About Absolute Humidity
Absolute humidity is the total mass of water vapor present in a given volume of air, expressed in grams per cubic meter (g/m³). Unlike relative humidity — which only measures how close the air is to saturation at its current temperature — absolute humidity is a direct measure of how much moisture the air actually contains, independent of temperature.
The formula
This calculator uses the Magnus (August-Roche-Magnus) approximation for saturation vapor pressure, combined with the ideal gas law for water vapor. First, the saturation vapor pressure Es (in hectopascals) is estimated from air temperature T (in °C):
Es = 6.112 × e(17.62 × T) / (243.12 + T)
The actual (partial) vapor pressure Pv is then found by scaling the saturation pressure by relative humidity RH (as a percentage):
Pv = Es × (RH / 100)
Finally, absolute humidity AH (in g/m³) is calculated from the actual vapor pressure and the temperature in Kelvin:
AH = 216.7 × Pv / (273.15 + T)
The dew point — the temperature air would need to cool to for the existing vapor to saturate it — is found by algebraically inverting the Magnus formula using the actual vapor pressure Pv:
Tdew = (243.12 × ln(Pv / 6.112)) / (17.62 − ln(Pv / 6.112))
Working with the inputs
- Enter air temperature in either Celsius or Fahrenheit — the calculator converts internally to Celsius before applying the formula.
- Relative humidity must be between 0% (bone dry) and 100% (fully saturated, i.e. fog or cloud).
- Absolute humidity results are reported in grams of water vapor per cubic meter of air (g/m³), the standard SI-derived unit for this quantity.
Knowing the limits
The Magnus approximation used here is accurate to within about 0.1–0.3% for temperatures roughly between −20°C and 50°C, which covers virtually all everyday weather and indoor-climate conditions. It becomes less reliable well below freezing (where saturation should technically be computed over ice rather than liquid water) or at extreme temperatures outside that range. It also assumes standard atmospheric pressure; at high altitude, actual vapor content can differ slightly from this estimate.