Excavation & Earthwork
How to Calculate Excavation Volume
Excavation volume step by step: rectangular digs, trenches with sloped sides, average-depth method for uneven ground, soil swell, and truckload planning.
Excavation estimates go wrong in two places: the geometry (sloped sides and variable depth are not box shapes) and the physics (soil expands when dug). Master both and your skip, truck, and disposal budgets land on target.
Shape 1: Rectangular pad or basement
A 24 × 30 ft foundation dug 4 ft deep: 24 × 30 × 4 = 2,880 ft³ = 106.7 yd³ in place. If the sides will be laid back for safety (they should be, beyond 4–5 ft depth), add the sloped wedges - see shape 3.
Shape 2: Vertical-wall trench
A utility trench 2 ft wide, 3 ft deep, 60 ft long: 2 × 3 × 60 = 360 ft³ = 13.3 yd³. Vertical walls are only safe (and legal, if anyone enters) up to limited depths - OSHA requires protection at 5 ft.
Shape 3: Sloped sides - the trapezoid method
Sloping trench walls for safety adds real volume. A drain trench with 2 ft bottom, sides laid back 1:1 (45°), 3 ft deep: top width = 2 + 2×3 = 8 ft. Volume = (8+2)/2 × 3 × 60 ÷ 27 = 33.3 yd³ - 2.5× the vertical-wall estimate. Sloping is a major volume driver; never quote a deep trench from bottom width alone.
Shape 4: Variable depth - the grid method
Real ground isn't level. Stake the area, measure the cut depth at each corner and the center (more points for bigger areas), and average:
| Point | Cut depth |
|---|---|
| NW corner | 1.2 ft |
| NE corner | 1.8 ft |
| SW corner | 2.4 ft |
| SE corner | 3.0 ft |
| Center | 2.1 ft |
| Average | 2.1 ft |
A 20 × 40 ft area × 2.1 ft average = 1,680 ft³ = 62 yd³ bank. The more irregular the ground, the more measurement points you need - professionals model this with survey data, but a 9-point grid gets a homeowner remarkably close.
Then apply swell - always
Soil expands 12–50% when excavated. The hole's volume is not the pile's volume:
| Soil | Swell | 62 yd³ bank becomes |
|---|---|---|
| Sand/gravel | 12% | 69 yd³ loose |
| Mixed earth | 25% | 78 yd³ loose |
| Clay | 30% | 81 yd³ loose |
Full explanation in Soil Swell Explained; truck planning in How Many Dump-Truck Loads?
All three shapes plus swell factors are built into the excavation calculator.
Open the Excavation CalculatorCommon mistakes
Quoting disposal from bank volume. Skips and trucks fill with loose soil. Budget disposal at bank × swell or you'll book ~25% too few loads.
Ignoring the slope wedges. Any excavation deeper than ~4 ft that a person will enter needs laid-back sides or shoring; laid-back sides multiply volume, as shown above.
Forgetting working room. Foundation digs need 2–3 ft of clearance beyond the wall line for formwork and waterproofing - that perimeter band is often 30%+ of the total dig.
Topsoil in the same pile. Strip and stockpile topsoil separately; it's valuable for finish grading and shouldn't pay disposal fees.
When to call a professional
Excavations deeper than 5 ft, near foundations or property lines, below the water table, or in ground that has been filled or shows instability need an experienced contractor - and for structural support or steep temporary slopes, a geotechnical engineer. Machine work near buried utilities is a job for pros with locating experience even after 811 marks.
Frequently Asked Questions
How do I calculate excavation volume in cubic yards?
Length x width x depth in feet, divided by 27. A 20 x 30 ft pad dug 2 ft deep is 20 x 30 x 2 / 27 = 44.4 cubic yards in place - about 55 cubic yards loose after swell.
How do I calculate a trench with sloped sides?
Average the top and bottom widths, then treat it as a rectangle: (top + bottom) / 2 x depth x length. A trench 4 ft wide at top, 2 ft at bottom, 3 ft deep, 50 ft long = 3 x 3 x 50 = 450 ft3 = 16.7 yd3.
What if the excavation depth varies?
Use the average-depth method: measure depth at a grid of points (corners plus center minimum), average them, and multiply by the plan area. For long cuts, average depths at regular stations along the run.
How much does excavation cost?
Machine excavation typically runs $50-200 per hour or $5-15 per cubic yard depending on access, soil, and disposal distance. Hauling and disposal often cost as much as the digging - estimate loose volume for that part.
Should I calculate bank volume or loose volume?
Both, for different purposes: bank (in-place) volume measures the dig and is what excavation contractors usually price; loose volume (bank x 1.12-1.5 swell factor) sizes trucks, bins, and disposal fees.