Estimate heat index or dew point from air temperature and relative humidity, or estimate relative humidity from air temperature and dew point.

Temperature with humidity calculator showing 90 degrees Fahrenheit at 65 percent relative humidity feels like 102.65 degrees Fahrenheit

Estimate the NWS heat index from shaded air temperature and relative humidity. This is not a wind-chill or individual safety assessment.

Weather estimate range: −40 to 50°C (−40 to 122°F).

Display settings

Related Calculators

Dew Point (Approximation) Formula

The following rule of thumb gives a rough dew point estimate from air temperature and relative humidity. It is most useful at higher humidity and is not a heat-index calculation. The calculator above uses the more detailed Magnus liquid-water approximation, so its dew-point answer can differ.

Td = T - ((100 - H)) / (5)

Variables:

  • Td is the dew point temperature (approx.) in degrees Celsius (°C)
  • T is the air (dry-bulb) temperature in degrees Celsius (°C)
  • H is the relative humidity in percent (%)

To estimate the dew point, subtract the quotient of (100 minus the relative humidity) divided by 5 from the air temperature (with temperatures expressed in °C).

What is the Effect of Humidity on Temperature?

Relative humidity describes how close the air is to being saturated with water vapor at a given temperature. Humidity does not directly change the air temperature measured by a thermometer, but it does affect other temperature-related quantities—especially the dew point, which is the temperature at which the air becomes saturated (100% relative humidity) and condensation can begin. Higher humidity generally means a higher dew point, which is associated with “muggier” conditions and increased condensation risk. The Heat index mode above uses the NWS method for shade and light wind. It is not wind chill or a personal safe-exposure assessment.

How to Calculate Dew Point (Approx.)?

The following steps outline how to estimate the dew point from air temperature and relative humidity using the approximation above.

  1. Measure the air temperature (T).
  2. Measure the relative humidity (H) in percent (%).
  3. Convert the temperature to degrees Celsius (°C) if it is not already in °C.
  4. Use the formula Td = T − (100 − H) / 5 to estimate the dew point.
  5. Compare this rough estimate with Dew point mode above, which uses the Magnus approximation rather than the rule of thumb.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Air Temperature (T) = 30°C

Relative Humidity (H) = 60%

Rule-of-thumb dew point: Td ≈ 30 − (100 − 60)/5 = 22°C. The calculator’s Magnus estimate for the same inputs is about 21.4°C.