How to Use the Backfill & Surplus Soil Calculator
On an earthwork job, soil that comes out of an excavation is reused for backfill, and whatever is left over has to be hauled off as surplus soil. This calculator takes total excavation volume, the volume occupied by footings or below-grade structures that won't be backfilled, and the backfill compaction rate, then works out how much soil actually goes back in and how much surplus has to leave the site.
Subtracting the structure fill volume from total excavation volume gives the void that needs to be backfilled. Because excavated soil is loose and shrinks once it's compacted, you need to place more soil than that raw void volume — dividing by the compaction rate (say, 90%) gives the actual placed volume required to hit the target density.
Total excavation volume minus that placed backfill volume is the surplus that must be hauled away. If the placed backfill volume comes out higher than the excavation volume, it means there isn't enough soil on site and extra material needs to be imported. Either way, this gives you a number to plan hauling logistics and disposal costs around.
Frequently Asked Questions
Surplus soil equals total excavation volume minus the backfill volume actually placed. The backfill volume placed already accounts for compaction, so it takes more loose soil than the void's raw volume.
Excavated soil is loose and its volume shrinks once compacted. To fill the void at the target compaction, you need to place more loose soil, divided by the compaction rate.
This calculator gives a rough quantity estimate only. Actual soil moisture content, gradation, and compaction behavior vary, so have a geotechnical or civil engineer confirm final quantities.