Absolute Humidity Calculator
Absolute humidity tells you how much water vapor air actually contains, measured in grams per cubic meter, rather than as a percentage of saturation. To find it, compute the saturation vapor pressure at the air temperature using the Magnus formula, scale it by the relative humidity, then convert that vapor pressure to a vapor mass per unit volume with the ideal gas relationship. Enter the air temperature in Celsius and the relative humidity as a percentage and this calculator returns absolute humidity, the saturation vapor pressure, and the actual vapor pressure.
Absolute humidity formula
es = 6.112 x exp(17.67 x T / (T + 243.5)) hPa
e = es x (RH / 100)
AH = 2.1674 x e / (273.15 + T) x 100 g/m3
T is the air temperature in degrees Celsius and RH is the relative humidity in percent. The Magnus formula gives saturation vapor pressure; the ideal gas relationship for water vapor converts the actual vapor pressure to grams per cubic meter.
Humidity notes
- Absolute humidity is a mass per volume, independent of how full the air is relative to saturation.
- Saturation vapor pressure roughly doubles for every 10 to 11 degrees of warming.
- Comfortable indoor air near 20 C and 50 percent RH holds about 8.6 g/m3.
- The Magnus coefficients used here are for temperatures above freezing over liquid water.
- Convert Fahrenheit to Celsius before entering: (F - 32) x 5/9.
Absolute humidity: frequently asked questions
What is absolute humidity?
Absolute humidity is the mass of water vapor present in a given volume of air, expressed in grams per cubic meter. Unlike relative humidity, which is a percentage of saturation, absolute humidity is a direct measure of how much water the air actually holds.
How is absolute humidity calculated from temperature and relative humidity?
First find the saturation vapor pressure with the Magnus formula es = 6.112 x exp(17.67 x T / (T + 243.5)) in hectopascals, with T in Celsius. Multiply by relative humidity as a fraction to get the actual vapor pressure, then apply AH = 2.1674 x e / (273.15 + T) x 100, giving grams per cubic meter.
Why does absolute humidity rise so steeply with temperature?
Warm air can hold far more water vapor than cool air. Saturation vapor pressure roughly doubles for every 10 to 11 degrees Celsius of warming, so even at the same relative humidity, warm air contains much more water by mass. That is why a hot, humid day feels so heavy.
What is a typical absolute humidity value?
Comfortable indoor air at 20 degrees Celsius and 50 percent relative humidity holds about 8.6 grams per cubic meter. Hot tropical air can exceed 25 grams per cubic meter, while cold winter air may carry only 2 to 3 grams per cubic meter even at high relative humidity.
Which temperature scale should I use?
Enter temperature in degrees Celsius, which is what the Magnus coefficients in this formula assume. To convert from Fahrenheit, subtract 32 and multiply by 5/9. Relative humidity is entered as a percentage from 0 to 100.
Official sources
- NOAA National Weather Service: Weather Calculators.
- NOAA National Weather Service: Vapor Pressure (PDF).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.