Formula: V = (A₁ + A₂) / 2 × L. For two cross-sections with areas A₁ and A₂ separated by distance L along the alignment, the volume of earthwork between them equals the average area times L. Sum across all pairs along the alignment for the total volume.
Average end area is exact for linear prisms (constant cross-section). For sections that change in shape between stations, it's an approximation. The prismoidal formula (V = L/6 × (A₁ + 4Aₘ + A₂)) is the more accurate refinement that adds the midpoint cross-section area.
Common applications: roadway cut and fill, ditch and trench excavation, channel sizing. Cross-sections are typically taken at regular stations (every 25, 50, or 100 ft along the alignment) plus extra sections at grade changes or alignment shifts.