Dew Point Calculator: Find Out When Your Windows Will Fog
A dew point calculator turns room temperature and relative humidity into the temperature at which water starts to condense. At 70°F and 40% humidity the dew point is about 45°F, so any window glass colder than 45°F will fog or drip.
That makes the glass temperature the number that decides whether your windows stay dry. Put in what your hygrometer reads and add the temperature of the pane if you can measure it. The result gives the highest humidity that glass can take before it gets wet.
- Dew point and relative humidity in °F or °C
- Window check: will this glass fog?
- The highest humidity your coldest pane can take
Dew point and window condensation calculator
Enter the room temperature and humidity. Add the glass temperature to see whether your windows will fog.
1. What do you know?
2. Room air
3. Window glass (optional)
Read it with a surface or infrared thermometer pointed at the inside of the pane.
Dew point chart for indoor air
At a typical 68 to 72°F room, the dew point climbs from the high 20s °F at 20% humidity to about 60°F at 70% humidity, so read across from your room temperature to your humidity and any surface colder than that figure collects water.
| Room temp | 20% RH | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH |
|---|---|---|---|---|---|---|
| 60°F | 19°F | 28°F | 36°F | 41°F | 46°F | 50°F |
| 65°F | 23°F | 33°F | 40°F | 46°F | 51°F | 55°F |
| 68°F | 25°F | 35°F | 43°F | 49°F | 54°F | 58°F |
| 70°F | 27°F | 37°F | 45°F | 51°F | 55°F | 60°F |
| 72°F | 29°F | 39°F | 46°F | 52°F | 57°F | 62°F |
| 75°F | 31°F | 41°F | 49°F | 55°F | 60°F | 65°F |
I calculated every cell with the calculator’s own equation, rounded. The chart covers indoor temperatures only. It is accurate enough for deciding what to do about windows and walls, though it is not a substitute for a calibrated instrument in a lab, a museum case or a grow tent. I added the glass check to the calculator because a chart like this can’t tell you which of your windows will be the one that fogs.
How cold can the glass get before it fogs?
In a 70°F room, glass at 50°F stays dry up to about 49% humidity, glass at 45°F up to about 40%, glass at 40°F up to about 33%, and glass at 35°F up to about 27%.
Colder glass means a lower humidity ceiling, which is why the same room fogs on the coldest mornings and not on mild ones.
- Glass at 55°F: dry below about 58% humidity at 70°F, about 63% at 68°F.
- Glass at 45°F: dry below about 40% at 70°F, about 43% at 68°F.
- Glass at 35°F: dry below about 27% at 70°F, about 29% at 68°F.
- Glass at 30°F: dry below about 22% at 70°F, and the water that does form will freeze.
Turning the heat down shifts the ceiling in your favor: the same 40°F pane can take about 35% humidity in a 68°F room against about 33% at 70°F, because cooler air carries less water at the same reading.
North Dakota State University Extension recommends 30 to 40% humidity indoors in winter. It says anything above about 40% raises the potential for condensation on windows and other cool surfaces. The ceilings above show why: once the glass drops into the low 40s °F, 40% is already too much.
People who have fought wet windows reach the same conclusion. A reply in an r/DIY thread on window condensation put it plainly:
“Its all about dew point with the surface temp of your inside windows. Humidity level of the inside air is condensing on the temp of the inside glass.”
– r/DIY, December 2023 (82 upvotes)
Check your hygrometer before you trust the dew point calculator
The calculator is only as good as the humidity reading you give it. NDSU Extension warns that mechanical hygrometers are frequently off by more than 20 percent, and says electronic meters tend to be more accurate but should also be calibrated.
NDSU’s salt test takes a plastic bag, a cup, half a cup of water and a quarter cup of table salt. Seal the hygrometer in the bag with the cup, leave it about 12 hours at room temperature, and it should read about 75%. If it reads 68%, add 7 points to everything it tells you afterwards.
When the windows are already wet
Lower the humidity, warm the glass, or both. NDSU’s fix for winter condensation is humidity under 40% plus a warmer window surface, such as one covered with plastic. It also suggests shutting off humidifiers and running exhaust fans while cooking, bathing and showering.
The plastic-film fix also works for renters who can’t replace their windows. One on r/Netherlands described it on old single glazing:
“I have lived for some years in a rental space with single layer glass and found a good solution to be transparent window film. You apply it on the wood and creates an insulating air layer in between. Went from overflowing windowsills to not even a drop of water.”
– r/Netherlands, October 2024 (17 upvotes)
A dehumidifier won’t get you there in winter.
NDSU says dehumidifiers generally bring humidity down only to about 50%, which is above the ceiling for almost any cold pane in the list above. My guides to condensation on windows and how to lower humidity in a house go through the rest of the options.
The same arithmetic applies to plants. If you run a humidifier for tropical houseplants, the calculator’s maximum for your coldest window is the number to set the humidistat under on cold nights, and raising humidity for plants covers ways to give them moisture locally instead.
Dew point questions
How do you calculate dew point?
Dew point is calculated from temperature and relative humidity with the Magnus approximation. The University of Miami’s calculator publishes the version used here, with constants 17.625 and 243.04 from Alduchov and Eskridge (1996): first find γ = ln(RH/100) + 17.625T / (243.04 + T), then the dew point is 243.04γ / (17.625 − γ), with T in °C.
For 21°C (about 70°F) and 40% humidity, γ works out to about 0.49 and the dew point to about 6.9°C, or 44.4°F.
What dew point causes condensation on windows?
No single dew point causes window condensation; water forms whenever the glass is colder than the room air’s dew point. At 70°F and 40% humidity that threshold is about 45°F, so a pane at 40°F fogs and one at 50°F stays dry.
What’s worse, humidity or dew point?
Dew point, because it stays put when the air warms or cools. Warm air at 70°F and 40% humidity to 75°F and the reading drops to about 34%. Its dew point stays at about 45°F, and that is what decides whether a cold surface gets wet. To go the other way, switch the calculator to Find the humidity and it works as a relative humidity calculator from a temperature and a dew point.
What dew point feels comfortable?
Outdoors in summer, the National Weather Service office in La Crosse, Wisconsin, describes dew points of 55°F or lower as dry and comfortable, 55 to 65°F as sticky with muggy evenings, and 65°F or higher as oppressive. Relative humidity can’t tell you that on its own, because 50% feels very different at 60°F and at 90°F.
What does a dew point of 70 mean?
A dew point of 70°F means the air holds enough water to condense on anything below 70°F. If that air comes indoors and is cooled to 75°F, its humidity reads about 84%, and an air-conditioned surface such as a duct or a cold drink glass will drip.