Trench Calculator: Calculate Excavation and Backfill for Trenches

Use our free trench calculator to determine trench volume, loose excavation volume, and backfill requirements for utility and foundation trenches. Built for excavation contractors, it accounts for soil swell so you know both how much material to haul away and how much you'll need to bring back for backfill.

How to use this calculator.

1. Enter trench length. Measure the full run of the trench, in feet.
2. Enter trench width. Measure the excavated width, in inches.
3. Enter trench depth. Measure the excavated depth, in feet.
4. Set the swell factor. Soil expands when excavated (default 25% for common earth). Clay runs 30–40%, sand 10–15%, rock 50–60%.
5. Review your results. See bank volume (in-place), loose volume (for hauling), and backfill volume.

How do you calculate trench volume and backfill?

Bank volume is the trench's in-place volume before excavation. Once dug, that same soil expands (swells) into a larger loose volume for hauling. Backfill volume assumes roughly 70% of the bank volume returns to the trench, since the pipe or utility and compaction settlement take up the rest.

Bank Volume (cu yd) = Length × (Width ÷ 12) × Depth ÷ 27

Loose Volume (cu yd) = Bank Volume × (1 + Swell % ÷ 100)

Backfill Volume (cu yd) = Bank Volume × 0.7

Where:

• Width ÷ 12: Converts trench width from inches to feet so all three dimensions are in the same unit before multiplying.
• Swell Factor: The percentage a soil expands once excavated from its natural, compacted state. It determines how many truckloads you need to remove the spoil.
• 0.7 backfill factor: An industry rule of thumb assuming pipe, bedding, and compaction consume about 30% of the original bank volume, so only 70% of what came out goes back in.

Example calculation.

Let's estimate excavation and backfill for a water service line installation.

Scenario: A 100 ft trench, 18 inches wide, 4 ft deep, in common earth with a 25% swell factor.

1. Convert width to feet: 18 in ÷ 12 = 1.5 ft
2. Calculate bank volume: 100 ft × 1.5 ft × 4 ft ÷ 27 = 600 ÷ 27 = 22.22 cu yd
3. Calculate loose volume: 22.22 cu yd × 1.25 = 27.78 cu yd
4. Calculate backfill volume: 22.22 cu yd × 0.7 = 15.56 cu yd

Result: This trench yields 22.22 cu ydof bank-measure spoil, which expands to 27.78 cu yd for hauling, while only 15.56 cu yd returns to the trench as backfill — the remaining 6.66 cu yd (after swell) must be hauled off site.

Tips from the field.

• Call 811 before you dig. Every utility trench starts with a locate request; hitting a buried line is dangerous and can trigger significant repair and fine costs.
• OSHA requires protective systems at 5 ft. Trenches 5 feet deep or greater need shoring, shielding, or sloped/benched walls before anyone enters — this is a hard legal requirement, not a suggestion.
• Match swell factor to actual soil. Clay swells 30–45%, silt 25–35%, sand 10–20%. Guessing low means you'll run out of truck capacity mid-haul.
• Depth targets vary by utility. Electric and gas typically run 18–24 inches, water lines go 24–48 inches (below the local frost line), and sewer depends on required slope.
• Budget $5–$15 per linear foot for excavation alone.Rock or difficult soil can double that; backfill and compaction are separate line items on top.

Disclaimer: This calculator provides estimates for planning purposes only. Trench safety — shoring, sloping, and benching per OSHA requirements — is the contractor's responsibility. PavementCalculator.com assumes no liability for material overages, shortages, or safety compliance decisions.