold rather than a sensation. If the inner face of a wall, a window frame or a cold bridge sits below the room’s dew point, moisture collects there and mould follows; wiping it changes nothing while the surface stays cold, and the fix is either insulation or less moisture in the room. Coating datasheets use the same threshold from the other side: most paints and industrial coatings require the substrate to be at least 3 °C above dew point before application, because film applied over condensed moisture lifts later.
What people use it for
- Working out whether a cold wall will condense and grow mould
- Meeting a coating datasheet rule that the substrate stays 3 °C above dew point
- Setting a dehumidifier target for a basement or a crawl space
- Judging whether a tent or a windscreen will be wet at dawn
- Deciding whether a muggy day is genuinely humid or only hot
Questions
The temperature air has to be cooled to before it saturates and water condenses out. It is computed here from temperature and humidity with the Magnus approximation, good to about ±0.4 °C.
It is absolute. Sixty percent humidity means something different at 10 °C than at 30 °C; the dew point needs no such context.
Below about 13 °C feels dry, 13 to 16 comfortable, 18 to 21 sticky, above 21 oppressive.
No. At most it equals it, which is 100 percent humidity — fog, or cloud.
Its inner surface is below the room’s dew point. Either raise the surface temperature with better glazing, or lower the indoor humidity.
A coating applied over a surface below dew point traps condensed moisture under the film, which lifts later. Most datasheets require the substrate to be at least 3 °C above it.