The Asaoka method is a useful tool for interpreting and extrapolating field settlement observations. The graphical procedure to estimate end-of-primary (EOP) settlement and coefficient of consolidation is simple, however, mathematical deduction of the method by Asaoka is not. The Asaoka method is deduced here using simple algebra for one-dimensional compression with and without vertical drains. The Terzaghi theory of one-dimensional consolidation, and the Barron equal strain theory of consolidation modified to include effects of smear, well resistance and vertical water flow through soil, are used. The Asaoka graphical procedure is applied to settlement observations at Skå-Edeby test area I with three spacings of sand drains and area IV without vertical drains. The Asaoka method is not recommended for predicting secondary settlement.