Dew Point Calculator
Choose what to find. Enter the air temperature and the relative humidity to get the dew point, or the temperature and the dew point to get the relative humidity.
Your numbers
How we calculate it
The dew point is the temperature at which the air is full of water vapor and dew starts to form. The calculator uses the Magnus formula: with γ = ln(RH ÷ 100) + 17.62 × T ÷ (243.12 + T), the dew point is 243.12 × γ ÷ (17.62 − γ), where T is in °C. To go the other way, RH = 100 × exp(17.62 × Td ÷ (243.12 + Td) − 17.62 × T ÷ (243.12 + T)). Fahrenheit values are converted first.
For example, air at 25 °C with 60% humidity has a dew point of 16.7 °C, and air at 20 °C with 50% humidity has a dew point of 9.3 °C. At 100% humidity the dew point is the same as the air temperature.
Assumptions and limits
- Weather figures are estimates for general information. Check your local weather service for warnings and advice.
- The formula is accurate to a few tenths of a degree from −45 °C to 60 °C (−49 °F to 140 °F), which is the range accepted.
- The dew point can never be higher than the air temperature. Many people find the air muggy when the dew point is above about 65 °F (18 °C), and dry and comfortable when it is lower.
Frequently asked questions
What is the dew point?
It is the temperature to which air must cool for the water vapor in it to condense into dew or fog. The closer the dew point is to the air temperature, the more humid the air.
What is the difference between relative humidity and dew point?
Relative humidity says how full of water the air is compared with the most it could hold at its temperature, so it changes as the temperature changes. The dew point is an absolute measure of the moisture and does not change when the air warms up.
Why does the dew point matter?
It tells you how humid it feels and whether dew, frost or fog will form. If the surface temperature of a window or a pipe is below the dew point, water will condense on it.
Related calculators
- Heat Index CalculatorEnter the air temperature and the relative humidity.
- Wind Chill CalculatorEnter the air temperature and the wind speed.
- Temperature ConverterEnter a temperature in any scale and see it in Celsius, Fahrenheit and Kelvin at once.
- Percent Change CalculatorEnter a starting value and an ending value to see the percent increase or decrease and the absolute difference.
Worked example (a fixed example using the starting numbers, it does not change as you type)
Inputs: Find the = Dew point (from temperature and humidity); Units = US (°F and mph); Air temperature (°F) = 77; Relative humidity (%) = 60.
- Dew point 62.0 °F
- Also in °C 16.7 °C
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