Relative Humidity Calculator

Created by Luis Hoyos
Based on research by
Mark G. Lawrence The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications Bulletin of the American Meteorological Society 86(2) (01 Feb 2005)
Last updated: Oct 16, 2022

If you're searching for how to find relative humidity, this relative humidity calculator is what you need. This tool will help you to:

  • Calculate the relative humidity, given the air temperature and dew point;
  • Calculate the dew point, given the air temperature and relative humidity;
  • Predict the air temperature corresponding to some specific relative humidity and dew point.

Once you've calculated relative humidity, you can use it in our wet-bulb temperature calculator. You can also check our mixing ratio of air calculator to learn how to calculate relative humidity from air's actual and saturation mixing ratios. Additionally, you can use the result of this calculator in our heat index calculator to know the influence of relative humidity on how hot the day feels to us.

What is relative humidity?

Air can hold only a limited amount of water, and the relative humidity measures how close we are to reaching that limit amount. We can define relative humidity as the ratio of water vapor (moisture) present in the air to the total amount of water vapor the air can hold at a given temperature. Relative humidity goes from 0 (dry air) to 100% (saturated air).

Mathematically, we can express relative humidity (RH) as the ratio of vapor pressure (Pv) at a given temperature (T) to the water saturation pressure at that temperature (Psat @T):

RH = Pv/Psat @T

Notice that for 100% RH (saturated air), the vapor pressure equals the saturation pressure at the given temperature.

Although this formula is simple, relative humidity is usually calculated using the equation presented in the following section.

How do I find relative humidity?

Our calculator uses the following equation for relative humidity calculation:

RH%=100%×[e17.625×Dp243.04+Dpe17.625×T243.04+T]RH\% = 100 \% \times \left [ \frac{e^{\frac{17.625 \times D_p}{243.04 + D_p}}}{e^{\frac{17.625 \times T}{243.04 + T}}} \right ]

where:

  • RHRH% — Relative humidity (calculated as a percentage);
  • DpD_p – Dew point temperature (in °C); and
  • TT – Air temperature (in °C).

How do I calculate wet-bulb temperature?

Wet-bulb temperature is the lowest temperature a portion of air can reach by evaporative cooling. One of the applications of relative humidity is calculating the wet-bub temperature (TW) using the following equation:

Tw = T × arctan[0.151977(RH% + 8.313659)1/2]
+ arctan(T + RH%) - arctan(RH% - 1.676331)
+ 0.00391838(RH%)3/2arctan(0.023101RH%) - 4.686035

where RH% is expressed as a percentage.

Hopefully, now you know the relative humidity equation and some applications. Thanks for reading.

Luis Hoyos
Temperature
°F
Dew point
°F
Relative humidity
%
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