Sidebar 3: How to Calculate Enthalpy


To calculate enthalpy, a simplifying assumption is made, followed by a series of calculations.

By definition, the degree of saturation () of air can be calculated using Equation 12 from Chapter 6 of the ASHRAE Handbook of Fundamentals, 2001, which is outlined below. 

However, the degree of saturation and relative humidity are approximately the same at temperatures below 100F.  This simplifying assumption is then applied to the previous equation by substituting relative humidity for m, obtaining values for Ws from a psychometrics chart and calculating W.  Rewriting the equation above, we get:

W = %RH x Ws



W = specific humidity (lbwater / lbdry air)

%RH = relative humidity of the air

Ws = specific humidity at saturation (lbwater / lbdry air)


Finally, enthalpy is calculated using Equation 32 from Chapter 6 of the 2001 ASHRAE Handbook of Fundamentals.


H = (0.24 x T) + [W x (1061 + 0.444 x T)]



H = enthalpy (Btu/lb)

T = dry-bulb temperature (F)

W = specific humidity (lbwater / lbdry air)