Enter the partial pressure of water vapor in the air and the saturation pressure of water vapor at the same temperature into the calculator to determine the relative humidity. This calculator can also evaluate any of the variables given the others are known.

Psychrometrics Formula

The following formula is used to calculate the relative humidity in a psychrometrics calculator. 

RH = (Pw / Ps) * 100

Variables:

  • RH is the relative humidity
  • Pw is the partial pressure of water vapor in the air
  • Ps is the saturation pressure of water vapor at the same temperature

To calculate the relative humidity, divide the partial pressure of water vapor in the air by the saturation pressure of water vapor at the same temperature. Multiply the result by 100 to get the relative humidity in percentage.

What is a Psychrometrics?

Psychrometrics is the study of the physical and thermodynamic properties of gas-vapor mixtures. The term comes from the Greek words ‘psycho’ meaning cold and ‘metrics’ meaning measurement. It is primarily used in the field of engineering, particularly in HVAC (heating, ventilation, and air conditioning) to understand the behavior of air under varying conditions of temperature, pressure, and humidity. This helps in designing and optimizing air conditioning and heating systems.

How to Calculate Psychrometrics?

The following steps outline how to calculate the Relative Humidity using the formula RH = (Pw / Ps) * 100.

  1. First, determine the partial pressure of water vapor in the air (Pw).
  2. Next, determine the saturation pressure of water vapor at the same temperature (Ps).
  3. Next, use the formula RH = (Pw / Ps) * 100 to calculate the Relative Humidity.
  4. Finally, calculate the Relative Humidity.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

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

Relative Humidity (RH) = 75%

Partial pressure of water vapor in the air (Pw) = 12 kPa

Saturation pressure of water vapor at the same temperature (Ps) = 16 kPa