Soil Compaction Calculator

How the soil compaction calculator works

Soil changes volume twice on its way from one hole to another. Dig it and it swells, because the grains lose their interlock and trap air. Roll it back into place and it shrinks again, often to less than it occupied in the ground. This calculator takes the loose volume you have (a stockpile, a truckload, a spoil heap) and works out how much compacted fill it makes. It does that in two steps: loose volume ÷ swell factor gives the bank volume, meaning what the soil occupied undisturbed, and bank volume × compaction factor gives the compacted volume.

The factors are the averages published in the US Army's earthmoving manual, FM 5-434, Table 1-1. Clay swells by a factor of 1.43 and compacts to 0.90 of its bank volume; loam (common earth) is 1.25 and 0.90; sand and gravel are 1.11 and 0.95; blasted rock is 1.50 and 1.30. Topsoil uses the loam figures. Chalk, which is a soft limestone, uses the manual's row for material comparable to lime rock. Treat that as the least certain row: weak chalk breaks down under the roller and can finish nearer its bank volume, so chalk fill is worth testing, or taking a figure from a geotechnical engineer.

Take the default: 13 yd³ of loose clay. Divide by 1.43 and you get 9.09 yd³ of bank clay; multiply by 0.90 and it compacts to 8.18 yd³, a drop of 37.1% from the loose pile. On the metric side, 10 m³ of loose clay is 6.99 m³ in the bank and 6.29 m³ compacted. Rock behaves very differently. The same 13 yd³ of blasted rock is 8.67 yd³ in the bank but 11.27 yd³ compacted, because broken rock never packs back as tightly as the solid rock it came from.

Swell, shrinkage and why the numbers differ

Three volumes matter on any earthworks job, and mixing them up is the commonest estimating mistake. Bank volume is what you measure in the ground and what excavation is usually priced on. Loose volume is what trucks, skips and dumpsters carry. Compacted volume is what fills a trench, raft or embankment once it has been rolled in layers. The ratio between any two depends on the material.

Cohesive soils lose the most volume between the truck and the finished fill. FM 5-434 puts loose to compacted at 0.63 for clay, 0.72 for loam, 0.86 for sand and gravel, and 0.87 for blasted rock. Read the other way, every cubic metre of compacted clay needs about 1.59 m³ delivered loose. That is why a trench dug in clay and backfilled with its own spoil ends up short: 5 m³ dug from the bank bulks to 7.15 m³ in the heap, but compacts to only 4.5 m³, leaving 0.5 m³ to find. Dig 5 m³ of rock and you finish with 6.5 m³ compacted and 1.5 m³ surplus.

What this tool does not do

It converts one volume into another using published averages. It does not know your soil's moisture content, grading or organic content, and all three shift the real figures. It doesn't set a compaction specification, check bearing capacity, design an embankment or say whether a material is suitable as fill at all. The manual itself notes that its figures are averages and that a specific material should be tested when exact values matter.

The compacted figures assume the material is placed and compacted properly, in thin layers at a suitable moisture content, to the density the job calls for. Fill that is tipped and tracked over will settle further over months, sometimes a lot further, so treat the output as the volume you would reach with proper compaction, not with casual placement.

On-site considerations for soil compaction

Compaction is specified and checked against a laboratory test. The common reference is the Proctor test (ASTM D698, with equivalents in other national standards), which finds the maximum dry density a soil can reach and the moisture content at which it gets there. A specification will usually ask for a percentage of that density, checked in the field. Soil that is too dry won't compact to specification however many passes it gets; soil that is too wet turns to mud under the roller.

Getting the soil there means working in excavations, and they collapse without warning. In the United States, OSHA's 29 CFR 1926.652 requires a protective system for excavations 5 ft (1.5 m) deep or more unless the ground is stable rock. In Great Britain, the HSE's excavation guidance is blunter still: no ground can be relied upon to stand unsupported in all circumstances. Check the rules where you are, and locate buried services before anyone digs.

Codes and compliance

Where fill supports a building, a slab or a road, it stops being a quantity question. Building codes such as the International Building Code, the UK Building Regulations and their equivalents elsewhere expect foundations to sit on ground or engineered fill that can carry the load, and a geotechnical engineer normally sets the fill specification and the testing regime. Drainage runs, retaining walls and highway works often have their own bedding and backfill rules. For anything structural, take the factors from the site investigation, not from this page.

Before you order

Ordering runs the same maths backwards. For 20 m³ of compacted clay fill, dividing by 0.90 gives 22.22 m³ of bank material, and multiplying by 1.43 gives 31.78 m³ to haul loose. For 30 yd³ of compacted loam, the same steps give 33.33 yd³ bank and 41.67 yd³ loose. The result shows this ratio as "Loose to Order per Unit Compacted", which is the compacted-to-loose column of the same FM 5-434 table. Ask your supplier or haulier which measure their price uses. Imported fill is usually sold loose or by weight, and a quote for "30 yards" of loose loam will not fill a 30 yd³ compacted void.

Adjusting the defaults

The calculator opens at 13 yd³ of loose clay on the US side and 10 m³ on the metric side (13 yd³ is about 9.94 m³, so the two are close, not identical). Change the volume to your stockpile or load and pick the nearest material. Topsoil for lawns and planting beds is firmed rather than rolled, so it will not reach the compacted figure; for a garden, treat the result as the least it could settle to and expect to end up with more. If your material sits between two rows, such as a sandy loam or a clay with a lot of stone in it, run both and treat the answer as a range. Measured factors from a site investigation or a supplier are more reliable than any table average.

Using this soil compaction calculator alongside other BuildMetricLab tools

Compaction sits between excavation and haulage in most earthworks estimates. The dumpster size calculator adds its own bulking allowance, so give it the bank (in-place) volume, not the loose figure, or the swell gets counted twice. The excavation volume calculator gives the bank volume of a hole, and the cut and fill calculator balances what comes out of the ground against what goes back in; a void to be backfilled is a compacted figure. Everything runs in your browser, nothing you type is sent anywhere, and the formula is on the page so you can repeat each step, as the examples above do.