Psychrometric Calculator | Humidity Ratio, Dew Point and Enthalpy
Estimate humidity ratio, dew point, specific enthalpy and specific volume from dry-bulb temperature and relative humidity. Explore a psychrometric chart at fixed atmospheric pressure of 101.325kPa.
Input Parameters
Dry-bulb temperature Tdb
°C
Relative humidity RH
%
Psychrometric Chart (Tdb – Humidity Ratio W)
While paused, moving a slider updates the results immediately.
Results
25.0
Dry-bulb temperature Tdb [°C]
9.9
Humidity ratio W [g/kg]
50
Relative humidity RH [%]
17.9
Wet-bulb temperature Twb [°C]
13.9
Dew-point temperature Tdp [°C]
50.3
Specific enthalpy h [kJ/kg]
Saturation curve (RH=100%)Constant-relative-humidity curvesState pointProcess line
Psychrometric Chart (Tdb – Humidity Ratio W)
Humidity ratio W
—g/kg
Enthalpy h
—kJ/kg
Dew-point temperature Tdp
—°C
Specific volume v
—m³/kg
Psychrometric chart (Tdb – W）
Theory & Key Formulas
Theory & Key Formulas
Magnus equation: $p_{sat}= 0.611\exp\!\left(\dfrac{17.27\,T}{T+237.3}\right)$ kPa
Humidity ratio: $W = 0.622\,\dfrac{p_v}{101.325 - p_v}$ kg/kg
Specific enthalpy: $h = 1.006T + W(2501 + 1.86T)$ kJ/kg
Specific volume: $v = \dfrac{0.2871(T+273.15)(1+1.608W)}{101.325}$ m³/kg
TRY THE SAME CONDITIONS
Worked example
At 25°C and 60% RH, the cards show humidity ratio 11.89g/kg dry air, enthalpy 55.4kJ/kg dry air, dew point 16.7°C and specific volume 0.8609m³/kg dry air. Converting the humidity ratio to kg/kg gives 0.01189.
Vapor pressure pv=(RH/100)·psat(T), W=0.622pv/(101.325−pv), and h=1.006T+W(2501+1.86T). Saturation pressure uses a Magnus-type approximation. Controls cover −10 to 50°C and 0–100% RH. Ice saturation and altitude-dependent pressure are not rigorously modeled.
Dew point is obtained by inverting the same Magnus equation used for saturation pressure. At 25°C and 100% RH, dew point and wet-bulb temperature both equal 25°C. At 0% RH there is no finite dew point, but wet-bulb temperature is still calculated from evaporative cooling rather than replaced by dry-bulb temperature. The wet-bulb model is a liquid-water approximation, not a subfreezing design model.
Questions and answers
Is humidity ratio measured in g/m³?
No. W is water-vapor mass per unit dry-air mass. The result card and chart use g/kg; the equations use kg/kg dry air. Volumetric absolute humidity in g/m³ is a different quantity.
Does moving temperature alone represent sensible heating?
The main RH input remains fixed, so humidity ratio also changes. For constant-moisture heating, RH must be adjusted. The upper process animation is a separate illustrative example.
Why is dew point absent at 0% RH?
Vapor pressure is zero, so the approximation has no finite dew point. This is different from missing input. Use validated psychrometric calculations for rigorous low-temperature or low-humidity properties.
Does the page calculate air-conditioner capacity?
One state is insufficient. Capacity calculations also need inlet/outlet conditions, dry-air mass flow and process assumptions, including condensate where relevant. This page does not automatically size HVAC equipment.