How do you calculate absolute humidity?

Work out the saturation vapour pressure at the air temperature using the Magnus formula, multiply it by the relative humidity to get the actual vapour pressure, then apply the gas law for water vapour: absolute humidity in g/m3 equals 216.7 times the vapour pressure in hPa divided by the temperature in kelvin. At 20 C and 65% relative humidity that is 216.7 x 15.19 / 293.15, which is about 11.2 g/m3.

What is the difference between absolute and relative humidity?

Absolute humidity is the actual mass of water vapour in a cubic metre of air, in grams. Relative humidity expresses that amount as a percentage of the most the air could hold at its current temperature. Because warm air holds more moisture, the same relative humidity means very different amounts of water at different temperatures: 65% at 20 C is about 11.2 g/m3, while 65% at 5 C is only about 4.4 g/m3.

How much water vapour can air hold?

It rises steeply with temperature. Saturated air holds about 4.8 g/m3 at 0 C, about 9.4 g/m3 at 10 C, about 17.3 g/m3 at 20 C and about 30.3 g/m3 at 30 C. That steep curve is why cooling moist air quickly produces condensation, and why heating a damp room drops its relative humidity even though the amount of water in the air has not changed.

Absolute Humidity Calculator

This absolute humidity calculator converts air temperature and relative humidity into the actual mass of water vapour in each cubic metre of air, in grams. Relative humidity is the figure you hear in weather forecasts, but it only tells you how full the air is compared with the maximum it could hold at its current temperature. Because warm air can hold far more moisture than cold air, 65 percent relative humidity on a 20 degree afternoon carries several times more water than 65 percent on a frosty morning. Absolute humidity strips the temperature out and gives you a physical quantity you can compare directly across rooms, days and seasons. That makes it the number to use when you are sizing a dehumidifier for a damp Kiwi bedroom, drying firewood or washing indoors, managing a greenhouse, curing paint, or checking whether opening the windows will actually dry the house out. The calculator uses the Magnus formula to work out the saturation vapour pressure at your temperature, scales it by the relative humidity to get the actual vapour pressure, then applies the gas law for water vapour to get grams per cubic metre. It also shows the maximum the air could hold, so you can see the headroom before condensation.

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Air temperature

°C

Relative humidity

11.2 g/m³

water vapour in each cubic metre of air

Saturation vapour pressure23.37 hPa

Actual vapour pressure15.19 hPa

At saturation17.3 g/m³

At 20°C saturated air holds about 17.3 g/m³ of water vapour, so at 65% relative humidity each cubic metre of air is carrying 11.2 g, which is 65% of the maximum.

Uses the Magnus formula (6.112, 17.67, 243.5 coefficients), which is accurate to within about half a percent between -45°C and 60°C at normal atmospheric pressure. Figures are for vapour over liquid water.

How it works

The Magnus formula gives the saturation vapour pressure of water in hectopascals: 6.112 times e raised to (17.67 x T / (T + 243.5)), where T is the air temperature in degrees Celsius. That is the pressure water vapour exerts when the air holds as much as it can. Multiplying by the relative humidity gives the actual vapour pressure. Water vapour behaves closely enough to an ideal gas that its density follows from the gas law: density equals vapour pressure divided by the specific gas constant for water vapour (461.5 J/kg/K) times the absolute temperature. With pressure in hPa and the result in grams, the constants collapse to a simple expression: absolute humidity in g/m³ equals 216.7 times the vapour pressure in hPa divided by the temperature in kelvin. The calculator also runs the same sum on the saturation pressure to show the ceiling for your temperature.

Worked example

At 20°C the exponent is 17.67 x 20 / 263.5 = 1.3412, so the saturation vapour pressure is 6.112 x e^1.3412 = 23.37 hPa. At 65% relative humidity the actual vapour pressure is 23.37 x 0.65 = 15.19 hPa. The temperature in kelvin is 293.15, so the absolute humidity is 216.7 x 15.19 / 293.15 = 11.2 g/m³. Saturated air at the same temperature would hold 216.7 x 23.37 / 293.15 = 17.3 g/m³, so this air is carrying about two-thirds of its ceiling. Cool it to about 13°C, the dew point, and it would reach 100% relative humidity and start condensing on windows and walls.

Related calculators

Dew Point Calculator: when condensation starts.
Vapour Pressure Calculator: vapour pressure in detail.
Air Density Calculator: density of moist air.
Heat Index Calculator: how hot it feels in humidity.

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