Absolute Humidity Calculator
Convert relative humidity and temperature to absolute humidity in g/m³ using vapor pressure and ideal gas law.
What Is Absolute Humidity?
Absolute humidity is the mass of water vapor in a given volume of air, usually expressed in grams per cubic meter (g/m³). Unlike relative humidity, it does not depend on temperature alone and directly measures how much moisture is present in the air.
Absolute Humidity Formula
This calculator uses the Magnus formula for saturation vapor pressure and converts it to absolute humidity:
$$AH = \frac{6.112 \cdot e^{\frac{17.67T}{T+243.5}} \cdot \frac{RH}{100} \cdot 216.74}{273.15 + T}$$where \(T\) is temperature in °C and \(RH\) is relative humidity in percent. At 25 °C and 50% RH, absolute humidity is about 11.5 g/m³.
Absolute vs Relative Humidity
Relative humidity compares current vapor pressure to the saturation value at that temperature. Warm air can hold more moisture, so the same absolute humidity corresponds to lower relative humidity at higher temperatures.
Frequently Asked Questions
What units does absolute humidity use?
Absolute humidity is typically expressed in grams per cubic meter (g/m³). Some engineering contexts use pounds per cubic foot.
How is absolute humidity different from relative humidity?
Relative humidity is a percentage of saturation at the current temperature. Absolute humidity is the actual mass of water vapor per unit volume.
What is the Magnus formula?
The Magnus formula estimates saturation vapor pressure over water as a function of temperature. It is accurate for typical meteorological temperatures.
Does pressure affect absolute humidity?
This simplified formula assumes near-standard atmospheric pressure. At very high altitudes, pressure corrections may be needed for precision work.
When is absolute humidity important?
HVAC design, drying processes, meteorology, and indoor air quality monitoring often need absolute humidity rather than relative humidity alone.
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