Dry volume is the combined volume of the dry ingredients (cement, sand, aggregate) needed to produce a given volume of finished wet concrete or mortar. It is larger than the wet volume because the dry materials contain air voids between particles that disappear once mixed with water and compacted — the paste fills the gaps and the mass shrinks.
To find the dry volume, the wet volume is multiplied by a bulking factor: about 1.54 for concrete and 1.33 for mortar. So 1 m³ of wet concrete needs roughly 1.54 m³ of dry materials, which are then split by the mix ratio into cement, sand and aggregate.
These factors are widely used site rules of thumb rather than fixed code values — the concrete factor commonly ranges from 1.52 to 1.57 depending on the aggregate grading and compaction. Using the right factor matters: applying 1.54 to mortar, or 1.33 to concrete, would give materially wrong quantities. This single step is the foundation of nearly every material estimate.