Backfill Calculator
Home & ConstructionFill needed around a finished structure, allowing for compaction shrinkage and reusing the excavated soil.
Runs entirely in your browser — nothing is uploaded
Backfill estimates go wrong because two corrections pull in opposite directions. Excavated soil swells, so there is more of it on the heap than came out of the hole. Placed soil shrinks when compacted, so filling a void takes more loose material than the void measures. Apply either backwards and you get a confident wrong answer.
What people do next
Features
- Void worked out as the excavation less whatever now sits in it.
- Compaction shrinkage applied to the quantity you have to place.
- Tells you whether the site is self-sufficient or fill has to be bought.
- Surplus for disposal, priced separately from imported fill.
- Compaction layers counted from the lift thickness.
- Runs entirely in your browser — nothing is uploaded.
How to use the Backfill Calculator
- 1Enter the excavated volume and the volume the structure now occupies.
- 2Choose the fill material and whether the excavated soil is reusable.
- 3Set the backfill depth and layer thickness.
- 4Read what has to be brought in, or carted away.
Frequently asked questions
How do I calculate backfill volume?
Take the excavated volume and subtract everything now sitting in the hole below ground — footings, blinding, pedestals, plinth beams. That difference is the void. The quantity you actually have to place is larger, because loose soil loses volume when compacted: about 15% more for common earth, 20–25% for clay.
Can I reuse the soil I dug out?
Usually, and it is the cheapest option — a site is normally self-sufficient in fill even after a structure takes up part of the hole, because the swell on excavation exceeds the shrinkage on compaction. The exceptions matter though: topsoil, organic material, waterlogged clay and anything contaminated should not go back against a foundation.
What is a compaction factor?
The proportion of volume lost when loose soil is compacted to its specified density. It is why placing 6 m³ of compacted fill needs about 6.9 m³ of loose soil for ordinary earth. Estimating the borrow quantity at the void volume is the reason a job runs out of fill two lifts from the top.
How thick should each layer be?
150–300 mm before compaction, depending on the plant. Fill dumped in one deep layer cannot be compacted through its full depth however many passes the roller makes — the bottom stays loose and the structure above it settles later. Each lift is compacted before the next goes on.
Does the backfill need to be tested?
On anything structural, yes. Compaction is normally specified as a percentage of maximum dry density from a Proctor test, and verified in the field. This tool works out quantities; it says nothing about whether the compaction you achieved is adequate.