Trench Backfill Calculator
A trench backfill calculator estimates fill volume by subtracting a pipe cylinder from a rectangular trench. Enter length, width, depth, and pipe outside diameter to get cubic yards and US short tons. The model assumes a constant section and one pipe fully within that section.
Backfill including allowance
60.12CY
- Trench volume
- 66.67 CY
- Pipe displacement
- 6.54 CY
- Weight including allowance
- 89.3 tons
Formula: CY = (L × W × D − pipe cylinder) ÷ 27; tons at 110 lb/ft³; waste applied last
Uses a generic density from the site’s material table. Confirm a product-specific value with the supplier.
How the calculation works
Gross trench volume is length × width × depth in feet. Pipe displacement is π × (outside diameter ÷ 2)² × length, with diameter converted to feet. Subtract the pipe from the trench, then divide by 27 for cubic yards. The selected material density converts this volume into tons.
A 100-foot trench, 3 feet wide and 6 feet deep, contains 66.67 cubic yards. An 18-inch outside-diameter pipe displaces 6.54 cubic yards, leaving 60.12 cubic yards before allowance. At the generic common-fill density of 110 lb/ft³, the fill weighs about 89.3 tons.
Gross trench volume and pipe displacement are shown before allowance. The backfill volume and weight include the percentage you enter. This is a simple quantity increase, not a calculated shrink or compaction model.
Calculate separately where width, depth, or material changes. The model does not account for sloped trench sides, multiple pipes, structures, or irregular excavation. To calculate fill above a separately measured embedment zone, enter only the remaining depth and set pipe diameter to zero; do not deduct the same pipe twice.
Reference values
| Material | Unit weight | Tons per CY |
|---|---|---|
| Common fill / borrow | 110 lb/ft³ | ≈ 1.49 |
| Sand | 100 lb/ft³ | ≈ 1.35 |
| Crushed stone, compacted | 100 lb/ft³ | ≈ 1.35 |
| Dense-graded aggregate | 135 lb/ft³ | ≈ 1.82 |
Frequently asked questions
Do I use inside or outside pipe diameter?
Outside diameter. The full exterior of the pipe displaces backfill, including its wall thickness.
Does this include pipe bedding?
A full trench section includes all space outside the pipe. If bedding uses a different material, calculate that zone separately and avoid counting it again as general backfill.
What does the allowance change?
It increases backfill volume and weight. Gross trench volume and pipe displacement remain before allowance.
Can it calculate a trench with sloped sides?
No. This model uses a rectangular section with constant width and depth. Calculate a different cross-section separately.