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Backfill Volume Calculator

Backfill volume, soil required, surplus to remove and truck loads.

Quick answer

With the default inputs: Pit length 10 · Pit width 8 · Pit depth 3 etc.Compacted backfill 160.00 m³

Edit the inputs below to recalculate.
Length LDepth HWidth W
Length and width are the excavation plan at the base; depth is the backfill thickness, not the dig depth.

Inputs

m³
Volume taken by footings, pile caps and basement
Loose-to-natural factor, usually 1.1-1.3
Compacted-to-natural ratio, usually 1.02-1.08
m³

Result

Compacted backfill
160.00m³
Excavation volume240.00m³
Structure volume80.00m³
Natural soil required152.38m³
Loose volume to import182.86m³
Surplus (natural)87.62m³
Surplus to cart away105.14m³
Truck loads out9loads
Truck loads in0loads

Backfill is compacted in layers no more than 300 mm loose, to the specified density (typically 0.93-0.96). Ks and Ks′ should be measured; they vary widely by soil type, and sand and clay must not be mixed.

Formula

Pit volumeV = length × width × depth
BackfillV = pit − structure
Natural soilV = backfill ÷ Ks′
Loose volumeV = natural × Ks
SurplusV = pit − natural
TrucksN = ceil(loose ÷ capacity)
A 10 × 8 × 3 m pit with 80 m³ of structure, Ks 1.20, Ks′ 1.05 and 12 m³ trucks: 160.00 m³ of backfill needs 152.38 m³ of natural soil (182.86 m³ loose), leaving 87.62 m³ surplus — 105.14 m³ loose, or 9 loads.

How to work it out

  1. Take the compacted volume needed: pit volume less foundations, blinding and basement structure.
  2. Deduct what is not backfilled — the working space is only outside the footing, and internal fill is measured to the underside of the slab.
  3. Convert compacted to loose using the bulking factor: loose ≈ compacted × 1.30 for ordinary soil.
  4. Work out the number of layers from loose thickness per layer (200-300 mm); tighter compaction targets mean more layers.
  5. Separate imported fill from re-used arisings — re-use only needs handling, imported fill is bought by loose volume.

Where it goes wrong

Material and cost

Imported fill runs 30-60 per m³ loose including haulage, with distance the dominant variable; re-handling arisings is 8-20 per m³ and layered compaction labour 15-30 per m³. Raising the compaction target from 0.90 to 0.94 can add 30% to plant hours. A good cut-and-fill balance with stockpiling on site removes the import entirely — that is the biggest lever in earthworks cost.

On site

Basis

FAQ

What exactly are the bulking factors?
Natural soil is compacted in situ; digging loosens and expands it, and that ratio is Ks (loose ÷ natural), typically 1.1-1.3. Recompacted soil shrinks back but not exactly to the original, giving Ks′ (compacted ÷ natural), usually 1.02-1.08. Cart-away volumes use loose figures, backfill uses compacted — mixing them costs about 20%.
Why backfill in layers?
Compaction energy only reaches a limited depth. A thick lift stays loose at the bottom, and later settlement drags everything above it down, cracking slabs and shearing pipes. Codes require layers no more than 300 mm loose, each compacted and tested before the next. This is the commonest source of failures.
What if the surplus comes out negative?
That means the excavated soil is insufficient and soil must be imported — common with deep pits, small structure volumes or where good fill is specified. Note that site storage is limited, so surplus is often carted away and later bought back; the double haul usually costs more than the soil, so balance on site where possible.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Backfill Volume Calculator. 2026-10-25. https://jianzhujisuan.com/en/backfill

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.