Hole Volume Calculator
The volume of an excavation and the spoil it produces — for round or rectangular holes, trenches and pits.
How much dirt comes out of the hole.
How the hole volume calculator works
Excavated volume is straightforward geometry, but the spoil is not: soil bulks up when dug. A cubic metre in the ground becomes 1.2 to 1.4 cubic metres loose, which is why the pile always looks too big to have come out of the hole.
That swell factor is what decides how many skips or truckloads you need, and it is the number people forget.
Formula: V = area × depth; spoil = V × swell factor
Worked examples
| Inputs | Excavated volume | Note |
|---|---|---|
| A 3 × 1 m trench, 1.2 m deep | 3.6 | 3.6 m³, 4.5 m³ of spoil |
| Clay instead | 3.6 | swells 40% |
| Four 600 mm post holes | 1.357 | 1.36 m³ |
FAQFrequently asked questions
Why is there more spoil than hole?
Because soil bulks when disturbed. Common earth swells about 25%, clay 40% and rock 50%.
How many skips will I need?
A standard 6 m³ builder's skip takes about 4.8 m³ of in-ground soil once the swell is accounted for.
How much does the soil weigh?
Around 1.6 tonnes per cubic metre in the ground — wet clay is heavier, dry sand lighter. Most skips have a weight limit well before they are full.
Do I need to allow for batter?
For anything deep enough to need battered or shored sides, yes — the top of the excavation is wider than the base and the volume rises accordingly.
What about backfilling?
You need the in-ground volume, not the loose one, and it must be compacted in layers. Loose backfill settles and takes the surface with it.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.