Backfill Volume Calculator
Backfill volume, soil required, surplus to remove and truck loads.
Inputs
Result
Formula
| Pit volume | V = length × width × depth |
| Backfill | V = pit − structure |
| Natural soil | V = backfill ÷ Ks′ |
| Loose volume | V = natural × Ks |
| Surplus | V = pit − natural |
| Trucks | N = 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.
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.
Related calculators
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.