Question 1

What is the soil swell factor?

Accepted Answer

The swell factor is the percentage increase in volume when soil is excavated from the ground. When you dig soil, it breaks apart and loosens, taking up more space than it did in the ground. For example, clay with a 35% swell factor means 10 m³ in the ground becomes 13.5 m³ after excavation. Swell factors range from 15% for sand to 60% for blasted rock.

Question 2

What is the soil shrink factor?

Accepted Answer

The shrink factor is the percentage decrease in volume when soil is mechanically compacted below its original bank (in-situ) density. When soil is placed as fill and compacted with rollers or tampers, it becomes denser than it was in the ground. For example, clay with a 15% shrink factor means 10 m³ of bank volume compacts to 8.5 m³. This matters for fill specifications and ordering backfill material.

Question 3

What is the difference between bank, loose, and compacted volume?

Accepted Answer

Bank (in-situ) volume is soil as it sits undisturbed in the ground. Loose volume is soil after excavation — it is larger because the soil expands when broken up. Compacted volume is soil after mechanical compaction — it is smaller than bank volume because the soil is compressed beyond its natural state. Always use loose volume for hauling estimates and compacted volume for fill specifications.

Question 4

How do I convert bank volume to loose volume?

Accepted Answer

Multiply the bank volume by (1 + swell factor / 100). For example, 10 m³ of clay (35% swell): 10 × 1.35 = 13.5 m³ loose volume. This calculator handles the conversion automatically for any soil type — just enter the bank volume and select your soil type.

Question 5

Why does soil swell when excavated?

Accepted Answer

Soil swells because the excavation process breaks the natural bonds between soil particles, introducing air voids. In the ground, particles are packed tightly with minimal air space. When dug up, the soil loosens and the gaps between particles increase, causing the total volume to expand. The amount of swell depends on soil type — clay swells more because its dense, cohesive structure breaks into larger clumps with more air space.

Question 6

Which soil type has the highest swell factor?

Accepted Answer

Blasted rock has the highest swell factor at around 60%, meaning 10 m³ of rock in the ground becomes 16 m³ after blasting. This is followed by heavy clay at 35%, light clay at 30%, and topsoil/loam at 25%. Sand has the lowest swell factor at 15% because its granular particles do not break apart as dramatically as clay or rock.

Soil Type	Swell Factor	Shrink Factor
Sand (Dry)	15%	10%
Sand (Wet)	20%	12%
Gravel	15%	8%
Topsoil/Loam	25%	10%
Clay (Light)	30%	12%
Clay (Heavy)	35%	15%
Rock (Blasted)	60%	20%

Soil Swell & Shrink Calculator

Volume States

How it works and assumptions

Three Volume States

Conversion Formulas

Typical Swell and Shrink Factors

Frequently Asked Questions

Sources and revision