For a particular application, such as a cavity wall, which is built up of a concrete block outer leaf, 50mm clear cavity, Kingpan Kooltherm K8 Cavity Board, dense block and skim coated plaster board, then the R-Value for each of the elements would be calculated in order to work out the U-Value.
U Value is the reciprocal of all resistances of the materials found in the building element. To calculate the U-Value of the building element the R-Value of all the different components that make up that element will be considered.
U-Value = 1/(Sum of all R-Value)
U-Value (of building element) = 1 / (Rso + Rsi + R1 + R2 …)
where Rso is the fixed external resistance, Rsi is the fixed internal resistance and R1, R2 etc are resistivity of all elements within the application including that of cavities within the construction.
This is the basic formula and by using this you could work out the U-Value for a particular application, however there are also other factors which need to be taken into account including thermal bridging factors of fixings or stud work.