Backfill Calculator
Calculate backfill volume around a structure and the material you need to import, using whichever compaction convention your source figure came from.
Formula
Backfill is the excavation less the structure. How much material you need on top of that depends on which convention your percentage comes from.
- Excavation (m³) = Le × We × De
- Structure (m³) = Ls × Ws × Hs
- Compacted backfill (m³) = Excavation − Structure
- Bank-based: Material (m³) = Compacted ÷ (1 − shrinkage % ÷ 100)
- Allowance: Material (m³) = Compacted × (1 + allowance % ÷ 100)
Worked example
A 10 m × 6 m × 1.5 m excavation containing an 8 m × 4 m × 1.5 m structure, with a 15% figure.
- Excavation = 10 × 6 × 1.5 = 90 m³
- Structure = 8 × 4 × 1.5 = 48 m³
- Compacted backfill = 90 − 48 = 42 m³
- Bank-based: 42 ÷ (1 − 0.15) = 49.41 m³ measured in the ground
- Allowance: 42 × 1.15 = 48.30 m³ measured loose
42 m³ compacted. The material to bring in is 49.41 m³ bank measure or 48.30 m³ under the allowance convention — the two are not interchangeable.
Assumptions and limitations
- Both the excavation and the structure are treated as rectangular prisms. Complex shapes should be broken into parts.
- The two conventions are genuinely different and the calculator does not guess which one you meant. A shrinkage factor quoted in a geotechnical report is bank-based; a delivery allowance agreed with a supplier is usually an uplift.
- Any granular drainage layer, geotextile or protective surround is not deducted and should be handled separately if it is significant.
What this calculator does not do
- Does not check whether excavated material is suitable for reuse as fill.
- Does not size drainage layers, geotextiles or protective surrounds.
- Does not verify compaction requirements or lift thicknesses.
Practical tips
- Check which convention your percentage came from before you use it. At 25% the two conventions differ by more than 6%, and the bank-based one always needs more material.
- Work out whether excavated material can be reused before ordering imported fill. Reuse usually depends on the material meeting the specification for the fill class.
- Backfill against a structure has to be placed evenly on both sides where that applies, otherwise the unbalanced pressure can move the structure.
Frequently asked questions
What is the difference between the two methods?
A bank-based shrinkage factor describes volume lost when in-ground material is compacted, so you divide by (1 − factor). An allowance is an uplift you add to the compacted volume, so you multiply by (1 + allowance). For 42 m³ compacted at 15%, the first gives 49.41 m³ and the second 48.30 m³.
Which one should I use?
Whichever matches where your percentage came from. Figures in geotechnical reports are bank-based by definition. A rule-of-thumb delivery allowance agreed with a supplier is usually an uplift. Using a bank-based figure as an uplift under-orders material.
What is the difference between swell and shrinkage?
Swell describes soil expanding when you dig it out, measured against the bank volume. Shrinkage describes material losing volume when compacted, also measured against the bank volume. Both are referenced to the in-ground state.
Can I reuse the material I excavated?
Often yes, if it meets the specification for the fill class and is at a workable moisture content. Unsuitable material, contaminated material and highly plastic clays are commonly excluded. The specification decides.
Related calculators
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- Trench Volume Calculator Calculate trench excavation, pipe bedding and backfill for a pipe run in one pass.
- Road Base & Subbase Calculator Calculate compacted volume and delivered tonnage of granular base or subbase, including the bulking allowance between loose and compacted material.
- Concrete Footing Calculator Calculate concrete for strip footings or a set of identical pad footings.
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.