Aggregates typically contain pores, which can be either permeable or impermeable. Considering the pores in the aggregates, the specific gravity of aggregates is defined in three different forms, namely, bulk or gross specific gravity, apparent specific gravity, and absolute specific gravity.

Bulk or gross specific gravity is calculated by taking the ratio of the mass of aggregates in the saturated surface-dry state to the total volume that includes both the solids and the voids within the aggregates. Apparent specific gravity, on the other hand, defines the density of aggregates as the ratio of the dry mass of the aggregates to the volume of the aggregates, but only accounting for the impermeable voids. Absolute specific gravity is determined by the ratio of the dry mass of aggregates to the volume of the aggregates, excluding all voids.

When preparing concrete mixtures, it is not essential to ascertain the true specific gravity of an aggregate. Natural aggregates are inherently porous; as a result, it is important to consider the total volume occupied by the aggregate particles, including the pores within them. Thus, the measurement of apparent specific gravity is crucial. The apparent specific gravity for most commonly used rocks typically ranges between 2.60 and 2.70, with specific values being approximately 2.69 for granite, 2.65 for sandstone, and 2.60 for dense limestone.