TryBuildCalcPit Backfill Calculator (Structure Deduction, Compaction, Truckloads & Cost)
Calculate backfill volume after structure deduction.
🕒 Last updated: July 30, 2026
Inputs
Choose the source excavation shape. Trench and rectangular use length × width × depth; pit uses top and bottom dimensions.
ℹ️Use the finished bottom size of the pit.
ℹ️Top dimensions should include side slopes or working space.
ℹ️Use this for repeated trenches, pits, or identical excavation bays.
Structure Deduction
ℹ️Enter footing, wall, pipe envelope, tank, or other built volume inside the excavation.
Select Yes if part of the structure volume is hollow and should not be deducted as solid structure.
Compaction
ℹ️Adds extra loose material to achieve the calculated compacted backfill volume. Typical planning range: 5-15%.
ℹ️Used only to convert volume into an approximate weight in tons — does not affect volume, compaction, or cost.
ℹ️Use the usable volume carried per trip. Typical: 3-10 m³ per trip (about 105-350 cft per trip).
ℹ️Optional buffer for unknowns — supplier density variance, delivery charges, or material substitution. Commonly 5-15%.
Backfill required after structure deduction is 31.649 m³. With compaction allowance, plan for 35.447 m³, requiring 8 truck loads and costing around ₹10,634.
Calculations
Excavation Volume: 40.649 m³ (~65.04 t)
Structure Deduction: 9.000 m³ (~14.40 t)
Net Backfill: 31.649 m³ (~50.64 t)
Compaction Added: +3.80 m³ (~6.08 t)
Adjusted Backfill: 35.447 m³ (~56.72 t)
Estimates
Truck Loads: 8
Estimated Cost: ₹10,634
Approximate results for planning only. Verify with a professional.
Poor compaction and drainage behind a wall are common causes of cracking. Wall Cracks: Causes and Remediation →
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Backfill Calculator.
Sloped pit backfill estimate
This page uses pit excavation geometry — a bottom of 4 m x 3 m widening to a top of 6 m x 5 m over a 2 m depth — so backfill can be estimated for a sloped-side excavation, using the frustum (truncated pyramid) volume formula instead of a simple box. That gives 40.65 m³ of excavated volume, and a 3 m x 2 m x 1.5 m structure (9 m³) deducted from it leaves 31.65 m³ of net void.
A 12% compaction allowance takes the adjusted backfill to 35.45 m³, requiring 8 truck loads at 5 m³ capacity and costing roughly 10,634 at 300 per m³.
- Bottom 4 m x 3 m, top 6 m x 5 m, depth 2 m → 40.65 m³ excavated (frustum formula).
- Structure deduction: 3 m x 2 m x 1.5 m (9 m³).
- Compaction allowance: 12% → 35.45 m³ adjusted backfill, 8 truck loads.
How does backfill quantity calculation work?
The calculation starts with excavation volume, deducts the solid volume occupied by the structure, then adds a compaction allowance to estimate the loose fill material to arrange.
Step 1 — Calculate Excavation Volume
Pit Volume = Depth ÷ 3 × (Bottom Area + Top Area + √(Bottom Area × Top Area))
If Overbreak is used: Excavation Volume = above × (1 + Overbreak %)
Use rectangular or trench mode for uniform sides. Use pit mode when top dimensions are larger than bottom dimensions due to side slopes. If the source excavation was actually dug larger than its theoretical size, the Overbreak allowance inflates this volume first, so the backfill void calculated below matches the real hole — not just the design dimensions.
Step 2 — Calculate Structure Deduction
Structure Volume = Gross Structure Volume − Void Volume
Void volume is deducted only when Include Voids is set to Yes. This is useful for hollow tanks, ducts, pipes, or other structure volumes that should not be treated as solid displacement.
Step 3 — Calculate Net Backfill
This gives the actual void space to be filled around the completed structure.
Step 4 — Add Compaction Allowance
The adjusted backfill is the loose material volume to order — it is larger than the net void because loose fill needs to be compacted down to fill that void at its final density.
Step 5 — Calculate Truck Loads and Cost
If Truck Capacity is entered in cft: Truck Capacity (m³) = Truck Capacity (cft) ÷ 35.3147
Total Cost = Adjusted Backfill (m³) × Cost per m³
Truck loads are rounded up because fill is normally transported in complete trips. Truck capacity and cost rate can each be entered in m³ or cft — a cft figure is converted to its m³ equivalent first, so the result is identical either way.
Step 6 — Add Contingency (Optional)
Total Cost = Base Cost + Contingency Amount
Contingency is an optional buffer on top of the base cost to cover unexpected variance — supplier density differences, delivery surcharges, or material substitution. It defaults to 0%; a commonly used range is 5-15%.
Example backfill calculation
This example uses the active calculator inputs above and follows the same steps from the formula section — showing the default scenario if you haven't changed anything, or your own live inputs once you do.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Pit dimensions | 4 × 3 (m) bottom, 6 × 5 (m) top, 2 m deep | Sets the frustum excavation volume |
| Structure deduction | 3 × 2 × 1.5 (m), × 1 | Volume occupied by the structure, subtracted from excavation volume |
| Compaction factor | 12% | Converts net backfill void into loose material to order |
| Equipment & cost | 5 m³/trip, ₹300 per m³ | Sets truck loads and total cost |
Step 1 — Calculate Excavation Volume
| Calculation | Substitution | Result |
|---|---|---|
| Frustum volume | 2/3 × (bottom + top + √(bottom×top)) × 1 | 40.649 m³ |
Step 2 — Calculate Structure Deduction
| Calculation | Substitution | Result |
|---|---|---|
| Gross structure volume | 3 × 2 × 1.5 × 1 | 9.000 m³ |
| Structure volume | Gross | 9.000 m³ |
Step 3 & 4 — Net Backfill and Compaction
| Calculation | Substitution | Result |
|---|---|---|
| Net backfill | 40.649 − 9.000 | 31.649 m³ |
| Adjusted backfill | 31.649 × (1 + 12/100) | 35.447 m³ |
Step 5 & 6 — Truck Loads, Cost & Contingency
| Calculation | Substitution | Result |
|---|---|---|
| Truck loads | 35.447 ÷ 5.00 m³ | 8 trucks |
| Base cost | 35.447 × rate | ₹10,634 |
| Total cost | Base cost | ₹10,634 |
Therefore, this job needs approximately 35.447 m³ of loose backfill material, requiring 8 truck loads and costing around ₹10,634.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Backfill Calculator.