How to Calculate Excavation Volume
Quick answer
Excavation volume for a rectangular dig-out is Length × Width × Average Depth, measured in bank (in-ground) cubic metres. To work out cartage, multiply by a bulking factor (commonly 20–30 %) to get loose volume, then divide by truck capacity for loads.
Bank volume vs loose volume
The single biggest mistake in excavation quantities is mixing up bank and loose measure. Bank volume is the material measured in the ground, before it's disturbed. The moment the digger breaks it out, the material bulks up — air gets between the particles and the same bank cubic metre now takes up more space on the truck.
- —Bank m³ — what you measure off the drawings and what the excavation is priced on
- —Loose m³ — what you cart, tip and stockpile
- —Compacted m³ — what fill becomes after compaction (usually less than bank)
The calculation
Use the average depth across the cut. If the depth varies a lot, split the excavation into sections and calculate each one — averaging one end at 0.5 m and the other at 3 m across a long cut will bite you.
Formulas
Bank volume = Length × Width × Average depth
Loose volume = bank volume × (1 + bulking %)
Truck loads = loose volume ÷ truck capacity (round up)
Worked example
Worked example
Trench for a foundation: 42 m long × 3.2 m wide × 1.4 m average depth
Bank volume = 42 × 3.2 × 1.4 = 188.16 m³
Bulking at 25 % → loose volume = 188.16 × 1.25 = 235.2 m³
9 m³ trucks → 235.2 ÷ 9 = 26.1 → 27 loads
Choosing a bulking factor
Bulking depends on the material. As a starting point: sands and gravels around 10–15 %, mixed fill around 20–25 %, clays commonly 25–30 %, and solid rock can exceed 50 % once blasted or ripped. If you cart the same ground regularly, your weighbridge history is the best factor you'll ever get.
- —Check payload too — wet clay usually hits the truck's legal weight before the bin is full
- —Allow for batters and over-dig in your width and depth, they're not in the plan dimensions
- —Topsoil strip is usually a separate item with its own depth
Pricing the whole earthworks package? Fieldlink turns quantities into estimates with your own plant, labour and cartage rates.
See Earthworks Estimating