Dew Point Calculator
Find the dew point from air temperature and relative humidity, and see how close a surface is to sweating.
This dew point calculator gives the temperature where the air in a room starts to drop water. Enter the air temperature and the relative humidity, and the dew point appears as you type.
A basic dew point tool stops at one number. This one has a second box for a surface, since a wet pipe or wall is the real worry. Type in its temperature and the calculator says whether it’ll sweat.
How the Dew Point Calculator Works
The calculator uses the Magnus approximation with the Alduchov and Eskridge constants. It’s a standard formula and stays within a fraction of a degree for normal indoor and outdoor air.
I checked it against reference values and against saturated air, where the dew point has to equal the air temperature.
With the example values (80°F air at 65% humidity), the dew point comes out near 67°F. Any surface at or below that collects water. The 65°F basement wall in the example sits 2.2°F under it, so it sweats.
Dew Point Chart: Temperature and Humidity
This chart comes from the same calculator, so it matches the tool. Find the air temperature on the left and the humidity across the top. The number inside is the surface temperature where water starts to form.
| Air temperature | 40% humidity | 50% humidity | 60% humidity | 70% humidity | 80% humidity |
|---|---|---|---|---|---|
| 60 F | 36 F | 41 F | 46 F | 50 F | 54 F |
| 70 F | 45 F | 51 F | 56 F | 60 F | 64 F |
| 75 F | 49 F | 55 F | 60 F | 65 F | 68 F |
| 80 F | 54 F | 60 F | 65 F | 69 F | 73 F |
| 90 F | 62 F | 69 F | 74 F | 79 F | 83 F |
Look at the 80°F row. At 40% humidity the dew point is 54°F, and at 70% it’s 69°F. The air temperature is the same, yet a surface has to be 15 degrees colder to stay dry.
How Humid Can the Air Get Before a Surface Sweats?
Turn the question around and the number gets more useful. Every surface has a highest humidity the room can reach before water forms on it. I worked it out for four common cold spots.
The surface temperatures are examples. Measure your own and run the calculator.
| Surface | Air at 70 F | Air at 75 F | Air at 80 F |
|---|---|---|---|
| Cold water pipe, about 50 F | 49% | 41% | 35% |
| Basement wall, about 60 F | 71% | 60% | 51% |
| Basement floor, about 65 F | 84% | 71% | 60% |
| Cold-weather window glass, about 40 F | 34% | 28% | 24% |
A 60°F basement wall in 75°F air starts to sweat at about 60% humidity. That’s right at the line EPA gives for a whole house. The same wall in 70°F air can take about 71%.
A cold water pipe at 50°F has almost no margin. It sweats at about 41% humidity in 75°F air.
EPA’s moisture guidance explains why the cold spot matters more than the room reading. The highest humidity in a room is always next to the coldest surface. EPA calls it the first condensing surface, and water shows up there first.
Three Cases Worked Through
- Humid summer day, basement wall: 80°F air at 65% humidity against a 65°F wall. The dew point is 67.2°F and the wall is 2.2°F below it, so water forms. A dehumidifier that drops the room to 55% moves the dew point to about 62.4°F, and the wall stays dry.
- Cold water pipe in a hot space: 90°F air at 70% humidity against a 55°F pipe. The dew point is 78.9°F, and the pipe is 23.9°F below it. That’s why cold water pipes drip in hot, humid spaces.
- A wall that stays dry but isn’t safe: 70°F air at 45% humidity against a 60°F wall. The dew point is 47.7°F, so no water forms. But the air touching the wall sits at about 64% humidity, above EPA’s 60% line.
Why does that last case matter? EPA notes that mold doesn’t need standing water. It can grow when high humidity at a surface lets enough moisture build up. A cold wall with no visible condensation isn’t automatically fine.
Which Problem Is It: Cold Surface or Wet Air?
EPA’s guidance splits indoor moisture problems into two types, and the fix for one can make the other worse. A cold surface needs to get warmer. Wet air needs to get drier. Three readings tell you which one you’ve got.
- Measure the air temperature and humidity in the middle of the room, and run them through the calculator.
- Point an infrared thermometer at the damp spot and read the surface temperature.
- Compare the two. A surface below the room’s dew point is a cold-surface problem. A warmer surface with a high dew point everywhere is a wet-air problem.
For a cold-surface problem, EPA’s fixes are to raise the temperature near the surface, or cut the heat loss through it with insulation or by closing cracks. EPA says this type may not respond very well to more ventilation.
For a wet-air problem, the fixes are source control, dilution and dehumidification. EPA adds that dilution only helps in the heating season, when cold outdoor air tends to hold less moisture.
If the readings point to a basement or crawl space, three guides go deeper: what humidity mold grows at in a basement, managing crawl space humidity in the right order and sizing a crawl space dehumidifier.
Water that seems to come from nowhere? Read why there’s water in a basement with no leak, which covers the four usual sources.
Is Humidity or Dew Point Worse?
For comfort outdoors, dew point is the better number. Relative humidity swings with the temperature all day, while the dew point mostly tracks how much water is in the air. One commenter on r/explainlikeimfive put it this way.
“I’m going to tell you, in terms of your comfort outdoors, the Dew Point is much more important than relative humidity. Anything above 70 degree dew point makes it feel sticky and heavy outside, no matter the relative humidity.”
– r/explainlikeimfive, August 2026 (30 upvotes)
Indoors, building problems need both numbers plus the surface temperature. EPA’s own example shows why. Air at 70°F and 40% humidity holds roughly twice the moisture of air at 50°F and 40% humidity, though the humidity reading is the same.
Dew Point Questions
How do I calculate the dew point?
Use the calculator above. By hand, the air temperature and relative humidity go into the Magnus formula, which gives the temperature where the air reaches 100% humidity. The math isn’t friendly, so a calculator is the practical way.
What does a dew point of 70 mean?
Any surface at 70°F or below gets wet from the air around it. Commenters in the Reddit thread above call anything over 70°F sticky outdoors. Indoors it’s a threshold for your coldest surface, which is why the table above matters more than the weather app.
What humidity should I keep indoors?
EPA advises keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. Check the table above to see whether your coldest surface can handle the number you’re at.
