How the spreadsheet grid method works
Lay a grid over the site (often 5 or 10 m). At each node, take the existing level from the survey and the proposed level from the design. The depth is proposed minus existing; the volume of each square is its area times the average depth at its four corners. Add up the positive squares for fill and the negative ones for cut.
Where it goes wrong
| Problem | What happens | What software does |
|---|---|---|
| Batters | Usually left out, or added as a guessed percentage. On a 1-2 m platform they can be 10-20% of the volume. | Runs each batter out to the ground at its slope and includes it. |
| Squares that cross the cut/fill line | Averaging corners mixes cut and fill, so both are understated. | Splits each piece where the depth changes sign and integrates exactly. |
| Falls and corners | Coarse grids miss the shape of the design and the ground between nodes. | Uses the surfaces themselves, not samples. |
| Formation | Proposed levels are finished levels; the dig is to formation, under the build-up. | Measures to formation for each element. |
| Topsoil strip | The bulk dig starts below the topsoil; fill has to make up the strip. | Measures from the stripped level and reports the strip separately. |
| Balance | Cut = fill is not a balance: dug material compacts. | Applies shrinkage, bulking and density by material. |
When a spreadsheet is enough
For a small, flat pad with no batters to speak of, a fine grid gets close. Earthworks shows the grid method beside its exact figure in every report: on the Hollins Farm sample the 0.5 m grid agrees with the exact overlay to within 0.01%, because the grid is fine and the shapes are simple. On a 10 m grid with batters left out, the difference is much bigger.