Absolute Humidity Calculator
Use our absolute humidity calculator to calculate absolute humidity (lb/cu ft) from air temperature and relative humidity. Also returns actual vapor pressure (psi). Includes the absolute humidity formula, equation, and steps for how to calculate absolute humidity.
What Is Absolute Humidity?
Absolute humidity is the amount of water vapor in the air per unit volume. It tells you how much water vapor is actually present, usually expressed as a mass-per-volume value.
Relative humidity (RH) is different: it depends on temperature and describes how close the air is to saturation. Warmer air can hold more water vapor, so the same absolute humidity can produce a different relative humidity at a different temperature.
This calculator uses air temperature (°F) and relative humidity (%) to estimate actual vapor pressure (psi) and absolute humidity (lb/cu ft).
Absolute Humidity Formula
To calculate absolute humidity from temperature and relative humidity, you can compute saturation vapor pressure at the air temperature, scale it by RH to get actual vapor pressure, then convert vapor pressure into a vapor density using the ideal gas relationship.
Convert air temperature to °C for the saturation vapor pressure step.
T is in °C and e_s is in hPa.
e is the actual vapor pressure (same units as e_s).
1 hPa ≈ 0.0145038 psi.
Use e in pascals (Pa), T in kelvin (K), and R_v = 461.5 J/(kg·K). The result ρ_v is in kg/m^3.
These convert hPa → Pa and °C → K for the absolute humidity step.
1 kg/m^3 ≈ 0.06242796 lb/ft^3.
How to Calculate Absolute Humidity
- 1
Enter relative humidity (%).
- 2
Enter air temperature (°F).
- 3
The calculator estimates saturation vapor pressure at that temperature, then multiplies by RH to get actual vapor pressure (psi).
- 4
It converts vapor pressure into vapor density and outputs absolute humidity in lb/cu ft.
Frequently Asked Questions
A common approach is: compute saturation vapor pressure from temperature, multiply by RH to get actual vapor pressure, then convert vapor pressure into vapor density using the ideal gas relationship.
One standard form is ρ_v = e / (R_v × T), where e is actual vapor pressure (Pa), T is temperature (K), and R_v is the water vapor gas constant.
Absolute humidity is usually derived from measurements like temperature and humidity (or dew point) using psychrometric relationships. Dedicated sensors typically measure relative humidity and temperature, then absolute humidity is computed.
Actual vapor pressure is a direct measure of how much water vapor is in the air, and it is an intermediate step used to compute absolute humidity.
Yes. You can implement the same steps: compute saturation vapor pressure from temperature, multiply by RH, then use ρ_v = e / (R_v × T) with unit conversions.