Dew point calculator
The dew point is the single most useful number in a damp house: any surface colder than it is actively collecting water. Room humidity tells you how the air feels; the dew point tells you what the walls are doing.
Enter your numbers
Normal margin
Only genuinely cold surfaces will reach the dew point.
- Dew point
- 47.5 °F
- Margin to dew point
- 22.5 °F
- Moisture in the air
- 48 gr/lb
What the dew point tells you
Air holds water as vapor, and how much it can hold depends almost entirely on its temperature. Cool that air and its capacity falls; cool it far enough and it can no longer hold what it has. The temperature where that happens is the dew point, and any water above that limit leaves the air as liquid — on your windows, on the cold corner behind the wardrobe, inside the wall if it gets that far.
This is why condensation is not really a humidity problem. It is a temperature problem that humidity sets the threshold for. A room at 70 °F and 45% humidity has a dew point around 47 °F, so nothing in a normally heated house will condense. Push that same room to 65% humidity and the dew point climbs past 57 °F — now the window glass on a cold night is wet every morning.
The margin is what matters
Compare the dew point to the coldest surface in the room, not to the room temperature. An uninsulated exterior wall typically runs 3–7 °F below room air; a window reveal or a single-glazed pane on a freezing night runs much colder than that.
A margin above 10 °F means you are comfortably clear. Under 5 °F means cold surfaces are already condensing. Under 2 °F means the room itself is close to saturation and everything cool is wet.
Using it to find the real problem
An inexpensive infrared thermometer turns this from theory into a diagnosis. Read the dew point here, then point the thermometer at the corner that keeps growing mold, at the window reveal, at the wall behind the sofa. Any reading below the dew point figure is a surface that is actively collecting water.
That single comparison tells you which fix you need: if surfaces are cold, you need insulation and heat; if the dew point itself is high, you need ventilation or dehumidification.
Common questions
What is a good dew point for indoors?
For a comfortable home in the 30–50% humidity band at normal room temperature, the dew point lands roughly between 40 °F and 52 °F. What matters more than the absolute figure is that it stays several degrees below your coldest interior surface.
Is dew point the same as humidity?
No. Relative humidity is a percentage of what the air could hold at its current temperature, so it changes as the air warms or cools even though the water content has not. Dew point is an absolute measure — it only changes when moisture is actually added or removed.
Why do my windows fog up in the morning?
Overnight the glass cools below the dew point of your indoor air, so water leaves the air onto the coldest surface in the room. Either lower the humidity or raise the glass temperature — better glazing, or moving heat toward the window.