Air Mixing Ratio Calculator

Mixing ratio (w) is the mass of water vapor per unit mass of DRY air, usually expressed in g/kg (grams of water vapor per kilogram of dry air). The formula is w = 0.622 x e / (p - e), where e = actual vapor pressure and p = atmospheric pressure. Example: 25 C temperature, 50% RH, 1013.25 hPa pressure gives w = 9.877 g/kg.

Mixing ratio (w) divides the mass of water vapor by the mass of DRY air only, while specific humidity (q) divides it by the mass of total MOIST air (dry + vapor): q = w / (1 + w). The value of q is always slightly smaller than w. In this calculator’s anchor example (w = 9.877 g/kg), q = 9.781 g/kg — a difference of only about 1%. In the troposphere the difference between the two rarely exceeds 2-3%.

Relative humidity is the RATIO of actual vapor pressure to saturation vapor pressure at that temperature (RH = 100 x e / e_s) — its value is relative to temperature. Mixing ratio is the AMOUNT of water vapor actually contained. RH can change just because the temperature changes (with no vapor added or removed), while mixing ratio does not change during heating/cooling as long as there is no condensation or evaporation. That is why weather models use mixing ratio, not RH, as the humidity variable.

The number 0.622 is the ratio of the molar mass of water vapor (H2O = 18.01528 g/mol) to the average molar mass of dry air (28.9647 g/mol): 18.01528 / 28.9647 = 0.62197, rounded to 0.622. This constant appears when deriving w = vapor_mass / dry_mass from the ideal gas law for both components.

In SI the unit is kg/kg (dimensionless), but in meteorological practice it is almost always written g/kg so the numbers are easy to read: 9.877 g/kg means 0.009877 kg of water vapor per kg of dry air. As another example, a vapor pressure e = 2.0 kPa at 1013.25 hPa gives w = 12.525 g/kg. This calculator displays both, plus specific humidity as a comparison readout.

Look at the formula’s denominator: w = 0.622 x e / (p - e). If atmospheric pressure p falls (for example at a high plateau of 850 hPa) while vapor pressure e stays the same, the denominator shrinks so w grows. At 25 C and 50% RH, w = 9.877 g/kg at sea level (1013.25 hPa) becomes 11.810 g/kg at 850 hPa — the same mass of vapor occupies thinner air.

Yes — that is the difference between relative and absolute humidity. Cold air can only hold a little water vapor: at 0 C when saturated (RH 100%), w is only 3.772 g/kg, while at 30 C when saturated, w reaches 27.185 g/kg. Hot desert air with RH 10% can contain more water vapor (in g/kg) than saturated polar air.

The dew point is the temperature at which the saturation vapor pressure e_s(Td) equals the actual vapor pressure e — that is, the temperature at which the air becomes saturated. From the mixing ratio, you can compute e back (this calculator’s inverse mode), then find Td from that e. See our dew point calculator for the Td steps.

VPD = e_s - e, the difference between saturation and actual vapor pressure — a measure of the air’s drying power that matters for plants and greenhouses. Mixing ratio tells you the AMOUNT of water vapor present, while VPD tells you HOW FAR the air is from saturation. Both use the same e; this calculator computes the e, our VPD calculator computes the deficit.