Absolute humidity calculator
How many grams of water are in a cubic metre of air.
Fill in the fields and the result will appear here automatically.
Convert relative humidity to water-vapour density (g/m³ moist air) and mixing ratio (g/kg dry air). Their denominators differ. Heating without water exchange preserves mass mixing ratio, while absolute humidity can change as volume expands.
How it works
Formula and logic
es=6.1078·10^(7.5 t/(t+237.3)) hPa, e=RH·es/100, T=t+273.15 K. Absolute humidity=216.7 e/T g/m³; mixing ratio=621.97 e/(p −e) g/kg dry air. Maximum=216.7 es/T refers to saturation over liquid water.
Example
At 20 °C,50% RH and 1013.25 hPa:≈8.642 g/m³ and 7.260 g/kg dry air. RH=0 makes both rows zero; saturation pressure at 20 °C remains≈23.381 hPa.
Fields and units
- Temperature — °C
- Relative humidity — %
- Atmospheric pressure — hPa
How to use
- — Enter local absolute pressure and RH at the stated temperature.
- — Do not substitute g/m³ for g/kg in a dry-mass balance.
- — Room water inventory is density difference×volume; hourly supply needs airflow and moisture fluxes.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- OU CAPS: liquid-water Tetens coefficients
- Limitation
- Liquid-water Tetens, not a separate ice model; no universal accuracy is claimed. The t>−237.3 °C branch excludes the singularity but does not validate all extrapolation. Require p>0,RH 0–100%,e<p: at e=p the dry component and mixing-ratio denominator vanish.
FAQ
Why are homes dry in winter?
Convert relative humidity to water-vapour density (g/m³ moist air) and mixing ratio (g/kg dry air). Their denominators differ. Heating without water exchange preserves mass mixing ratio, while absolute humidity can change as volume expands. es=6.1078·10^(7.5 t/(t+237.3)) hPa, e=RH·es/100, T=t+273.15 K. Absolute humidity=216.7 e/T g/m³; mixing ratio=621.97 e/(p −e) g/kg dry air. Maximum=216.7 es/T refers to saturation over liquid water.
Why is absolute humidity more useful than relative?
Convert relative humidity to water-vapour density (g/m³ moist air) and mixing ratio (g/kg dry air). Their denominators differ. Heating without water exchange preserves mass mixing ratio, while absolute humidity can change as volume expands.
What is the mixing ratio?
Grams of water per kilogram of dry air. Unlike absolute humidity it does not change on heating or cooling without condensation, which is why ventilation calculations use it.
How does this relate to dew point?
Dew point is the temperature where the current vapour partial pressure equals saturation pressure. Compare it with surface temperature. Vapour mass density in g/m³ and its pressure also depend on temperature, so they are not interchangeable without stated conditions.