The direct‐derivation (DD) method is a new technique for quantitative phase analysis (QPA) Toraya (2016). J. Appl. Cryst. 49, 1508–1516. A simple equation, called the intensity–composition (IC) formula, is used to derive weight fractions of individual components (wk; k = 1–K) in a mixture. Two kinds of parameters are required as input data of the formula. One is the parameter Sk, which is the sum of observed powder diffraction intensities for each component, measured in a wide 2gθ range and corrected for the Lorentz–polarization factor. The other is the parameter ak−1, defined by ak−1 = Mk−1∑nik2, where Mk is the chemical formula weight and nik is the number of electrons belonging to the ith atom in the chemical formula unit. The parameter ak−1 was originally derived by using the relationship between the peak height and the integrated value of the peak at the origin of the Patterson function, implicitly assuming the presence of periodic structures like crystals. In this study, the formula has been derived theoretically from a general assemblage of atoms resembling amorphous material, and the same expression as the original formula has been obtained. The physical meaning of ak−1, which represents `the total scattering power per chemical formula weight', has been reconfirmed in the present formulation. The IC formula has been tested experimentally by using two‐, three‐ and four‐component mixtures containing SiO2 or GeO2 glass powder. In the whole‐powder‐pattern fitting (WPPF) procedure, incorporated into the DD method, a background‐subtracted halo pattern is directly fitted as one of the components in the mixture, together with profile models for crystalline components. In the WPPF, an interaction was observed between the parameters of the background function (BGF) and the parameter for scaling the halo pattern, and this resulted in systematic deviations of wk from weighed values. The deviations were ≤0.7% in the case of binary mixtures when the BGF was fixed at the correct background height, supporting the hypothesis that the DD method is applicable to the QPA of amorphous components. The direct‐derivation method has been applied to the quantitative phase analysis of mixtures containing amorphous components. A theoretical basis has been given to the formula for deriving individual weight fractions from observed intensity data. The formula has been experimentally tested by whole‐powder‐pattern fitting to the observed patterns of mixtures containing amorphous components.