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Absolute Humidity Calculator

Convert temperature and relative humidity into absolute humidity.

Result

13.82 g/m³
19.00 hPa

How it is calculated

Three steps, with temperature T in °C and relative humidity RH in percent (0 to 100):

Saturation vapour pressure: es = 6.112 × exp(17.67 T / (T + 243.5)) hPa

Actual vapour pressure: e = es × RH / 100 hPa

Absolute humidity: AH = 2.1674 × (e × 100) / (273.15 + T) g/m³

The constants are the Magnus approximation for saturation pressure; the last step applies the ideal gas law for water vapour, with e converted from hPa to Pa. A relative humidity below 0 or above 100 gives no result. Both the absolute humidity and the vapour pressure are shown with two decimals.

Worked example

Air at 25 °C and 60 % relative humidity.

  1. es = 6.112 × exp(17.67 × 25 / 268.5) ≈ 31.67 hPa
  2. e = 31.67 × 0.60 ≈ 19.00 hPa
  3. AH = 2.1674 × 1900 / 298.15 ≈ 13.82 g/m³

Each cubic metre of that air holds about 13.82 g of water vapour, at a vapour pressure of 19.00 hPa.

Try this example →

Common mistakes

  • Entering relative humidity as a fraction such as 0.6: the field expects percent, so 0.6 means 0.6 % and gives a tiny result.
  • Entering Fahrenheit or Kelvin: temperature must be in °C.
  • Comparing two rooms by relative humidity alone: warm air at 50 % RH holds far more water than cold air at 50 % RH, which is why absolute humidity differs.
  • Assuming the value stays constant when the air is heated: absolute humidity stays the same only until water is added or removed, while relative humidity falls.

Sources

About this calculator

This calculator converts air temperature and relative humidity into absolute humidity — the actual mass of water vapour per cubic metre of air (g/m³) — using the Clausius–Clapeyron-derived saturation vapour pressure formula (commonly the Magnus/Tetens approximation) combined with the ideal gas law. It also reports the actual vapour pressure in the air.

HVAC engineers, meteorologists, greenhouse growers, and museum or archive conservators use it because relative humidity alone doesn't say how much water is actually in the air — absolute humidity is what matters for condensation risk, drying rates, and climate control.

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Comments (1)

  • Editorial team

    Absolute humidity is what a humidifier or dehumidifier actually moves, while relative humidity depends on temperature. Warm the same room air and relative humidity falls, yet the grams per cubic metre stay the same. That is why this tool asks for both temperature and relative humidity. Enter relative humidity as a percentage from 0 to 100. If you compare indoor and outdoor air, which pair of readings would you like to see side by side?

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