Earthwork cut & fill: the average-end-area method

Before you can quote a grading job or order trucks for a trench, you need a volume — and most earthwork estimates start from the same shortcut: measure the cross-sectional area at two points, average them, and multiply by the distance between them. It's not a perfect model of the ground, but it's the standard quick method contractors have used for a long time, and it's easy to check by hand.

The method

Take the cross-sectional area at each end of a run — Area A and Area B, both in square feet — and the distance between them. Volume = ((Area A + Area B) ÷ 2) × distance, which gives cubic feet. Divide by 27 to get cubic yards, the unit most quotes and truck capacities use. That's the bank volume — the volume of the soil while it's still in the ground, undisturbed. Once you dig it, it loosens and takes up more space (the technical term is swell or bulking), so the volume you actually have to haul is bank volume times (1 + swell % ÷ 100).

A worked example: one trench run

A trench run has a cross-section of 95 sq ft at the start and 145 sq ft at the end, measured 120 ft apart. Swell for this soil type is estimated at 18%.

1. Volume: ((95 + 145) ÷ 2) × 120 = 120 × 120 = 14,400 cubic feet.
2. Bank volume: 14,400 ÷ 27 ≈ 533.33 cubic yards.
3. Loose (haul) volume: 533.33 × 1.18 ≈ 629.33 cubic yards.

With a 12-cubic-yard dump truck, that's 629.33 ÷ 12 ≈ 52.4 — round up to 53 truckloads to haul it all off site. Quoting the bank volume instead would leave you more than 90 cubic yards short of what actually needs hauling.

Chaining multiple sections

Long runs are usually measured at several stations, not just two, and each consecutive pair is its own average-end-area calculation, summed. Say the trench continues past the 120 ft mark: the cross-section narrows to 60 sq ft another 80 ft further on, same 18% swell.

1. Second segment: ((145 + 60) ÷ 2) × 80 = 102.5 × 80 = 8,200 cubic feet → 8,200 ÷ 27 ≈ 303.70 bank cubic yards → 303.70 × 1.18 ≈ 358.37 loose cubic yards.
2. Total bank volume: 533.33 + 303.70 = 837.03 cubic yards.
3. Total loose volume: 629.33 + 358.37 = 987.70 cubic yards.

Add as many segments as you have stations — the method doesn't change, it just repeats.

Where the error creeps in

The average-end-area method assumes the ground changes in a fairly straight line between the two measured sections. On irregular terrain with a lot happening between stations, that's an approximation, not an exact volume — the fix is to measure more, closer-together sections, not a fancier formula. Swell percentage is also soil-specific: loose sand swells far less than clay. If you don't know your soil's swell factor, a geotechnical reference or your excavation contractor's experience on that soil type is the place to get it, not a guess.

Run your own numbers

The free Earthwork Cut & Fill Volume tool takes your two cross-section areas, the distance, and a swell percentage, and returns cubic feet, bank cubic yards, and loose cubic yards instantly, entirely on your device.