Excavation calculator

Dig volume in bank and loose cubic yards, the soil swell factor by material, and the truckload count taken off the loose volume a dump body is actually rated in — not the hole.

01 How the math works

Start with the hole. Bank volume is length × width × average depth, in consistent units, divided by 27 to read it in cubic yards. That is the soil in the ground, in its natural, undisturbed state — what the trade calls bank cubic yards.

Dug soil breaks apart and traps air, so its volume grows. The increase is the swell factor, and it is the step most dirt calculators skip. Loose volume is the bank volume times one plus the swell percentage. The Caterpillar Performance Handbook publishes the reciprocal, the load factor, which runs the other way: multiply a loose truck volume by the load factor to get back to bank yards.

Swell runs about 12% for sand and gravel, 25% for common earth, 30% for clay, and 50% for blasted rock. A dump body is rated in loose cubic yards, so the truckload count has to come off the loose volume, never the bank volume of the hole.

02 Worked example

Take the 24 ft × 24 ft garage footprint from our concrete scenarios (7.32 m × 7.32 m), excavated 12 in (0.30 m) of common earth to reach subgrade.

Bank volume: 24 × 24 × 1 = 576 ft³ = 21.33 yd³ (16.31 m³) sitting in the ground. Common earth swells 25%, so the loose spoil is 21.33 × 1.25 = 26.67 loose yd³ (20.39 m³) — the volume that actually goes in the truck. At a 12 yd³ (9.17 m³) tandem-axle body, that is ceil(26.67 ÷ 12) = 3 truckloads.

Size the loads off the bank volume instead — 21.33 ÷ 12 = 1.78, call it 2 — and you have booked one truck short. The load factor here is 0.800: a full 12 yd³ loose load carries back to 9.6 bank yd³.

03 When this calculator is wrong

The result uses a single mid-range swell figure per material. Swell is not a constant — it moves with moisture, grading, and how the soil is handled, and the Caterpillar Performance Handbook tables that these figures come from state as much. For engineered cut-and-fill, use the swell and shrink factors from the project soils report, not a generic table.

• Vertical walls are assumed; real cuts are sloped. OSHA 29 CFR 1926 Subpart P requires any trench or excavation over 5 ft (1.52 m) deep to be sloped, benched, or shored. A sloped-back cut removes more soil than the neat box — at the Type C maximum 1.5:1 slope, a 10 ft (3.05 m) deep cut opens 15 ft (4.57 m) wider on each side at the top. Enter the average top-to-bottom dimensions, not the trench-floor footprint, or the volume reads low.
• Swell is the haul factor, not the fill factor. Spoil reused as compacted fill ends up below its bank volume, because compaction squeezes out more void than the bank held. Common earth shrinks about 10% and clay about 12%. The alternative result row applies that; the loose-volume figure is for the truck, the compacted figure is for the fill, and swapping them is a classic over- or under-order.
• It ignores water and over-dig. A dig below the water table hauls out heavier, wetter spoil and the sides slump, so loose volume runs higher than the dry swell figure. Machine over-dig and hand-trimming add a few percent more on top. Neither is in the geometry.
• Rock swell is a wide band. The 50% figure is for well-blasted rock. Poorly fragmented rock swells more, and the actual number depends on the blast and the fragment size — treat rock as the least certain row in the table.

04 What to do with the result

Book trucks off the loose cubic-yard figure and round up, every time. A dump body is rated in loose volume, so the truckload count above already matches how the hauler bills — but confirm the body's rated capacity rather than assuming 12 yd³, because tandem-axle bodies run 10–14 loose yd³ and a tri-axle carries more. One short-booked load means a second trip at a second trip's price, which is the whole reason to haul off loose, not bank.

05 Common questions

What is the difference between bank and loose cubic yards?

Bank cubic yards (BCY) is soil measured in the ground, undisturbed. Loose cubic yards (LCY) is the same soil after it is dug, broken up, and piled or loaded — larger, because it now holds trapped air. The gap between them is the swell factor: common earth at 25% swell means 1 bank yard becomes 1.25 loose yards.

How much does soil swell when excavated?

Roughly 12% for sand and gravel, 25% for common earth, 30% for clay, and 50% for blasted rock, per the Caterpillar Performance Handbook material tables. These are mid-range planning figures; moisture and composition move them, so an engineered job uses the soils report.

How many cubic yards does a dump truck hold?

A typical tandem-axle ("ten-wheeler") dump truck carries about 10–14 loose yd³ (7.6–10.7 m³); tri-axle and larger bodies carry more. The rating is loose volume, which is why the truckload count has to be figured from loose cubic yards, not from the bank volume of the hole.

Why is my excavated pile bigger than the hole?

Because the soil swelled. The hole is bank volume; the pile is loose volume, 12–50% larger depending on the material. It is the same soil and the same weight — only the void space changed.

Can I use the excavated soil as backfill?

Yes, but it shrinks when compacted: common earth drops about 10% and clay about 12% below its bank volume. So the loose haul-off figure over-states what the soil will fill once it is placed and compacted. Use the compacted figure for fill planning and the loose figure for trucking.