Enter your indoor temperature and relative humidity to get the dew point: the temperature at which the moisture in the air turns into water. Add the temperature of a surface, such as the inside of a window or an exterior wall, and the calculator tells you whether condensation will form on it.
Dew point
53.6 °F
Add a surface temperature (for example, the inside of a window) to check the condensation risk.
Above 60% relative humidity, the risk of condensation and mold rises. Ventilate or dehumidify.
Calculated with the Magnus formula (coefficients by Alduchov and Eskridge, 1996), a standard approximation in meteorology. Nothing you enter is stored.
What the dew point tells you
Air can only hold so much water vapor, and the colder it is, the less it holds. When indoor air touches a surface colder than its dew point, the extra moisture condenses as droplets. That is why the same room can have dry walls but wet windows: the glass is the coldest surface. The US National Weather Service explains that the dew point is a direct measure of how much moisture the air contains, while relative humidity also depends on temperature.
How to use the calculator
- Read the temperature and relative humidity from a hygrometer placed in the middle of the room, away from windows and heat sources.
- If you want to check a specific spot, measure its surface temperature with an infrared thermometer: the lower corner of a window, a cold wall corner or the wall behind a wardrobe.
- Compare: if the surface is at or below the dew point, it will get wet. Within about 4 °F (2 °C) of it, the coldest spots nearby usually will too.
Dew point at common indoor conditions
| Room temperature | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|---|
| 62 °F | 37 °F | 43 °F | 48 °F | 52 °F | 56 °F |
| 66 °F | 41 °F | 47 °F | 52 °F | 56 °F | 60 °F |
| 70 °F | 45 °F | 51 °F | 55 °F | 60 °F | 64 °F |
| 74 °F | 48 °F | 54 °F | 59 °F | 64 °F | 67 °F |
Read it this way: in a room at 70 °F and 60% relative humidity, any surface colder than about 55 °F will collect water. Single-pane glass and the frames of older windows often drop below that in winter.
If the calculator shows a condensation risk
- Lower the humidity: run exhaust fans while cooking and showering, and avoid drying laundry indoors. The EPA recommends keeping indoor humidity below 60%, ideally between 30% and 50%.
- Warm the cold surface: keep heating steady, open curtains at night and leave a gap between furniture and exterior walls.
- Wipe condensation promptly and dry wet materials within 24 to 48 hours, as the EPA advises, so mold has no time to grow.
Frequently asked questions
Is a high dew point always a problem?
No. It only matters in relation to your coldest surfaces. A high dew point in a well-insulated room with double glazing may cause no condensation at all, while a modest one can wet single-pane windows on a cold night.
How accurate is the result?
The formula itself is precise enough for home use; the limit is your measurements. Cheap hygrometers can be off by several percentage points of humidity, and surfaces can be colder in corners than where you measured, so treat values near the dew point as a warning.
Why does condensation appear mostly in the morning?
At night the outdoor temperature drops, so windows and exterior walls get colder, while people sleeping in closed bedrooms keep adding moisture to the air. Both push surfaces below the dew point by early morning.
Sources
- US National Weather Service. Why dew point vs. humidity.
- US EPA. A brief guide to mold, moisture and your home.
- Alduchov, O. A. and Eskridge, R. E. (1996). Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology.