Backfilling Quantity Calculator

Backfilling is the process of placing soil back around foundations, footings, pipes and retaining walls after the structure has been built. Ordering or hauling the wrong amount of fill is expensive, so site engineers calculate the backfill quantity from the excavated volume, the volume occupied by the structure and the compaction behaviour of the soil.

How to use the calculator

1. Enter the excavation dimensions (length, width and depth) or the total excavated volume.
2. Enter the volume of the structure that will remain in the pit, such as the footing, column stub and PCC bed.
3. Set the compaction or shrinkage factor for the fill material.
4. Read the compacted backfill volume and the loose volume to order or haul.

The basic formula

Backfill (compacted) = Excavated volume - Volume of structure below ground level. Because soil compacts, the loose material required is larger: Loose volume = Compacted volume × compaction factor. Typical factors range from about 1.1 to 1.3 depending on the soil and compaction effort.

Worked example

A footing pit is 2.0 m × 2.0 m × 1.5 m = 6.0 m³. The footing, PCC bed and column stub occupy 1.4 m³. Compacted backfill = 6.0 - 1.4 = 4.6 m³. With a compaction factor of 1.2, you need about 5.5 m³ of loose soil.

Site tips

• Place and compact backfill in layers of 150 to 300 mm, as specified.
• Do not use excavated material containing organic matter or large boulders.
• Keep a record of layer tests, such as field density tests, for quality control.