There is nothing in any aircraft manual on how to correct for humidity when calculating density altitude. The formulas all assume 0% humidity. We know that humidity is almost always much higher than that, and that it decreases the density of the air. And therefore, it increases density altitude -- but by how much, and how do we correct for it? The following information comes from unofficial sources and "is to be used for educational purposes only." That having been said, there are two ways to do DA calculations and correct for humidity. If you have access to the Internet, go to wahiduddin.net/calc/calc_da.htm. You can calculate the prevailing density altitude by entering the current temperature, dew point, and other information. (Try entering a very low dew point first, and then the real one to see the difference.) If you do not have access to a computer, here are some quick guidelines derived from performing many calculations across a wide spectrum of temperatures, humidity levels and altitudes. After you complete your normal calculation, add the following percentages to the density altitude figure to correct for humidity. For Sea Level -- add 32%; for 3,000 feet -- add 14.5%; for 5,000 feet -- add 11%. These figures are calculated based on 100% humidity, but there is no point in splitting hairs, so use the full value as an add-on to your normal density altitude calculation -- and BE SAFE!