List the R-values of every component of your wall. The R-value is a measure of thermal resistance used by the building industry. Knowing the R-values is the easiest way to derive the U-value. You can get the R-value from the manufacturer for items such as insulation. For simple, common items such as studs or sheathing, the typical values can be found in tables published in books or online.
Create an "area weighted average" R-value. This represents the proportion of space occupied by each element of your wall, multiplied by the element's R-value. For example, if every 18 inches of wall is composed of 16 inches of insulation and 2 inches of framing studs, then your area weighted average R-value equals 16 times the R-value of the insulation divided by 18, plus 2 times the R-value of the framing stud divided by 18.
Add the R-values of all the other stand-alone elements of the wall -- such as brick or exterior sheathing -- to the area weighted average. This sum represents your total R-value.
Calculate the reciprocal of the total R-value by dividing 1 by it. U is the reciprocal of the R-value. This can be expressed as U =1/R.