Dredging quantity calculation is a fundamental task in marine and river engineering, directly affecting project cost, schedule, and environmental impact. It involves determining the volume of material to be removed from a waterway, harbor, channel, or basin to achieve the required design depth and width, while allowing for tolerances, over‑dredging, and possible sedimentation during the works.The process begins with establishing a clear reference system. A design seabed level (or riverbed level) is defined relative to a recognized datum, such as chart datum or a local tidal benchmark. Existing bathymetric data are then collected through hydrographic surveys, typically using single‑beam or multi‑beam echo sounders combined with precise positioning systems. These survey results provide a three‑dimensional model of the current bed surface.Dredging quantities are generally calculated by comparing the existing bed surface with the design surface within the project boundaries. Two common methods are used. The first is the cross‑section method, where transverse sections are generated at regular intervals along the project area. For each section, the area between the existing and design profiles is determined, often by numerical integration, and then multiplied by the spacing between sections to obtain the volume. The second method is the digital terrain model (DTM) approach, where both existing and design beds are represented as grids or triangulated irregular networks. Software then computes the volume between the two surfaces across all grid cells or triangles.Several adjustments are necessary to obtain realistic dredging quantities. First, side slopes must be considered. In cohesive or granular soils, vertical cuts are usually unstable, so design profiles include side slopes at specified ratios (for example, 1:3 or 1:5). The volume associated with cutting these slopes is part of the dredging quantity. Second, tolerances and overdredging allowances are included. Because dredging equipment cannot achieve a perfectly flat bed, a certain depth tolerance below the design level may be permitted. This results in an increased theoretical volume known as the overdredge allowance.Material characteristics also influence quantity calculations. Different strata, such as soft silt, sand, clay, or rock, may be present within the project area. It is often necessary to break down the total volume by material type, because excavation methods, production rates, and disposal options vary significantly. Geotechnical investigations and borehole logs help define the depth and extent of each layer, which are then integrated with the bathymetric model.Another important factor is the distinction between in‑situ volume and bulked volume. In‑situ volume refers to the material in its natural state on the seabed or riverbed, while bulked volume refers to the volume after excavation, when the material has loosened and expanded. Bulking factors, determined from experience or laboratory testing, are applied to convert in‑situ volumes to volumes relevant for transport, placement, or disposal site capacity design.Finally, dredging quantity calculations should be documented clearly, including survey dates, methods, grid resolution or cross‑section spacing, software used, assumed tolerances, and bulking factors. Transparent documentation allows verification, facilitates contract management, and supports later comparison between estimated and actually dredged quantities, as confirmed by post‑dredge surveys.