About the Absolute Humidity Calculator
This absolute humidity calculator tells you how much water vapour is actually in the air, as a mass per unit volume (grams per cubic metre). Enter the air temperature and relative humidity and it returns the vapour density, the most the air could hold at that temperature, the vapour pressure, the mixing ratio and specific humidity used in meteorology and HVAC, the dew point, and the total grams of water in a room of the size you give.
Absolute humidity is the number to compare when temperatures differ. It explains why cold winter air at 80% RH becomes desert-dry once heated indoors, helps size a humidifier or dehumidifier, and is used in drying, storage, cleanroom, greenhouse and virus-survival studies. The chart shows the saturation curve so you can see how sharply the air's capacity for water rises with temperature.
Vapour pressure comes from the Magnus formula (Alduchov–Eskridge constants) over liquid water, and density from the ideal gas law for water vapour. Results are accurate to within about 1% for normal weather and indoor conditions; below freezing the RH is assumed to be reported relative to water, as weather stations do.
With the default inputs, the absolute humidity is 8.62 g/m³. Change any value above to recalculate instantly.
How to use the absolute humidity calculator
- 1Pick °C or °F and enter the air temperature.
- 2Enter the relative humidity from your hygrometer or forecast.
- 3Optionally change the air pressure (for mixing ratio) and room volume.
- 4Read the absolute humidity and how close it is to saturation.
- 5Use the chart to compare air at different temperatures.
Formula and method
First the saturation vapour pressure at the air temperature is found with the Magnus formula, and multiplying by RH/100 gives the actual vapour pressure e. Treating water vapour as an ideal gas, its density is e divided by the specific gas constant of water vapour (461.5 J/kg·K) times the absolute temperature; with e in pascals this gives kg/m³, which the calculator reports in g/m³.
The mixing ratio w is grams of vapour per kilogram of dry air, and specific humidity q = 622e ÷ (P − 0.378e) is grams of vapour per kilogram of moist air. Both depend on pressure, while absolute humidity does not. The dew point is found by inverting the Magnus formula.
- AH
- Absolute humidity (water vapour density)
- e
- Vapour pressure
- R_v
- Gas constant of water vapour, 461.5 J/(kg·K)
- T_K
- Temperature in kelvin
- w
- Mixing ratio (g/kg dry air)
- P
- Air pressure (hPa)
Worked examples
Typical room: 20 °C at 50% RH
eₛ(20) = 23.33 hPa, so e = 11.67 hPa. AH = 1,166.7 Pa ÷ (461.5 × 293.15) = 8.62 g/m³, half the 17.25 g/m³ maximum. A 40 m³ room holds about 345 g of water vapour.
Humid summer: 30 °C at 70% RH
e = 0.70 × 42.37 = 29.66 hPa, giving 21.2 g/m³ – about two and a half times the room example – and a dew point of 23.9 °C.
Winter outdoor air: 0 °C at 80% RH
eₛ(0) = 6.11 hPa so e = 4.89 hPa and AH = 3.88 g/m³. Heated to 20 °C without adding moisture, that air would sit at only about 21% RH.
77 °F at 40% RH
77 °F is 25 °C, where saturation is 31.62 hPa. At 40% e = 12.65 hPa, so AH = 9.19 g/m³ and the dew point is 50.8 °F.
Frequently asked questions
What is the difference between absolute and relative humidity?+
Absolute humidity is the actual mass of water vapour in a volume of air, in g/m³. Relative humidity is that amount as a percentage of the maximum the air could hold at its temperature, so it changes whenever the temperature changes.
How do you convert relative humidity to absolute humidity?+
Find the saturation vapour pressure at the temperature, multiply by RH/100 to get the vapour pressure e, then divide e (in Pa) by 461.5 × the temperature in kelvin. Multiply by 1,000 to get grams per cubic metre.
What is a normal absolute humidity indoors?+
At typical indoor conditions of 20–22 °C and 30–50% RH, absolute humidity is roughly 5 to 10 g/m³. In heated winter homes it can fall to 3–5 g/m³, which feels dry.
Why is winter air so dry indoors?+
Cold air can hold very little water: at 0 °C saturation is under 5 g/m³. When that air is heated to 20 °C its moisture content stays the same but the capacity rises to over 17 g/m³, so the RH drops sharply.
What is the mixing ratio?+
The mixing ratio is the mass of water vapour per kilogram of dry air, usually in g/kg. Meteorologists like it because it stays constant when air rises or sinks without condensation, unlike absolute humidity which changes as air expands.