The absolute humidity calculator above answers a question relative humidity cannot: how much water is actually in this air? Relative humidity is a ratio. It tells you how close the air is to saturation at its current temperature, and because that saturation limit roughly doubles for every ten degrees Celsius of warming, the same percentage describes wildly different amounts of water depending on how warm the air is. Absolute humidity sidesteps the whole problem by reporting a mass concentration in grams of water vapour per cubic metre of air, which is a physical quantity you can weigh.
Arb Digital publishes this alongside a set of related atmospheric tools, and the boundary between them is worth stating plainly before you go further. The relative humidity calculator returns a percentage. The dew point calculator returns a temperature. This page returns neither — it returns a mass concentration in g/m³. All three describe the same moist air, but they are three different quantities with three different units, and mixing them up is the single most common error in humidity work.
What This Absolute Humidity Calculator Does
You give it a dry-bulb air temperature and a relative humidity. It computes the saturation vapour pressure at that temperature, scales it by the relative humidity to get the actual vapour pressure, then applies the ideal gas law to the water vapour alone to convert that pressure into a mass per unit volume. The headline figure is that mass concentration in grams per cubic metre.
The supporting grid gives four numbers the headline cannot. The saturation limit is the absolute humidity the same air would have at 100 % relative humidity, which is the ceiling temperature imposes on it. The room figure multiplies the concentration by the volume you entered, turning an abstract density into a total mass of water — usually a surprising one, because a normal bedroom routinely holds a litre of water in the air alone. The mixing ratio expresses the moisture as grams of vapour per kilogram of dry air, which is the form heating and ventilation work usually wants because it does not change when air is heated or cooled. The vapour pressure is the partial pressure of water vapour in hectopascals, the quantity every other humidity measure is ultimately derived from.
The subtitle under the headline also reports the dew point, because it makes the relationship visible: the dew point is simply the temperature at which the current absolute humidity would become the saturation limit. It is shown for orientation, not as this page's job.
How to Use It
- Set the temperature unit first. The dropdown changes how the temperature box is read, so pick Celsius or Fahrenheit before you type a number rather than after.
- Enter the dry-bulb air temperature. This is an ordinary thermometer reading, not a wet-bulb or dew-point reading. If you only have a dew point, set relative humidity to 100 % and enter the dew point as the temperature — the absolute humidity is the same either way.
- Enter the relative humidity. Whatever your hygrometer, thermostat or weather app reports. Values above 100 % are clamped, because supersaturated air is not a state a simple calculation can describe.
- Adjust the room volume if you care about total water. Multiply floor area by ceiling height. A 4 m by 4 m room with a 2.5 m ceiling is 40 m³, the default.
- Read the saturation limit next to the answer. The gap between the two is the room the air has left before condensation starts, and it is the number that predicts whether you will get fog on the windows tonight.
The Formula: How Absolute Humidity Is Calculated
Water vapour in air behaves closely enough to an ideal gas that the ideal gas law does the whole job. Rearranged for density, it gives absolute humidity = e × M ÷ (R × T), where e is the partial pressure of water vapour in pascals, M is the molar mass of water at 0.018015 kg/mol, R is the molar gas constant and T is the absolute temperature in kelvin. This tool uses the CODATA value of the molar gas constant published by NIST, 8.314462618 J mol⁻¹ K⁻¹, which is exact under the 2019 SI redefinition. Collapsing the constants gives the working form: absolute humidity in g/m³ = 2.16679 × e ÷ T, with e in pascals and T in kelvin.
Getting e requires the saturation vapour pressure first. This calculator uses the Buck equation, es = 6.1121 × exp((18.678 − T ÷ 234.5) × (T ÷ (257.14 + T))) in hectopascals with T in degrees Celsius, then multiplies by the relative humidity as a fraction. Buck is preferred over the older Magnus form here because it stays accurate to better than a tenth of a per cent across the range of ordinary weather.
Work through the defaults. At 22 °C the Buck equation gives a saturation vapour pressure of 26.44 hPa. At 60 % relative humidity the actual vapour pressure is 15.87 hPa, which is 1587 Pa. The absolute temperature is 295.15 K. So the absolute humidity is 2.16679 × 1587 ÷ 295.15 = 11.65 g/m³. In a 40 m³ bedroom that is 466 grams of water floating in the air — close to half a litre, held up by nothing but molecular motion.
Why Relative Humidity Misleads and Absolute Humidity Does Not
Sixty per cent relative humidity sounds like a fixed description of how damp air is. It is not. At 22 °C, 60 % relative humidity is 11.65 g/m³. At 0 °C, the same 60 % is only 2.91 g/m³ — a quarter as much water in the same volume of air. The percentage did not move; the physics under it did, because saturation vapour pressure rises roughly exponentially with temperature.
This is why cold outdoor air, however clammy it feels, is bone dry once you bring it indoors. Take that 0 °C air at 60 % relative humidity, heat it to 22 °C without adding any moisture, and its absolute humidity is unchanged at 2.91 g/m³ — but the saturation limit at 22 °C is 19.41 g/m³, so the relative humidity has collapsed to 15 %. That is the whole explanation for dry skin, static shocks and cracking furniture in heated buildings in winter, and none of it is visible if you only ever look at the percentage. The National Weather Service discussion of humidity from the Louisville office makes exactly this point, and defines absolute humidity in grams per cubic metre for the same reason.
Absolute Humidity Versus Mixing Ratio
Both describe how much water is in the air without reference to temperature, so they get treated as interchangeable. They are not, and the difference matters as soon as air moves between temperatures or altitudes.
Absolute humidity is mass of vapour per cubic metre of air. It has a volume in the denominator, so it changes when the air expands or contracts even though no water has been added or removed. Heat a sealed room and nothing happens; heat air that is free to expand and its absolute humidity falls, purely from the expansion. The mixing ratio, by contrast, is mass of vapour per kilogram of dry air. Mass does not change with temperature or pressure, so the mixing ratio is conserved through heating, cooling and vertical motion right up until condensation begins.
That conservation is why meteorology and building services prefer the mixing ratio for tracking air parcels. Absolute humidity earns its place where the volume genuinely matters: sizing a dehumidifier, or working out the vapour load in a fixed enclosure. If your problem has a cubic metre in it, use absolute humidity; if it follows air from one place to another, use the mixing ratio the grid also reports.
What the Saturation Limit Actually Predicts
The saturation figure in the grid is the ceiling on absolute humidity at your entered temperature. The gap between your actual value and that ceiling is often called the vapour pressure deficit when expressed in pressure terms, and it drives every evaporative process in the room: how fast washing dries, how fast a plant transpires, how fast a coat of paint or plaster cures. A small gap means slow drying no matter how much airflow you provide, because the air simply has nowhere to put more water.
The same gap predicts condensation, but only against the coldest surface present rather than the room air. A window pane on a winter night can sit ten degrees below room temperature. If the saturation limit at the pane's temperature falls below the room's actual absolute humidity, water condenses there. That is why a room can feel perfectly comfortable while streaming with condensation in one corner.
Where Pressure and Altitude Come In
Absolute humidity is unusual among humidity measures in barely caring about barometric pressure. The formula uses the partial pressure of water vapour, which is set by temperature and relative humidity alone, so moving the same air to a mountain top does not change the vapour pressure term at all. Only the modest expansion of the air affects it. Mixing ratio and specific humidity behave differently because their denominators involve the dry air, which does thin out with altitude — that is why the pressure box on this page feeds the mixing-ratio figure and nothing else.
The picture changes once you ask about the density of the moist air as a whole rather than its water content. Humid air is lighter than dry air at the same temperature and pressure, because a water molecule weighs less than the average nitrogen or oxygen molecule it displaces. That effect belongs to the air density calculator, which takes humidity as an input and returns the density of the mixture, and to the density altitude calculator, which restates that density as the altitude at which the standard atmosphere would match it. Use this page to get the vapour content, then hand it to those tools when the question is about lift, engine performance or buoyancy.
How This Differs From the Other Humidity Tools on This Site
Three pages here take temperature and moisture in and give a different quantity out, and choosing correctly saves a lot of confusion. The relative humidity calculator takes a dew point, a wet-bulb reading or a pair of vapour pressures and returns a percentage. The dew point calculator takes temperature and relative humidity and returns the temperature at which condensation starts. This page takes the same two inputs as the dew point tool but returns a mass concentration in grams per cubic metre, which is neither a percentage nor a temperature.
Alongside those, the vapour pressure calculator handles pure liquids by Antoine or Clausius–Clapeyron, and the temperature converter fixes scale problems before you start.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Treating absolute humidity as a percentage — it is a mass per unit volume in g/m³. If your answer has a per cent sign on it, you have computed relative humidity instead.
- Entering a dew point in the temperature box — the formula wants the dry-bulb air temperature. Entering a dew point with a relative humidity below 100 % double-counts the drying and understates the result.
- Comparing indoor and outdoor relative humidity directly — the two are usually at very different temperatures, so the percentages are not comparable. Convert both to absolute humidity first and the comparison becomes meaningful.
- Assuming absolute humidity is conserved when air is heated — it is not, because the volume changes. The mixing ratio is the quantity that survives heating and cooling unchanged.
- Predicting condensation from the room air temperature — condensation happens on the coldest surface, not in the middle of the room. Run the saturation figure at the surface temperature.
Related Free Tools From Arb Digital
Work the same air three ways with the relative humidity calculator and the dew point calculator, which return a percentage and a temperature where this page returns a concentration. For the density of the moist air itself, use the air density calculator, and for the aviation form of the same result the density altitude calculator. The vapour pressure calculator covers pure liquids, and the temperature converter handles scale changes before you start. Everything Arb Digital publishes is indexed on the free online tools hub.
Frequently Asked Questions
Absolute humidity is a mass concentration, reported in grams of water vapour per cubic metre of air. Relative humidity is a ratio, reported as a percentage of the maximum the air could hold at its current temperature. The same relative humidity corresponds to very different absolute humidities at different temperatures, which is why the percentage alone tells you little about how much water is present.
No. Dew point is a temperature and absolute humidity is a mass per unit volume. They are related, because the dew point is the temperature at which the current absolute humidity would equal the saturation limit, but they are different quantities with different units and are not interchangeable.
Comfortable heated interiors typically sit between about 6 and 12 grams per cubic metre. Below roughly 5 grams per cubic metre air feels dry and static builds up easily. Above about 14 grams per cubic metre condensation becomes likely on cold window surfaces, though the exact threshold depends entirely on how cold those surfaces are.
So it can turn a concentration into a total mass. Grams per cubic metre is hard to picture, but multiplying by the volume of an actual room gives the number of grams of water suspended in that room's air. It is usually far more than people expect, and it is the figure that matters when sizing a dehumidifier.
Barely. Absolute humidity depends on the partial pressure of water vapour, which is set by temperature and relative humidity, not by the total pressure of the air. The pressure input on this page is used only for the mixing-ratio figure, where the mass of dry air genuinely does depend on pressure.
The Buck equation over liquid water, which is accurate to better than a tenth of a per cent across ordinary weather temperatures. The result is multiplied by the relative humidity to get the actual vapour pressure, then converted to a mass concentration with the ideal gas law using the CODATA molar gas constant.
Use absolute humidity when the volume matters, such as sizing equipment for a fixed room or enclosure. Use mixing ratio when you are following air that will be heated, cooled or lifted, because mixing ratio is conserved through those changes and absolute humidity is not.
This tool is provided for educational and general reference use. It models idealised moist air over liquid water and does not account for supersaturation, ice-phase saturation below freezing, or instrument calibration, so treat its output as a physics result rather than a substitute for a measured reading.
