The Three States of Soil Volume
Every earthwork calculation needs to account for the fact that soil changes volume depending on its state. There are three volume states that matter in construction:
- Bank (in-situ) volume: The volume of soil in the ground, in its natural undisturbed state. This is what you measure when you calculate how much to dig.
- Loose volume: The volume of soil after excavation. Digging loosens the soil structure, introducing air voids between particles. Loose volume is always larger than bank volume. This is the volume you need to haul.
- Compacted volume: The volume of soil after mechanical compaction. Compacted volume is smaller than bank volume because compaction removes both the excavation-introduced voids and some of the original natural voids. This is the volume you get when backfilling and compacting.
What is the Swell Factor?
The swell factor (also called bulking factor) is the percentage increase in volume when soil is excavated from bank state to loose state. A swell factor of 30% means that 1 cubic yard of soil in the ground becomes 1.3 cubic yards when excavated.
Swell occurs because the excavation process disrupts the soil structure. Particles that were tightly interlocked in the ground separate and create air spaces. The amount of swell depends on the soil type, moisture content, and how tightly the particles were originally packed.
Typical Swell Factors by Soil Type
| Soil Type | Swell Factor | 1 cu yd bank = |
|---|---|---|
| Sand (dry) | 15% | 1.15 cu yd loose |
| Sand (wet) | 20% | 1.20 cu yd loose |
| Loam / Topsoil | 25% | 1.25 cu yd loose |
| Clay (medium) | 35% | 1.35 cu yd loose |
| Clay (heavy) | 40% | 1.40 cu yd loose |
| Rock (blasted) | 65% | 1.65 cu yd loose |
What is the Shrink Factor?
The shrink factor (also called compaction factor) is the percentage decrease in volume when soil is compacted beyond its natural bank state. A shrink factor of 15% means that 1 cubic yard of bank soil compacts to 0.85 cubic yards.
Shrink matters when you are backfilling or building embankments. If you need 100 cubic yards of compacted fill, you actually need to excavate more than 100 cubic yards of bank material to account for the volume loss during compaction.
The Canonical Rule: Always Start from Bank Volume
The most common mistake in earthwork calculations is applying factors to the wrong volume state. The correct approach is to always start from bank volume as the canonical base:
- Loose volume = bank volume × (1 + swell factor)
- Compacted volume = bank volume × (1 - shrink factor)
Never apply swell to loose volume or shrink to compacted volume. Doing so double-applies the factor and produces incorrect results. If you know the loose volume and need the compacted volume, first convert back to bank volume, then apply the shrink factor.
Practical Applications
Hauling Estimation
When estimating truck trips, you must use loose volume, not bank volume. A 10 cu yd dump truck can carry 10 cu yd of loose material. If you have 100 cu yd of bank clay (35% swell), you have 135 cu yd of loose material, requiring 14 truck trips — not 10.
Cut and Fill Balance
When balancing cut and fill on a grading project, you need to account for both swell and shrink. If you cut 100 cu yd of bank material and need to fill 80 cu yd of compacted material, you might think you have 20 cu yd of surplus. But the compacted fill actually requires more bank material: 80 / (1 - 0.15) = 94 cu yd of bank material to produce 80 cu yd of compacted fill. Your actual surplus is only 6 cu yd of bank material, which becomes about 8 cu yd of loose surplus to haul away.
Use the cut and fill calculator to handle this math automatically with independent swell and shrink factors.
Spoil Pile Planning
When you need to stage excavated material on site, the spoil pile calculator converts your bank volume to loose volume and models the pile as a cone to estimate the footprint and height. This helps you plan staging areas and ensure OSHA setback requirements are met.
Try the Calculator
Use our swell & shrink calculator to convert between bank, loose, and compacted volumes instantly. Enter any one volume state and see all three with soil-specific factors applied automatically.