How to use the dew point calculator
- Choose Fahrenheit or Celsius.
- Enter the air temperature and the relative humidity in percent. You can read both from a weather report or a hygrometer.
- Read the dew point in both units, how far it is below the air temperature, and a plain description of how humid that feels.
- Use the result to judge condensation, comfort and mold risk.
Formula
g = ln(RH / 100) + 17.62 x T / (243.12 + T)
Dew point = 243.12 x g / (17.62 - g)
T is the air temperature in degrees Celsius and RH is relative humidity in percent
F = C x 9 / 5 + 32
This is the Magnus formula with constants that are accurate to a fraction of a degree between about minus 45 and 60 degrees Celsius. For Fahrenheit input, the temperature is converted to Celsius first and the result is converted back.
Worked example
On a day of 80 F (26.67 C) at 60% relative humidity, g = ln 0.6 + 17.62 x 26.67 / 269.79 = 1.2308, so the dew point is 243.12 x 1.2308 / (17.62 - 1.2308) = 18.3 C, or 64.9 F.
At 20 C and 50% humidity the dew point is 9.3 C. At 100% humidity the air is saturated, so the dew point equals the air temperature.
In the first example the dew point is about 15 degrees below the air temperature, so a cold glass of water at 50 F would collect condensation while one at 70 F would not.
Dew point and relative humidity are different
Relative humidity says how full the air is compared with how much it could hold at its current temperature. Because warm air can hold more water, the same moisture gives a high humidity on a cool morning and a low humidity on a hot afternoon. The dew point does not change with the time of day in the same way. It is the temperature to which air must cool to become saturated, so it tells you directly how much moisture is in the air.
That is why weather forecasters and many people watch the dew point for how muggy it feels. A dew point of 50 F feels dry to most people, while one of 70 F feels heavy.
Where dew point helps
Anywhere surfaces can be colder than the air, the dew point tells you whether water will collect on them.
- Windows, pipes and cold drinks gather condensation when their surface is below the dew point.
- Basements and bathrooms are more likely to grow mold when surfaces stay near or below the dew point.
- Outdoor work such as painting, concrete finishing and car detailing goes better when surfaces are several degrees above the dew point.
- The description in the result (dry, comfortable, humid, sticky) is a common rule of thumb for people and not a standard.
Assumptions and limits
- The Magnus formula is used with b = 17.62 and c = 243.12 degrees Celsius, valid roughly from -45 to 60 degrees Celsius.
- Relative humidity must be between 1% and 100%.
- Below freezing the result is technically the frost point and not a dew point. The number is still useful for judging condensation.
- The comfort description is an everyday rule of thumb and changes from person to person.
Frequently asked questions
What is a comfortable dew point?
Many people feel comfortable when the dew point is below about 60 F, find it sticky from about 65 F and oppressive at 70 F or more. These are rules of thumb and personal preferences vary.
How do I calculate dew point from humidity?
Use the temperature in degrees Celsius and the relative humidity in the Magnus formula shown here, or enter both values and let the calculator do it. At 80 F and 60% humidity the dew point is about 65 F.
Why is condensation forming on my windows?
The glass surface is colder than the dew point of the room air. Raising the window temperature with better insulation or lowering the dew point with ventilation or a dehumidifier will reduce it.
Can the dew point be higher than the air temperature?
No. At most they are equal, which happens at 100% humidity and means fog or dew is forming. If your reading shows otherwise, one of the measurements is off.