Free calculator
Dew Point Calculator: Will Your Windows Fog Up?
Enter the temperature and relative humidity to get the dew point, then check whether a window, wall or pipe will collect condensation.
Humidity in the air is invisible until it meets something cold enough. The dew point tells you exactly how cold that is. It’s the number to watch when you run a humidifier in winter, when windows stream with water, or when a basement wall or cold-water pipe feels damp.
How the calculator works
It uses the Magnus formula with the coefficients recommended by Alduchov and Eskridge (1996), a = 17.625 and b = 243.04 °C:
γ = ln(RH ÷ 100) + a·T ÷ (b + T)
Dew point = b·γ ÷ (a − γ)
with temperatures in °C (°F values are converted first and back again). For the condensation check, the calculator compares the surface temperature with the dew point, and works out the highest indoor humidity that surface can take before it gets wet.
Worked examples
These results match published dew point tables to within 0.1 °F:
| Air temperature | Relative humidity | Dew point |
|---|---|---|
| 70 °F | 40% | 44.6 °F (7.0 °C) |
| 68 °F | 50% | 48.7 °F (9.3 °C) |
| 75 °F | 60% | 60.2 °F (15.7 °C) |
| 72 °F | 30% | 38.9 °F (3.8 °C) |
So in a 70 °F room at 40% humidity, any surface below 44.6 °F will collect water. A quick rule of thumb from the same research: above 50% humidity, the dew point drops about 1 °C for every 5% drop in relative humidity.
Windows: how much humidity is too much?
Windows are usually the coldest surface in a heated home, so in winter they set the practical limit for indoor humidity. NRC Canada calculated the inside glass temperature for a 70 °F room with a 15 mph wind outside:
| Outdoor temperature | Single glazing fogs above | Double glazing fogs above |
|---|---|---|
| 40 °F | 44% RH (glass at 47 °F) | 68% RH (glass at 59 °F) |
| 0 °F | 12% RH (glass at about 17 °F) | 41% RH (glass at about 45 °F) |
That’s why advice to keep indoor humidity between 30% and 50% (the EPA’s range) needs adjusting in a cold snap: with older windows, even 35% can mean wet sills. Read our guide to the ideal indoor humidity for how to set a humidifier through the seasons.
Tip: An infrared thermometer measures a window or wall’s surface temperature in seconds. Enter it in the optional field to see whether that surface will stay dry.
What to do about condensation
The EPA’s advice is to dry wet surfaces quickly and reduce the moisture source: lower the humidity, ventilate (bathroom and kitchen fans), insulate cold surfaces such as cold-water pipes, and keep the air moving near windows. If a humidifier is making a room damp or hazy, see why a humidifier makes the room foggy and whether to close the door when using a humidifier. Persistent dampness can lead to mold; our guide to the best air purifier for mold explains what a purifier can and can’t do about it.
Frequently asked questions
What is the dew point?
The dew point is the temperature at which air becomes saturated and water vapor starts to condense. Any surface at or below the dew point of the room air collects condensation: water droplets, or frost below freezing.
What is the dew point at 70 °F and 40% humidity?
About 44.6 °F (7.0 °C). A window pane colder than that will fog up.
Why do my windows get condensation in winter?
In cold weather the inside surface of the glass is the coldest surface in the room. When it drops below the dew point of the indoor air, water condenses on it. Lowering indoor humidity or warming the glass stops it.
What indoor humidity is too high in winter?
It depends on how cold it is outside and on your windows. In an NRC Canada calculation with a 70 °F room, double glazing starts to fog above about 41% relative humidity when it is 0 °F outside, and single glazing above about 12%.
How accurate is the calculator?
It uses the Magnus formula with the Alduchov and Eskridge (1996) coefficients, which is accurate to about 0.4% for saturation vapor pressure between −40 °C and 50 °C. That is far more precise than the hygrometers most homes use.
Related guides
Humidifiers
Ideal Indoor Humidity: What Level to Aim For in Every Season
Aim for 30–50% relative humidity, and lower in a cold snap. What the EPA and others say, how to read a hygrometer and when to run a humidifier.
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Why Is My Humidifier Making the Room Foggy? Causes and Fixes
A little mist above the nozzle is normal. A haze that fills the room means too much output, air near its dew point or tap-water minerals. Here's the fix.
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