Atmospheric Mixing Ratio Calculator
The water vapor mixing ratio is the mass of water vapor per kilogram of dry air, a conservative measure of atmospheric moisture that does not change when the air is merely warmed or cooled. This calculator takes the dew point in degrees Celsius and the station pressure in hectopascals, computes the actual vapor pressure from the Magnus saturation formula at the dew point, and returns the mixing ratio in grams per kilogram along with the vapor pressure itself. It is a standard derived quantity used in soundings, thermodynamic diagrams, and moisture transport analysis.
Mixing ratio formula
Vapor pressure e = 6.112 * exp(17.62 * Td / (243.12 + Td))
Mixing ratio w = 621.97 * e / (P - e) [g/kg]
Specific humidity q = 621.97 * e / (P - 0.378 * e) [g/kg]
Td = dew point (degrees C), P = station pressure (hPa)
The constant 621.97 g/kg is the molar mass ratio of water vapor to dry air times 1,000. Specific humidity differs from mixing ratio by using moist-air mass in the denominator.
Mixing ratio context
- Mixing ratio is conserved under dry adiabatic motion, making it a clean moisture tracer.
- The Magnus coefficients 17.62 and 243.12 degrees C are WMO-recommended for vapor over water.
- Station pressure (not sea-level pressure) is the correct denominator; it falls with elevation.
- Typical near-surface mixing ratios range from a few g/kg in cold dry air to over 20 g/kg in the humid tropics.
- Specific humidity and mixing ratio are nearly equal at low moisture content and diverge slightly as vapor rises.
Mixing ratio: frequently asked questions
What is the atmospheric mixing ratio?
The mixing ratio is the mass of water vapor per unit mass of dry air, usually expressed in grams of vapor per kilogram of dry air. Unlike relative humidity, it does not change when the air is simply heated or cooled, so it is a conservative tracer of moisture content.
How is mixing ratio calculated from the dew point?
First compute the saturation vapor pressure at the dew point using the Magnus formula, which gives the actual vapor pressure of the air. The mixing ratio then equals 621.97 times the vapor pressure divided by (station pressure minus vapor pressure), giving grams per kilogram.
Why use the dew point instead of relative humidity?
The dew point directly fixes the actual vapor pressure of the air, so it is the cleanest input for mixing ratio. Relative humidity needs the air temperature as well to back out the vapor pressure. Using the dew point keeps the calculation to two physical inputs.
What does the 621.97 constant represent?
It is the ratio of the molar mass of water vapor to that of dry air (about 0.62197), multiplied by 1,000 to express the result in grams per kilogram. It links partial pressures to mass ratios under the ideal gas approximation.
What pressure should I enter?
Enter the station pressure (the actual air pressure at your location) in hectopascals, which equals millibars. At sea level a typical value is about 1,013 hPa. At elevation the station pressure is lower, which raises the mixing ratio for a given dew point.
Official sources
- NOAA National Weather Service: Weather Calculators and formulas.
- American Meteorological Society: Glossary of Meteorology.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.