Absolute Humidity Calculator

What the absolute humidity calculator does

Absolute humidity tells you the actual amount of water vapour in the air, by mass, regardless of temperature. This calculator works it out from the air temperature and the relative humidity, and also reports the vapour pressure along the way.

Below is what absolute humidity is, how it differs from relative humidity, how the calculator arrives at it, and a worked example.

How to use it

1. Enter the air temperature in degrees Celsius.
2. Enter the relative humidity as a percentage.
3. Press Calculate for the absolute humidity and the actual vapour pressure, or Reset to clear it.

What absolute humidity is

Absolute humidity is the mass of water vapour present in a given volume of air, usually expressed in grams of water per cubic metre. It is a direct count of the moisture itself: how much water, physically, is floating in the air. Unlike relative humidity, it does not refer to how full the air is or how close to saturation, only to the plain quantity of vapour present.

This makes it the most concrete of the humidity measures. If you could squeeze all the water vapour out of a cubic metre of air and weigh it, absolute humidity is the number you would get. Because it counts the moisture directly rather than as a fraction of capacity, it stays meaningful even as the temperature changes around it, which is exactly what makes it useful for comparing one situation to another.

Absolute versus relative humidity

The difference between the two is worth getting clear. Relative humidity is a percentage, telling you how full the air is relative to the most it could hold at its current temperature. Absolute humidity is an actual mass, telling you how much water vapour is there, full stop. The first changes the moment you heat or cool the air, even if no water is added or removed; the second does not.

Picture a sealed room. If you warm the air without adding any moisture, the relative humidity falls, because the warmer air could hold more and is now less full. But the absolute humidity stays the same, because the actual amount of water vapour in the room has not changed. This is the heart of the distinction: relative humidity tracks comfort and saturation, while absolute humidity tracks the moisture itself.

How the calculator gets there

Finding the absolute humidity takes two steps. First the calculator works out how much water vapour the air actually contains, in terms of vapour pressure. It does this by computing the saturation vapour pressure, the most the air could hold at the given temperature, using a high-accuracy formula for water, then scaling it by the relative humidity to get the actual vapour pressure present.

Second, it converts that vapour pressure into a mass per volume. Water vapour, like any gas, links its pressure, temperature, and density through the gas law, so dividing the vapour pressure by the temperature and the gas constant for water vapour gives the density of the vapour, which is the absolute humidity. The calculator reports both the intermediate vapour pressure and the final absolute humidity, so you can see the moisture expressed in two natural ways.

Why absolute humidity is the fair comparison

Because absolute humidity counts the moisture directly, it lets you compare moisture levels across very different conditions on equal terms. Relative humidity can mislead here: cold winter air at 90 percent relative humidity actually holds very little water, while warm summer air at 50 percent holds a great deal more, even though the percentages suggest the reverse. Absolute humidity cuts through that by reporting the real amount.

This matters wherever the actual moisture content is what counts rather than how the air feels. It is used in drying and storage, where the goal is to control how much water is around; in museums and archives, where materials must be kept at steady moisture; and in comparing indoor and outdoor air, or one season against another. Whenever you need to know how much water is genuinely in the air rather than how saturated it is, absolute humidity is the figure to reach for, and this calculator provides it.

Units and precision

The calculator takes the temperature in degrees Celsius and the relative humidity as a percentage, and returns the absolute humidity in grams per cubic metre along with the actual vapour pressure in pascals. Internally it converts the temperature to kelvin and uses a precise saturation-pressure formula for water together with the specific gas constant for water vapour. The result is an accurate estimate of the moisture content. Absolute humidity is shown to three decimal places.

A worked example

Suppose the air is at 25 degrees Celsius with a relative humidity of 60 percent.

At 25 degrees, the saturation vapour pressure of water is about 3,170 pascals, so at 60 percent humidity the actual vapour pressure is about 1,900 pascals. Converting that to a mass per volume gives an absolute humidity of about 13.8 grams per cubic metre. That is the real weight of water vapour floating in each cubic metre of this air, a figure that would stay the same even if the relative humidity changed because the temperature did.

Questions people ask

What is absolute humidity?

The actual mass of water vapour in a given volume of air, usually grams per cubic metre. It counts the moisture directly, regardless of temperature or how close the air is to saturation.

How is it different from relative humidity?

Relative humidity is a percentage of the air's capacity and changes with temperature; absolute humidity is an actual mass and does not. Warming sealed air lowers its relative humidity but leaves its absolute humidity unchanged.

How is absolute humidity calculated?

By finding the actual vapour pressure, from the saturation pressure at that temperature scaled by the relative humidity, then converting that pressure to a vapour density using the gas law for water vapour.

Why use absolute humidity instead of relative?

Because it compares moisture fairly across different temperatures. Cold air at high relative humidity holds less water than warm air at moderate relative humidity, and absolute humidity reveals the real amounts.

References

A quick note on where the science comes from. The definition of absolute humidity and its calculation from vapour pressure and the gas law are standard meteorology and physics, described in the Wikipedia article on humidity and in Vaisala's humidity references, with saturation-pressure data for water following the International Association for the Properties of Water and Steam.

1. Wikipedia, Humidity (absolute humidity). https://en.wikipedia.org/wiki/Humidity
2. Vaisala, Humidity Conversion Formulas (technical reference). https://www.vaisala.com/en/lp/make-your-job-easier-humidity-conversion-formulas
3. International Association for the Properties of Water and Steam (IAPWS), formulations for the properties of water. http://www.iapws.org/