This paper presents a practical algebraic unified design method for the selection of capacitors in dc-dc switching converters. The model adopted accounts for the correlation existing between the capacitance C and the equivalent series resistance (ESR) of capacitors and provides reliable design formulas for the choice of commercial components. The application of the method is illustrated for the design of capacitors complying with maximum acceptable steady-state ripple voltage. A piecewise-defined algebraic function is introduced, which expresses the boundary of the capacitor acceptability region in the C-ESR plane. Numerical and experimental validations, referred to the design of capacitors of a single-ended primary inductor converter (SEPIC), prove the effectiveness of the method.