In this study, based on the idea of cutting particle packing from three to two dimensions, an indirect evaluation method for calculating packing voids and probabilities is proposed. Actual coal particles with average particle sizes of 244, 104, 45 and 17 mu m were used to build 20 stable three-particle packing and 35 unstable four-particle packing models. Three algorithms, the Law of Cosines, Heron Formula, Bretschneider's Formula were used to calculate irregular particle packing voids. Four indexes of cumulative void areas, Sigma PVT, Sigma PST, Sigma PVQ and Sigma PSQ (P is the probability, V-T, S-T, V-Q, S-Q are void areas both in three-particle and four-particle packing) were created. The accuracy of the method was testified by poly-nominal fit analysis. The correctness of this method was confirmed by two case studies of 96 factory CWS samples and 18 laboratory CWS samples. The method also provides a feasible way to evaluate particle packing in other industrial fields. (C) 2020 Elsevier B.V. All rights reserved.