Mixing Ratio of Air Calculator — Water Vapour in the Atmosphere
Enter air temperature, dew point and atmospheric pressure to find the water-vapour mixing ratio (grams of vapour per kilogram of dry air), saturation mixing ratio, relative humidity, specific humidity and the vapour-pressure deficit.
°C
°C
hPa
Grams of water vapour per kilogram of dry air
- 1
Actual vapour pressure e
6.112 × exp(17.67 × 10 ÷ (10 + 243.5)) = 12.272August-Roche-Magnus approximation evaluated at the dew point temperature. - 2
Denominator P − e
1,013.25 − 12.272 = 1,000.978 - 3
Mixing ratio w
621.97 × 12.272 ÷ 1,000.978 = 7.625
How does this calculator work?
The mixing ratio of air is w = 621.97 × e / (P − e) g/kg, where e is the actual vapour pressure derived from the dew point via the Magnus formula e = 6.112 × exp(17.67 × Td / (Td + 243.5)) hPa. Enter temperature, dew point and pressure to get mixing ratio, relative humidity and specific humidity.
Formula
How this is calculated
The mixing ratio (w) is a fundamental quantity in meteorology and atmospheric science. It expresses the mass of water vapour present in a parcel of air relative to the mass of the dry air that surrounds it, measured in grams per kilogram (g/kg). Unlike relative humidity, which changes with temperature, the mixing ratio stays constant as long as no water is added or removed — making it a conserved quantity useful for tracking air masses.
The calculation uses the August-Roche-Magnus approximation to find the actual vapour pressure e from the dew point temperature Td: e = 6.112 × exp(17.67 × Td / (Td + 243.5)) hPa. The dew point is the temperature to which air must be cooled (at constant pressure and moisture content) for saturation to occur, so it directly encodes the actual moisture content. Substituting e into the mixing-ratio formula w = (ε × e) / (P − e), where ε = M_water / M_dryair = 0.621 97, gives w in g/kg.
The saturation mixing ratio ws uses the saturation vapour pressure es computed from the actual air temperature T with the same formula. Relative humidity is e / es × 100 %. The calculator also reports specific humidity (w / (1 000 + w) converted to g/kg), which is the vapour mass fraction of the total moist air. The Magnus formula is accurate to within 0.1 % for temperatures between −40 °C and +60 °C and is the standard used by weather services.
Frequently asked questions
In temperate climates the mixing ratio ranges from about 2–5 g/kg in winter to 10–18 g/kg in summer. Tropical air near the surface can reach 20–25 g/kg. Arctic air in winter may be below 1 g/kg.
Dew point directly encodes the actual moisture content: the same dew point always gives the same mixing ratio regardless of temperature. Relative humidity depends on temperature — 50 % RH at 30 °C contains far more vapour than 50 % RH at 10 °C — so it is not conserved when air parcels rise or are heated.
The mixing ratio (w) is vapour mass relative to dry-air mass (g/kg dry air). Specific humidity (q) is vapour mass relative to total moist-air mass (g/kg moist air). They are close in value (q ≈ w / (1 000 + w) × 1 000) but specific humidity is always slightly smaller.
Also known as
TG we-Calculate Editorial Team. (2026). Mixing Ratio of Air Calculator — Water Vapour in the Atmosphere [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/mixing-ratio-of-air-calculator
TG we-Calculate Editorial Team. "Mixing Ratio of Air Calculator — Water Vapour in the Atmosphere." TG we-Calculate. 2026. https://we-calculate.com/calculator/mixing-ratio-of-air-calculator.
TG we-Calculate Editorial Team, "Mixing Ratio of Air Calculator — Water Vapour in the Atmosphere," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/mixing-ratio-of-air-calculator
@misc{wecalculate_mixing_ratio_of_air_calculator, title = {Mixing Ratio of Air Calculator — Water Vapour in the Atmosphere}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/mixing-ratio-of-air-calculator}}, year = {2026}, note = {TG we-Calculate} }
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