Absolute Humidity Calculator

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.

To learn more about this topic, keep reading this article to understand what it is and how to calculate absolute humidity (the formula).

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/(R_w × T)

, where:

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

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

, 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 Wagner and Pruß:

, 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
  • $a_6 = 1.80122502$.

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:

, 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.