Absolute humidity calculator
See how many grams of water vapour each cubic metre of air holds, useful for ventilation, drying and air conditioning.
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How to use it
- Enter air temperature, temperature unit and relative humidity.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
es = 6.1094 × exp(17.625T ÷ (T + 243.04)) hPa, e = es × RH ÷ 100, then AH = e ÷ (Rv × T in kelvin), with Rv = 461.52 J/(kg·K).
Example
Inputs: Air temperature = 25, Temperature unit = Celsius (°C), Relative humidity = 60 %. Result: Absolute humidity: 13.79 g/m³; Actual vapour pressure: 18.97 hPa; Saturation vapour pressure: 31.62 hPa.
Limits and caveats
- Treats water vapour as an ideal gas; the error is tiny in normal weather.
- Saturation pressure uses the Magnus formula over water, accurate to roughly 0.1 to 0.3%.
Questions about this tool
How is absolute humidity different from relative humidity?
Absolute is the actual mass of vapour per cubic metre; relative is that vapour as a share of the most the air can hold at that temperature.
How much water can air hold at 20°C?
About 17.3 g per cubic metre when fully saturated.
Does absolute humidity change when I heat a room?
It stays roughly the same if no vapour is added, while relative humidity falls.