Absolute Humidity Calculator

Created by Purnima Singh, PhD
Reviewed by Steven Wooding
Last updated: Feb 14, 2023

The absolute humidity calculator allows you to find out the absolute humidity from the relative humidity and air temperature.

• What is absolute humidity?
• What is the difference between absolute humidity and relative humidity?
• How to calculate absolute humidity.

You will also find an example of using our tool for converting relative humidity to absolute humidity and vice-versa.

If you are interested in calculating relative humidity using actual and saturation mixing ratios, our mixing ratio of air calculator can help you with that.

What is absolute humidity? – Absolute humidity definition

Absolute humidity measures the actual amount of water vapor present in a given volume of air. We can express absolute humidity ($AH$) as the mass of water vapor ($m$) per cubic unit volume of air ($V$).

$\!\scriptsize AH = \frac{m}{V}$

Relative humidity

Relative humidity is the amount of water vapor present in the air expressed as a percentage of the maximum possible amount of water vapor that the air can hold at a given temperature.

We know that the air is a mixture of several gases, including water vapor, each with its own vapor pressure.

According to the definition of relative humidity, we can express it as the ratio of the actual vapor pressure of water $P_a$ to the saturation vapor pressure $P_{s}$ at a given temperature, i.e.,

$\!\scriptsize RH = 100 \times \frac{P_a}{P_{s}}$

🙋 Our vapor pressure of water calculator is a handy tool that can help you determine the vapor pressure of water and ice.

Absolute humidity vs relative humidity

Both relative humidity and absolute humidity measure the water vapor levels in the atmosphere. However, it is worth noting that absolute humidity does not take into account the air temperature. It only depends on the amount of water vapor.

Relative humidity, on the other hand, depends on the air temperature.

We express relative humidity as a percentage, whereas absolute humidity is expressed in units of g/m3 or kg/m3.

Now that we understand the essential difference between absolute humidity and relative humidity, let us learn how to calculate the value of one if we know the other.

How to calculate absolute humidity from relative humidity and temperature

If we know the relative humidity and air temperature, we can easily calculate the absolute humidity using the equation:

$\!\scriptsize AH = \frac{RH \times P_s}{R_w \times T \times 100}$

where:

• $RH$ – Relative humidity;
• $P_s$ – Saturation vapor pressure, measured in pascal (Pa) ;
• $R_w= 461.5~ \rm{J/(kg⋅K)}$ – Specific gas constant for water vapor; and
• $T$ – Temperature, measured in kelvin (K).

According to the equation proposed by , the saturation vapor pressure of water at a given temperature $T$ can be written as:

$\!\scriptsize {\begin{gather*} P_s \end{gather*}} = {\begin{gather*} P_c\exp\!\! \end{gather*}} \left[ \frac{T_c}{T} \left ( {\begin{gather*} a_1\tau + a_2\tau^{1.5} + a_3\tau^3 + \\ a_4\tau^{3.5} + a_5\tau^{4} + a_6\tau^{7.5} \end{gather*}} \right ) \right ]$

where:

• $P_c = 22.064 \ \rm {MPa}$ – Critical pressure for water;
• $T_c = 647.096\ \rm K$ – Critical Temperature for water;
• $a_1, a_2, .., a_6$ – Empirical constants; and
• $\tau = 1 - \frac{T}{T_c}$.

How to use the absolute humidity calculator

Now let's see how to calculate the absolute humidity from relative humidity and temperature using our calculator. We will consider an example where the relative humidity is 60%, and the air temperature is 32 °C.

1. Enter the values of relative humidity (60%) and air temperature (32 °C).

2. The calculator will display the actual vapor pressure (2856 Pa) and the absolute humidity (20.278 g/m3).

3. If you want to calculate relative humidity, just enter the values of absolute humidity and temperature. Note: you may have to reset the calculator by clicking the reload button between calculations.

FAQ

How do I calculate absolute humidity from vapor pressure?

To calculate the absolute humidity from actual vapor pressure, proceed as follows:

1. Multiply the air temperature T with specific gas constant for water vapor, 461.5 J/(kg⋅K).

2. Divide the actual vapor pressure P by the value obtained from step 1.

3. Congrats! You have calculated absolute humidity from actual vapor pressure.

How do I calculate absolute humidity from relative humidity?

To calculate absolute humidity from relative humidity, follow the given instructions:

1. Determine the air temperature (T) and relative humidity (RH).
2. Find out the saturation vapor pressure (P).
3. Calculate the absolute humidity using the absolute humidity formula:
AH = (RH × P)/(461.5 J/(kg⋅K) × T × 100)

What does 40% humidity means?

We express relative humidity as a percentage, and it measures the saturation level of air with water. Hence, relative humidity of 40% means that air is 40% saturated with water.

What is the unit of absolute humidity?

Absolute humidity is the mass of water vapor present in a unit volume of air. Hence, we can express it in terms of the units of the expression mass/volume, i.e., g/m3 or kg/m3.

Purnima Singh, PhD
Enter one of the humidity values and temperature
Relative humidity
%
Air temperature
°F
Actual vapor pressure
psi
Absolute humidity
lb/cu ft
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