How this calculator works
The dew point is the temperature at which air becomes saturated and water starts to condense. It depends on the air temperature and how much moisture is in it. The calculator uses the Magnus formula: it works out the quantity γ = ln(RH ÷ 100) + a × T ÷ (b + T) from the temperature T in °C, with a = 17.625 and b = 243.04, and then the dew point is b × γ ÷ (a − γ). The reverse formula gives the humidity from a dew point.
Worked example: air at 70°F and 50% relative humidity has a dew point of 50.5°F: any surface colder than that collects condensation. At 80% humidity the dew point is 63.5°F, and at 30% it is 37.1°F. In Celsius, 21°C and 50% gives 10.2°C. Going the other way, 70°F air with a dew point of 50°F is 49% relative humidity. A 68°F room at 40% humidity has a dew point of 42.8°F, so a window colder than that will fog.
Dew point (°F) by temperature and relative humidity
Air at this temperature and humidity condenses on surfaces colder than the dew point.
| Air temperature | 30% | 40% | 50% | 60% | 70% | 80% |
|---|---|---|---|---|---|---|
| 50°F | 20°F | 27°F | 32°F | 37°F | 41°F | 44°F |
| 60°F | 28°F | 36°F | 41°F | 46°F | 50°F | 54°F |
| 68°F | 35°F | 43°F | 49°F | 54°F | 58°F | 62°F |
| 70°F | 37°F | 45°F | 51°F | 55°F | 60°F | 64°F |
| 75°F | 41°F | 49°F | 55°F | 60°F | 65°F | 68°F |
| 80°F | 46°F | 54°F | 60°F | 65°F | 69°F | 73°F |
| 90°F | 54°F | 62°F | 69°F | 74°F | 79°F | 83°F |
Tips before you buy
- Keep indoor relative humidity between 30% and 50% (and below 60%) to avoid mold, dust mites and condensation. Use a cheap hygrometer to check.
- In winter, the colder the outside air, the lower the indoor humidity must be to stop windows fogging. Single-pane glass and uninsulated corners are the first to drip.
- Dew point does not change when the air warms or cools without gaining or losing moisture, while relative humidity does. That makes it the better measure of how humid the air feels.
- A basement or crawl space with cold walls in summer can condense moisture from warm humid air: a dehumidifier or better ventilation solves it.
- Outdoors, a dew point above 65°F feels muggy, and above 70°F is oppressive.
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Frequently asked questions
What is the dew point?
The temperature at which air is saturated with moisture and water begins to condense on surfaces. Air at 70°F and 50% humidity has a dew point of 51°F.
How do you calculate the dew point from humidity?
Use the Magnus formula (see above) or this calculator. A common shortcut is to take the temperature and subtract (100 − RH) ÷ 5 in °C, which gives 11°C for 21°C and 50%, close to the exact 10.2°C.
What humidity level causes condensation on windows?
Condensation forms when the glass is colder than the dew point of the room air. In a 68°F room at 40% humidity the dew point is 43°F, so single-pane glass colder than that will fog.
What is a good indoor humidity?
Between 30% and 50%, and under 60% to prevent mold. Higher humidity raises the dew point, so cold surfaces are more likely to get wet.
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.
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