Bulk density refers to the mass of aggregate particles that would fill a unit volume. The concept of bulk density originates from the inability to pack aggregate particles in a manner that completely eliminates void spaces. Hence, the term bulk refers to the volume that encompasses both the aggregates and the voids. This measurement is crucial when aggregates are batched by volume and is used to convert quantities by mass to volume.

Most natural mineral aggregates, like sand and gravel, typically exhibit a bulk density ranging from 95 to 100 lb/ft³ and are used to produce concrete of normal weight with a unit weight of about 150 lb/ft³. Aggregates with a bulk density of less than 70 lb/ft³ are classified as lightweight, while those with a density exceeding 130 ft³ are considered heavyweight.

The bulk density of aggregate particles is influenced by their size distribution; although aggregates of a single size can only be compacted to a limited degree, introducing smaller particles into the gaps between larger ones can increase the bulk density. The optimal bulk density of a mix of fine and coarse aggregates is reached when fine aggregates constitute approximately 35 to 40 percent of the total aggregate mass.