Dew Point Chart: Temperature, Humidity and When Windows Fog
Part of the project planner: Planning heating, cooling and ventilation.
The dew point is the temperature at which air becomes saturated and water starts to condense. Air at 70°F and 50% relative humidity has a dew point of 50.5°F: any surface colder than that gets wet. At 80% humidity the dew point rises to 63.5°F, and at 30% it drops to 37.1°F. The dew point calculator works out your own air, in °F or °C, and also the humidity from a dew point.
Dew point chart (°F)
By air temperature (rows) and relative humidity (columns):
| Air temperature | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|
| 50°F | 10° | 20° | 27° | 32° | 37° | 41° | 44° | 47° |
| 60°F | 19° | 28° | 36° | 41° | 46° | 50° | 54° | 57° |
| 65°F | 23° | 33° | 40° | 46° | 51° | 55° | 59° | 62° |
| 68°F | 25° | 35° | 43° | 49° | 54° | 58° | 62° | 65° |
| 70°F | 27° | 37° | 45° | 51° | 55° | 60° | 64° | 67° |
| 72°F | 29° | 39° | 46° | 52° | 57° | 62° | 65° | 69° |
| 75°F | 31° | 41° | 49° | 55° | 60° | 65° | 68° | 72° |
| 80°F | 35° | 46° | 54° | 60° | 65° | 69° | 73° | 77° |
| 85°F | 39° | 50° | 58° | 64° | 70° | 74° | 78° | 82° |
| 90°F | 44° | 54° | 62° | 69° | 74° | 79° | 83° | 87° |
Dew point chart (°C)
| Air temperature | 30% | 40% | 50% | 60% | 70% | 80% |
|---|---|---|---|---|---|---|
| 10°C | -6.8° | -3.0° | 0.0° | 2.6° | 4.8° | 6.7° |
| 15°C | -2.4° | 1.5° | 4.7° | 7.3° | 9.6° | 11.6° |
| 18°C | 0.2° | 4.2° | 7.4° | 10.1° | 12.4° | 14.5° |
| 20°C | 1.9° | 6.0° | 9.3° | 12.0° | 14.4° | 16.4° |
| 21°C | 2.8° | 6.9° | 10.2° | 12.9° | 15.3° | 17.4° |
| 22°C | 3.6° | 7.8° | 11.1° | 13.9° | 16.3° | 18.4° |
| 25°C | 6.2° | 10.5° | 13.9° | 16.7° | 19.1° | 21.3° |
| 30°C | 10.5° | 14.9° | 18.4° | 21.4° | 23.9° | 26.2° |
How humid it feels
| Dew point (°F) | How it feels |
|---|---|
| Below 50° | Dry and comfortable |
| 50° to 60° | Comfortable |
| 60° to 65° | A little sticky |
| 65° to 70° | Muggy and uncomfortable |
| Above 70° | Oppressive |
When do windows fog?
Condensation forms when a surface is colder than the dew point of the air touching it. Roughly, the inside surface of single-pane glass is only about a quarter of the way from the outdoor to the indoor temperature. For a 68°F room, the highest humidity before a single-pane window starts to fog is about:
| Outdoor temperature | Glass surface (approx.) | Highest indoor humidity |
|---|---|---|
| 10°F | 25°F | 19% |
| 20°F | 32°F | 26% |
| 30°F | 40°F | 35% |
| 40°F | 47°F | 46% |
| 50°F | 55°F | 62% |
Double-glazed and low-e windows have warmer inner surfaces, so they tolerate more humidity. The figures are rough guides: the real surface temperature depends on the window, the wind and the room.
Controlling indoor humidity
- Use exhaust fans in bathrooms and over the range, and vent them outdoors: see the CFM calculator for the airflow.
- Run a dehumidifier in damp basements and in summer: see the dehumidifier size calculator.
- Fix leaks and dry out wet materials; keep the ground around the house sloping away from it.
- Do not dry laundry indoors without ventilation, and remember that houseplants and aquariums add moisture.
- Check the humidity with a digital hygrometer, which costs very little.
Dew point from the Magnus formula; accurate to a fraction of a degree in normal indoor conditions.
Frequently asked questions
What is the dew point?
The temperature at which air is saturated and water begins to condense on surfaces. Air at 70°F and 50% humidity has a dew point of 51°F.
What is the dew point at 70 degrees and 50% humidity?
50.5°F, which is 10.3°C for air at 21.1°C and 50% humidity.
How do you find relative humidity from the dew point?
With the temperature and the dew point: 70°F air with a dew point of 50°F is 49% relative humidity.
Why do my windows fog up in winter?
The glass is colder than the dew point of the room air, so moisture condenses on it. Air at 68°F and 40% humidity has a dew point of 43°F, and single-pane glass on a cold day is colder than that.
What humidity should I keep indoors?
Between 30% and 50%, and below 60% to avoid mold. In very cold weather, lower humidity may be needed to stop windows fogging.
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