Absolute Humidity Calculator
How much water a cubic metre of air actually holds — absolute humidity in grams — from temperature and relative humidity, with the vapour pressure, the mixing ratio, the dew point, and how much more the air could hold before saturating.
Relative humidity says how full the air is; absolute humidity says how much water is in it, and the two diverge because warm air holds far more.
How the absolute humidity calculator works
Relative humidity says how full the air is; absolute humidity says how much water is in it, and the two diverge because warm air holds far more. The saturation vapour pressure at the temperature comes from the Magnus formula, the relative humidity scales it to the actual vapour pressure, and the ideal gas law turns that into grams per cubic metre. Air at 30 °C and 50% holds more water than air at 10 °C and 100%.
Formula: AH = 216.7 × p_v / T (g/m³, p_v in hPa, T in K); p_v = RH × 6.1078 × 10^(7.5t/(t+237.3))
Worked examples
| Inputs | Absolute humidity (g/m³) | Note |
|---|---|---|
| 24 °C at 55% | 11.967 | about 12 g/m³ |
| A cold day at 100% | 6.796 | less water than the warm room |
| Tropical | 28.698 | very humid |
FAQFrequently asked questions
What is absolute humidity?
The mass of water vapour in a volume of air, in grams per cubic metre. Unlike relative humidity it does not change when the air warms or cools — until it condenses.
Why does warm air hold more water?
Because saturation vapour pressure rises steeply with temperature — roughly doubling every 10 °C. Air at 30 °C can hold four times as much water as air at 10 °C.
What is the mixing ratio?
Grams of water per kilogram of dry air — the meteorologist's measure, conserved as air rises and cools until condensation. It needs the pressure, which is why that is an input.
Why does this matter indoors?
Condensation and mould happen where warm humid air meets a surface below its dew point. The dew point row is the temperature a window or wall must stay above.
How accurate is the Magnus formula?
Within about 0.1% for liquid water between −40 and 50 °C. Over ice the constants differ slightly; the page uses the liquid-water form throughout.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.