How to Use the Excavation Volume Calculator
Excavations with sloped sides are dug narrower at the bottom and wider at the top for slope stability, which means a simple length x width x depth calculation understates the real volume. This calculator takes bottom length, width, depth, and side slope ratio, then returns both the top opening dimensions and the total excavation volume.
A sloped excavation forms a frustum (truncated pyramid) shape — narrower at the bottom, wider at the top — so this calculator uses the frustum volume formula V = h/3 x (A1 + A2 + sqrt(A1 x A2)), where A1 is the bottom area, A2 is the top area, and h is the depth. This accounts for the full widening shape rather than just the bottom footprint.
The slope ratio is expressed as horizontal run per 1 unit of vertical depth — a ratio of 0.5 means the excavation widens by 0.5 ft for every 1 ft of depth. Safe slope ratios depend heavily on soil type and depth, so always confirm the appropriate ratio against your geotechnical report or OSHA excavation safety standards; this formula doesn't apply when shoring or trench boxes are used instead of sloping.
Frequently Asked Questions
It means 0.5 units of horizontal run for every 1 unit of vertical rise. So for every foot you dig down, the excavation widens by 0.5 ft on each side. The safe slope ratio depends on soil type and excavation depth, so follow your geotechnical report or applicable safety standards.
A sloped excavation widens toward the top, forming a frustum (truncated pyramid) shape. Multiplying only the bottom area by depth underestimates the true volume. This calculator uses the frustum formula, which accounts for both the bottom and top areas, for an accurate result.