Excavation & Earthwork
Soil Swell Explained: Bank, Loose, and Compacted Volume
Why dirt grows when you dig it and shrinks when you compact it: swell and shrinkage factors by soil type, the three volume states, and conversion math with examples.
Dig a perfectly measured 10-cubic-yard hole and you'll have roughly 12.5 cubic yards of dirt on the ground. Put it back and compact it, and the hole won't quite fill. This isn't measurement error - it's the defining physics of earthwork, and it's why haul-off orders, fill orders, and skip rentals so often miss.
The three states
Bank volume is soil as nature compacted it over centuries - dense, structured, minimal air. Excavation contractors price digging in bank yards because that's what the hole measures.
Loose volume is what exists after the bucket breaks that structure: particles jumbled with air voids. Trucks, skips, stockpiles, and disposal sites all deal in loose yards.
Compacted volume is soil after modern compaction equipment squeezes the air back out - usually tighter than the original bank state, which is why fill "shrinks."
Swell and shrinkage factors by material
| Material | Swell (bank→loose) | Shrinkage (bank→compacted) | 1 bank yd³ = loose yd³ |
|---|---|---|---|
| Sand | 10–15% | 0–5% | 1.12 |
| Gravel | 10–15% | 0–5% | 1.12 |
| Common earth | 20–30% | ~10% | 1.25 |
| Clay | 25–40% | ~10–15% | 1.30 |
| Topsoil | 25–35% | varies (don't compact as fill) | 1.30 |
| Blasted rock | 40–60% | swells even compacted (+25–30%) | 1.50 |
Rock is the interesting outlier: broken rock can never be packed back to solid-ledge density, so rock fill occupies more space than the bank rock it came from - permanently.
The two conversions you'll actually use
Haul-off estimate: Loose = Bank × (1 + swell). Dig 20 bank yards of clay → 26 loose yards → that's the number for trucks and disposal fees.
Fill purchase: Loose to buy = Compacted needed × (1 + swell) ÷ (1 − shrinkage compensation)… in practice, use the simple planning rule: order 1.25–1.4 loose yards per compacted yard needed, tighter for sand, looser for clay. To place 10 compacted yards of earth fill, order 13–14 loose yards.
The excavation calculator applies swell by soil type and plans truckloads from loose volume automatically.
Open the Excavation CalculatorWorked example: pool dig
- Pool excavation: 16 × 32 × 5.5 ft average = 2,816 ft³ = 104 bank yd³ in clay
- Loose: 104 × 1.30 = 135 loose yd³
- Haul with 12-yd³ tandems: 135 ÷ 12 = 11.3 → 12 loads (bank math would have said 9 - three trucks short)
- Backfill around shell needs 18 compacted yd³: order 18 × 1.35 ≈ 24 loose yards of suitable fill
Why this matters for money
Disposal is priced per loose yard or per load; fill is sold per loose yard; excavation labor per bank yard. A contract that doesn't specify which volume state it means is ambiguous by exactly 25% - enough to swallow the whole margin of a small job. When comparing quotes, confirm every volume's state.
When to call a professional
Structural fill under buildings must be placed in tested, compacted lifts of specified material - that's geotechnical territory with field density testing, not calculator territory. Same for any fill on slopes or over utilities. For landscaping and non-structural regrading, the factors above will serve you well.
Frequently Asked Questions
What is soil swell?
The volume increase when in-place soil is excavated and loosened. Digging breaks the soil's structure and adds air voids: sand swells about 12%, mixed earth 25%, clay 30%, and blasted rock up to 50%.
What are bank, loose, and compacted volume?
Bank: soil undisturbed in the ground. Loose: the same soil after digging, sitting in a pile or truck. Compacted: the soil after placement and compaction, often denser than bank. One bank cubic yard of earth is roughly 1.25 loose and 0.9 compacted.
Why does my fill order never match the hole?
Because you buy loose volume but the placed, compacted fill occupies less. Filling a 10-cubic-yard void with earth fill requires ordering roughly 12-14 loose cubic yards, depending on the material and compaction level.
Does soil shrink back if I re-compact it?
Yes - compaction removes the air digging introduced, and modern compaction typically makes fill denser than the original bank state. That's the shrinkage factor: earth fill compacts to about 90% of its bank volume.
Do swell factors matter for small projects?
Proportionally, more than for big ones: one skipped 25% swell on a 10-yard dig means an extra trip with a 6-yard truck. On any project with disposal fees or rented bins, use loose volume.