Absolute Humidity Calculator
Absolute humidity is the mass of water vapour in each cubic metre of air. At 20 °C and 50% relative humidity, air holds about 8.6 g of water per m³, half of the 17.3 g/m³ it could hold when saturated.
Air “glass” is 50% full
Absolute humidity
8.62 g/m³
Each cubic metre of this air holds 8.62 g of water vapour. At 20°C it could hold at most 17.25 g/m³.
Vapour pressure
11.67 hPa
Saturation pressure
23.33 hPa
Mixing ratio
7.25 g/kg
Water in 50 m³ room
431 g
Show the math
- 1
Find the saturation vapour pressure
es = 6.1094 × e^(17.625 × 20 ÷ (20 + 243.04)) = 23.33 hPa
This is the most water vapour the air can hold at this temperature, expressed as a pressure (Magnus formula). Warm air holds much more.
= 23.33 hPa
Read the steps as text
- Find the saturation vapour pressure. es = 6.1094 × e^(17.625 × 20 ÷ (20 + 243.04)) = 23.33 hPa This is the most water vapour the air can hold at this temperature, expressed as a pressure (Magnus formula). Warm air holds much more.
- Scale by the relative humidity. e = 50% × 23.33 = 11.67 hPa Relative humidity is the share of that maximum which is actually present.
- Convert the vapour pressure to grams per cubic metre. AH = 216.679 × 11.67 ÷ (20 + 273.15) = 8.62 g/m³ This comes from the ideal gas law for water vapour: 216.679 is the vapour's molar mass divided by the gas constant, with unit conversions.
- Compare with the most the air could hold. At 100% humidity: 216.679 × 23.33 ÷ 293.15 = 17.25 g/m³ Your air holds 8.62 of a possible 17.25 g/m³, which is the 50% relative humidity.
- Work out the mixing ratio. 622 × 11.67 ÷ (1,013.3 − 11.67) = 7.25 g/kg The mixing ratio is grams of water vapour per kilogram of dry air. 622 is the ratio of the molar masses of water and dry air, times 1000.
- Water vapour in your room. 8.62 g/m³ × 50 m³ = 431 g That is about 0.43 litres of water, spread through the air as invisible vapour.
Absolute, relative and specific humidity
Relative humidity tells you how full the air is compared with the most it can hold at its current temperature. Absolute humidity tells you how much water is actually there, in grams per cubic metre. The mixing ratio measures it against a kilogram of dry air instead. The glass in the calculator is filled to the relative humidity, so you can see the air's capacity and how much of it is being used.
Warm air can hold much more water vapour than cold air, roughly doubling for every 10 °C rise. That is why relative humidity changes during the day even when the amount of water in the air stays about the same.
How it is calculated
First the saturation vapour pressure is found with the Magnus formula: es = 6.1094 × e^(17.625 T ÷ (T + 243.04)), in hPa with T in °C. Multiplying by the relative humidity gives the actual vapour pressure. The ideal gas law then converts that to density: AH = 216.679 × e ÷ (T + 273.15) in g/m³. The mixing ratio is 622 × e ÷ (P − e) in g/kg, where P is the air pressure.
Enter the size of a room to see how many grams of water vapour it contains. A typical bedroom at 20 °C and 50% humidity holds a few hundred grams.
Why it matters at home
In winter, cold outdoor air holds very little water. When it is heated indoors, the same water content gives a low relative humidity, which is why heated homes feel dry. Guidance for comfort and to limit mould and dust mites is generally to keep indoor relative humidity between about 30% and 50%. Absolute humidity is also what dehumidifiers, humidifiers and clothes dryers actually move, so it helps to size them.
Frequently asked questions
- What is the difference between absolute and relative humidity?
- Absolute humidity is the amount of water vapour in grams per cubic metre. Relative humidity is that amount as a percentage of the maximum the air can hold at that temperature.
- How much water is in the air in my room?
- Multiply the absolute humidity by the room volume. At 20 °C and 50% humidity, a 50 m³ room holds about 430 g, less than half a litre.
- Why does indoor air feel dry in winter?
- Cold outdoor air holds little water. Once heated, the same amount of water is a small fraction of what warm air could hold, so relative humidity drops.
- What is a good indoor humidity?
- Around 30% to 50% relative humidity is commonly recommended for comfort and to limit mould growth.