Shape-related properties of irregular particles are of interest in many fields. The volume and dimensions of rocks, such as coarse and larger fine concrete aggregates, can be physically measured rather easily. However, the surface area is difficult to measure physically, if at all possible. A combination of computed tomography and spherical harmonic analysis can be used to calculate the surface areas of micrometer-sized to centimeter-sized particles. This paper compares the success of several approaches that use easy-to-measure properties, specifically the orthogonal length, width, and thickness of a particle and its volume to estimate its surface area. A training set of 3,359 particles varying widely in size and shape is used to develop regression equations between various shape parameters. These equations are then used to estimate surface areas of cement and aggregate particles in three different validation sets. Improvements are offered to some existing surface area estimation methods. Finally, a novel method is proposed that estimates surface area with a mean absolute percentage error of 2.8 to 3.8% and with 14% individual particle error.