Dew Point Calculator
Find the dew point from the air temperature and relative humidity, or the relative humidity from a known dew point, in Fahrenheit or Celsius.
At 77 °F and 60% relative humidity, the dew point is 62.1 °F (16.7 °C), the temperature at which that air would become saturated and start to form dew. The calculator uses the National Weather Service formula: find the vapor pressure in the air from the humidity, then the temperature at which that vapor pressure would be the saturation pressure.
This calculator is for weather and building estimates and is not a substitute for a calibrated instrument. It uses an approximation over liquid water for air from -40 to 50 °C, and it does not give the frost point.
Dew point
62.1 °F
- Dew point
- 62.1 °F / 16.7 °C
- Relative humidity
- 60.0%
- Air temperature minus dew point
- 14.9 °F
- Vapor pressure (actual / saturation)
- 19.00 / 31.67 mb
Computed over liquid water with the National Weather Service formula. Below freezing this is the dew point, not the frost point.
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About the Dew Point Calculator
Relative humidity says how full the air is compared with the water vapor it could hold at its current temperature, so it changes as the air warms and cools even when the moisture stays the same. The dew point does not. It is the temperature the air would have to cool to, with no change in its moisture, before it becomes saturated and dew or fog forms. That makes it the steadier measure of how much water vapor is in the air. The calculator follows the National Weather Service. The saturation vapor pressure in millibars is 6.112 times e raised to the power of 17.67 times T, divided by T plus 243.5, with T in degrees Celsius. The actual vapor pressure is the relative humidity times that, and the dew point is the temperature whose saturation pressure matches it. At 25 °C and 50% humidity the dew point is 13.9 °C. It also runs the other way: air at 77 °F with a 62 °F dew point is at 59.9% relative humidity.
Frequently asked questions
What is the difference between dew point and relative humidity?+
Relative humidity is a percentage of what the air could hold at its current temperature, so the same air reads higher in the cool morning and lower in the warm afternoon. The dew point depends only on the moisture in the air, so it stays put unless moisture is added or removed.
Can the dew point be higher than the air temperature?+
No. When air cools to its dew point it is saturated, at 100% relative humidity, and any extra moisture condenses out as dew, fog or cloud. The calculator rejects a dew point above the air temperature, and at 100% humidity it returns the air temperature itself.
Does it work below freezing?+
The formula is for vapor over liquid water, so below 0 °C (32 °F) it gives the dew point rather than the frost point, the temperature at which frost forms on a surface. In freezing air the frost point is a little higher than the dew point, so treat results below freezing as approximate for frost.
How do I convert a dew point between Celsius and Fahrenheit?+
Multiply the Celsius value by 9/5 and add 32; to go back, subtract 32 and multiply by 5/9. The calculator shows both, so a 16.7 °C dew point is 62.1 °F.
Formula: U.S. National Weather Service, El Paso office, Relative Humidity and Dewpoint Temperature from Temperature and Wet Bulb Temperature: saturation vapor pressure es = 6.112 x e^(17.67 T / (T + 243.5)) in millibars with T in degrees Celsius, relative humidity = e / es x 100, and dewpoint Td = 243.5 x ln(e / 6.112) / (17.67 - ln(e / 6.112)).
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