Absolute Humidity, Vapor Pressure & Mixing Ratio Calculator
Psychrometric properties from air temperature (°C, °F or K) and relative humidity, dew point, wet-bulb temperature or absolute humidity: saturation and actual vapour pressure, vapour pressure deficit, absolute humidity, mixing ratio, specific humidity, enthalpy, air density, heat index and the water to add or remove in a room — with a choice of Magnus, Arden Buck, Tetens or Wagner–Pruss formulas.
How it works
Magnus: e_s = 6.112 × e^(17.62 T ÷ (243.12 + T)) hPa (over ice 6.112 × e^(22.46 T ÷ (272.62 + T))) · Arden Buck: e_s = 6.1121 × e^((18.678 − T/234.5)(T ÷ (257.14 + T))) · Tetens: 6.1078 × e^(17.27 T ÷ (T + 237.3)) · Wagner–Pruss (IAPWS) for liquid water · e = RH × e_s, or e_s(dew point), or e = e_s(T_w) − 0.000653 (1 + 0.000944 T_w) p (T − T_w) from the wet bulb · absolute humidity = e ÷ (R_v × T_K), R_v = 461.5 J/(kg·K) · mixing ratio w = 621.97 e ÷ (p − e) g/kg · specific humidity q = 621.97 e ÷ (p − 0.378 e) · h = 1.006 T + w (2501 + 1.86 T)/1000 kJ/kg · VPD = e_s − e · standard-atmosphere pressure = 1013.25 (1 − 2.25577×10⁻⁵ h)^5.25588 hPa
Relative humidity tells you how close air is to saturation, not how much water it holds — warm air at 35% can contain more vapour than cold air at 90%. Absolute humidity (grams per cubic metre) and the mixing ratio (grams per kilogram of dry air) are the actual quantities. They come from the vapour pressure, which you can start from relative humidity, a dew point, a wet-bulb reading or a known absolute humidity; the saturation vapour pressure comes from your choice of formula — Magnus (the WMO-recommended constants), Arden Buck, Tetens or the Wagner–Pruss reference equation — optionally over ice below freezing. Temperatures can be entered in °C, °F or K, pressure in hPa, kPa, inHg, mmHg or psi, or estimated from your elevation. The results include dew and frost point, the true psychrometric wet-bulb temperature, vapour pressure deficit (used by growers and greenhouse controllers), specific humidity, enthalpy, air density, heat index and humidex. They are what humidifiers, dehumidifiers, dryers and HVAC coils actually move, so the calculator also estimates the water to add or remove to bring a room to a target humidity. A table shows what happens to the same air when it is heated or cooled — absolute humidity stays fixed while RH swings — and another compares the four formulas.
Worked example
Air at 22 °C and 35% RH (Magnus): saturation vapour pressure 26.37 hPa, actual 9.23 hPa, so the vapour pressure deficit is 1.71 kPa. Absolute humidity = 923 Pa ÷ (461.5 × 295.15 K) = 6.78 g/m³ against 19.36 g/m³ at saturation; mixing ratio 5.72 g/kg; dew point 5.8 °C; wet bulb 13.2 °C; enthalpy 36.7 kJ/kg. Raising a 60 m³ room to 45% means adding about 116 g of water — and keeping it there needs roughly that much again per air change. Cooled to 10 °C, the same air would be at 75% RH.
Sources: WMO Guide to Instruments and Methods of Observation (WMO-No. 8), Annex 4.B — humidity formulae · Buck, A. L. (1981) New equations for computing vapor pressure and enhancement factor, J. Appl. Meteor. · Wagner & Pruss (2002) IAPWS formulation for the thermodynamic properties of water · NWS — The heat index equation
Frequently asked questions
Why does the mixing ratio depend on pressure?
Because it is per kilogram of dry air: at altitude the same vapour pressure is a larger share of the thinner air. Absolute humidity in g/m³ does not depend on pressure. Switch on the elevation option if you do not have a barometer reading.
How much does ventilation change this?
Each air change replaces the room’s vapour with outdoor air’s; in winter that dries the room (outdoor air at −5 °C holds under 3.5 g/m³ even when saturated), in humid summers it adds moisture.
Which saturation formula should I use?
For everyday temperatures they agree within about half a percent, so the default Magnus formula is fine. Arden Buck is slightly more accurate from −40 to 50 °C, and Wagner–Pruss is the IAPWS reference for liquid water above freezing. The second table shows all four side by side for your temperature.
What is vapour pressure deficit (VPD)?
The difference between the saturation and the actual vapour pressure — how much more water the air could take up. It drives evaporation and plant transpiration, which is why greenhouse growers control VPD rather than RH. At 22 °C and 35% RH it is 1.71 kPa.
Is the wet-bulb temperature exact?
It solves the psychrometer equation with the WMO coefficient for a ventilated thermometer, so it matches a sling or aspirated psychrometer reading to within a few tenths of a degree. Earlier versions of this page used a rough one-third rule, which overstated it.
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