# Absolute Humidity Calculator

Created by Luis Hoyos
Last updated: Nov 05, 2022

Welcome to this absolute humidity calculator, a tool created to go from relative humidity to absolute humidity. If you know the actual vapor pressure instead of relative humidity, you can also input it and obtain the absolute humidity.

## What is and how to calculate absolute humidity?

#### What is absolute humidity?

Absolute humidity indicates the amount of water vapor in the air. We usually express it in grams of water vapor per cubic meter of air (g/m³). The absolute humidity of air can vary depending on temperature and pressure. At higher temperatures, the air can hold more water vapor. And at higher pressures, the air is denser and can also have more water vapor.

#### Absolute humidity formula

This calculator uses the following formula for absolute humidity calculation:

AH = Pₐ/(Rw × T)

, where:

• AH — Absolute humidity, calculated in kilograms per cubic meter (kg/m³);
• P — Actual vapor pressure, in pascals (Pa);
• T — Air absolute temperature, in kelvin (K); and
• Rw = 461.5 J/(kg K) — Specific gas constant for water vapor;

💡 To convert your result in kg/m³ to g/m³, divide it by 1000 (as 1000 g/m³ = 1 kg/m³), or use our density converter.

## Actual vapor pressure

Usually, we know the relative humidity instead of P, so we calculate the latter using the former:

$\scriptsize P_\text{a} = P_\text{s} \times (RH/100)$

, where:

• $RH$ — Relative humidity, expressed as a percentage; and
• $P_\text{s}$ — Saturation vapor pressure​, in $\text{Pa}$

For the saturation vapor pressure, we only need the temperature ($T$) in K and the following equation proposed by :

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

, where:

• $P_\text s$ — Saturation vapor pressure, in megapascals ($\text{MPa}$).
• $P_\text c = 22.064 \text{ MPa}$ — Critical pressure for water;
• $T_\text c = 647.096\text{ K}$ — Critical temperature for water;
• $\tau = 1 - \frac{T}{T_\text{c}}$; and
• $a_1, a_2, .., a_6$ — Empirical constants, whose values are:
• $a_1 = -7.85951783$;
• $a_2 = 1.84408259$;
• $a_3 = -11.7866497$;
• $a_4 = 22.6807411$;
• $a_5 = -15.9618719$; and
• $a_6 = 1.80122502$.

## Relative humidity

You can measure the RH using a hygrometer or obtain it with our relative humidity calculator if you know the dew point temperature. That calculator uses the following relationship to get the relative humidity:

$\scriptsize RH = 100 \% \times \left [ \frac{e^{\frac{17.625 \times D_p}{243.04 + D_p}}}{e^{\frac{17.625 \times T}{243.04 + T}}} \right ]$

, where:

• $RH$ — Relative humidity, expressed as a percentage;
• $D_p$ – Dew point temperature in degrees Celsius (°C); and
• $T$ – Air temperature, in °C.

That's it! Hopefully, now you know how to go from relative humidity to absolute humidity using the calculator and the formulas.

Luis Hoyos
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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