How to Calculate Earthwork Quantity for a Foundation

Earthwork is one of the first quantities anyone estimates on a project, and it's simple in principle — length × width × depth — but the details of measurement and the shape of the excavation are where most estimating mistakes actually happen.

The basic case: a simple pit or trench

For a straightforward rectangular pit with vertical sides (common for small isolated footings), the volume is simply:

• Volume = Length × Width × Depth

For a continuous trench (like a strip footing), you'd calculate the cross-sectional area and multiply by the total running length.

When sides aren't vertical: the trapezoidal method

In loose or unstable soil, excavation sides are often battered (sloped) rather than vertical, for safety. In that case the pit isn't a simple rectangular box — it's closer to a trapezoidal prism, and the volume is worked out as:

• Volume = Depth × [(Top Area + Bottom Area) / 2] — for a trapezoidal cross-section
• Or, more generally for larger areas with varying levels: the trapezoidal method takes the average of two end areas and multiplies by the distance between them, repeated across several sections.

A quick worked example

Say you're excavating a footing pit 1.5 m × 1.5 m at the bottom, 1.0 m deep, with sides battered out to 1.9 m × 1.9 m at the top (a common allowance for working space and slope stability):

• Bottom area = 1.5 × 1.5 = 2.25 sq.m
• Top area = 1.9 × 1.9 = 3.61 sq.m
• Volume = 1.0 × (2.25 + 3.61) / 2 = 2.93 cum

Compare that to a naive calculation using just the bottom dimensions (1.5 × 1.5 × 1.0 = 2.25 cum) — a 30% difference, which is exactly the kind of gap that causes budget overruns if the battering isn't accounted for.

Don't forget

• Working space: most estimates add 150–300 mm extra on each side of the actual footing for working room, which increases the excavated footprint.
• Backfill deduction: the volume you excavate isn't the same as the volume you dispose of — some of it goes back in as backfill around the footing once concrete is cured.