How the excavation calculator works
Digging is measured as the bank volume: the solid hole you leave behind. Once dug, the material loosens and takes up more space. The calculator applies a bulking factor for your soil type to get the loose volume you'll have to move, then works out the weight from typical in-ground densities.
Bulking and weight by material
| Material | Bulking | Weight per m³ dug |
|---|---|---|
| Topsoil | +25% | ~1.5 t |
| Clay / subsoil | +30% | ~1.9 t |
| Sand & gravel | +15% | ~1.8 t |
| Chalk | +50% | ~1.9 t |
| Broken concrete | +60% | ~2.4 t |
Getting rid of spoil
- Skips: limited by weight as much as by size. For soil, assume a 6-yard skip (about 4.6m³) holding no more than about 7 tonnes.
- Grab lorry: best value for larger amounts (roughly 8m³ or 16 tonnes a load) and no skip permit needed.
- Tonne bags: handy for small jobs and awkward access. They hold about 0.75m³, but don't overfill one with heavy clay.
- Keep clean topsoil separate: it's worth reusing in the garden or giving away.
- Before digging, check for buried cables and pipes, and be careful near trees and foundations. Deep trenches need support to stop the sides collapsing.
Worked example: digging out for a 6m × 4m patio
A patio base 6m × 4m dug 250mm deep in clay subsoil is 6m³ of solid ground. Clay bulks up by about 30%, so you'll have 7.8m³ of loose spoil weighing roughly 11.4 tonnes. That's two 6-yard skips (limited by weight as much as volume), one grab lorry load, or about 12 tonne bags. By barrow it's around 100 trips, which is worth knowing before you promise to have it done in a weekend.
Ten fence post holes 300mm across and 600mm deep add up to only about 0.4m³, around 0.8 tonnes of clay. That's easily spread in the garden or taken away in a few bags.
By hand or with a digger?
Hand digging is fine for post holes, footings for a small wall or a shed base. For a patio or driveway, hire a mini digger with or without a driver. It does in a morning what would take days with a spade, especially in clay. Remember the digger also needs access: measure gates and side passages before booking, as many micro diggers need a gap of around 0.75–1m.
Dig safely
- Find the services first. Look for inspection chambers, meter boxes and service covers to work out where pipes and cables run, and use a cable detector if you're unsure. Dig by hand near any suspected service.
- Support deep trenches. Trench sides can collapse without warning. Don't work in a trench deeper than about 1.2m (less in loose or wet ground) without proper support, and keep spoil heaps back from the edge.
- Mind the house and trees. Digging close to foundations, or cutting large tree roots, can cause movement. Get advice before digging below the level of nearby footings.
Common mistakes
- Ordering skips by volume alone. Soil hits the weight limit before the skip looks full. This calculator checks both.
- Mixing topsoil with subsoil. Good topsoil is valuable for beds and lawns. Strip it off first and keep it separate.
- Forgetting old concrete. Breaking out a slab or old patio produces far heavier, bulkier waste. Choose "broken concrete" above.
Once you know the spoil, the skip size calculator picks the skip. For what goes back in the hole, see the MOT Type 1, foundation and patio calculators.