Before a single shovel hits the ground, every digging project needs one number: how much earth has to come out. Order too few truckloads and the job stalls; order too many and you pay for air. That number is the excavation volume.

The Excavation Calculator on this page computes it for rectangular pits and sloped-side trapezoid pits alike. Enter the length, width, and depth, choose feet or meters, and it returns the volume in cubic feet, cubic yards, and cubic meters — plus the swollen spoil volume and the number of dump-truck loads to haul it away.

Below the calculator you will find how the math works, worked examples for common digs, and the mistakes that cost contractors real money.

What Does the Excavation Calculator Do?

The Excavation Calculator turns pit dimensions into dig volume and then into logistics. For a rectangular pit it multiplies length by width by depth. For a trapezoid pit with sloped sides it averages the bottom and top areas before multiplying by depth.

It then converts the result into the three units the industry uses, applies a 25-percent swell factor to estimate the loose spoil, and divides by a 10-cubic-yard truck capacity to give the number of hauls. One set of inputs, one complete hauling plan.

How to Use the Excavation Calculator

Pick the pit shape first: rectangular for vertical sides, trapezoid when the sides slope outward. Choose feet or meters for all measurements — the calculator converts internally, so keep every entry in the same unit.

Enter the bottom length, bottom width, and depth. For sloped sides, also enter the wider top length and top width at the surface. Press Calculate and read the volume rows, the spoil estimate, and the truck count.

Rectangular Pits: The Simple Case

A rectangular pit has vertical walls, so the volume is just a box. This suits shallow digs in stable soil, basement footprints, and any excavation with shoring or formwork holding the walls.

The formula is:

Volume = Length × Width × Depth

A 20 × 10 × 6 foot pit gives 20 × 10 × 6 = 1,200 cubic feet. Because walls are vertical, the top and bottom areas are identical and no averaging is needed.

Trapezoid Pits: When Sides Slope

Deep digs in loose soil cannot hold vertical walls, so the sides slope outward and the opening at the surface is wider than the bottom. The pit becomes a frustum — a box with slanted sides.

The formula is:

Volume = ((Bottom area + Top area) ÷ 2) × Depth

Averaging the two areas approximates the true frustum volume closely for typical side slopes. For example, a 20 × 10 foot bottom with a 26 × 16 foot top at 6 feet deep gives ((200 + 416) ÷ 2) × 6 = 1,848 cubic feet — over 50 percent more than the vertical-walled version.

Feet or Meters: Picking Your Unit

The calculator accepts feet or meters and converts meters to feet internally at 3.28084 feet per meter before doing the math. All results are then shown in cubic feet, cubic yards, and cubic meters regardless of your input unit.

Pick whichever unit your tape measure uses and stick with it for every field — mixing feet for length and meters for depth is the fastest way to get a nonsense answer. When in doubt, measure twice and enter once.

Worked Example: A 20 × 10 × 6 ft Foundation Pit

First: rectangular shape, feet, length 20, width 10, depth 6.

The volume is 20 × 10 × 6 = 1,200 cubic feet.

Then: divide by 27 for cubic yards — 1,200 ÷ 27 = 44.44 yd³. Multiply by 0.764555 for cubic meters: 33.98 m³.

With 25 percent swell the spoil is 44.44 × 1.25 = 55.56 yd³, which needs ⌈55.56 ÷ 10⌉ = 6 truckloads. The calculator shows each of these rows.

Worked Example: A Sloped-Side Pond Dig

First: trapezoid shape, feet, bottom 30 × 20, top 40 × 30, depth 8.

The bottom area is 600 ft² and the top area is 1,200 ft², averaging 900 ft².

Then: 900 × 8 = 7,200 cubic feet, or 266.67 cubic yards.

Swollen spoil is 333.33 yd³, needing 34 truckloads. Compare that with vertical walls — 30 × 20 × 8 = 4,800 ft³, or 177.78 yd³ — and the cost of safe slopes becomes very visible.

Worked Example: A Small Trench in Meters

First: rectangular shape, meters, length 12, width 1.5, depth 2.

Internally the calculator converts: 39.37 × 4.92 × 6.56 feet.

Then: the volume is 1,270.55 cubic feet, which is 47.06 cubic yards or 35.98 cubic meters.

Spoil with swell is 58.82 yd³ — 6 truckloads. Utility trenches are the classic small job where a quick estimate prevents ordering a half-empty truck.

Cubic Feet, Cubic Yards, Cubic Meters

Three units describe the same pile. Cubic feet are the raw math unit; cubic yards are the hauling unit, since dump trucks and disposal sites price by the yard; cubic meters are the standard on metric drawings.

The conversions are fixed: 1 cubic yard = 27 cubic feet, and 1 cubic yard = 0.764555 cubic meters. The calculator shows all three so the number on your screen matches the number on the quote, whatever unit the contractor speaks.

The 25-Percent Swell Factor

Undisturbed soil is compact; dug soil is fluffy. The same earth occupies roughly 25 percent more volume once excavated — this is the swell factor, and it is the number most DIY estimators forget.

The formula is:

Loose spoil = Bank volume × 1.25

Planning haulage on the bank volume instead of the swollen volume leaves you short by a truck or two on a typical residential dig. The calculator applies the factor automatically so the truck count reflects reality.

Common Excavation Mistakes

The costliest mistake is measuring the top of a sloped dig and treating it as the whole volume — that overstates the dig, while measuring only the bottom understates it. The trapezoid option exists to split the difference properly.

Others include forgetting the swell factor, rounding truckloads down instead of up, and ignoring overdig: footings and working room around a foundation add real volume beyond the neat rectangle on the plan. Add 5–10 percent contingency on top of the calculator’s number for a real quote.

Estimating Dump-Truck Loads

The calculator assumes a 10-cubic-yard truck, the common tandem-axle size. It divides the swollen spoil by 10 and rounds up, because you cannot order half a truckload.

Real capacity varies: small single-axle trucks carry 5–8 yards, large trailers up to 20. If your hauler uses a different size, scale the calculator’s truck count proportionally — the spoil volume in cubic yards is the number that transfers directly to any quote.

Where Excavation Calculations Are Useful

Foundations, basements, pools, ponds, septic systems, and utility trenches all start with this math. Landscapers use it for grading and topsoil orders; the same volume logic in reverse tells you how much fill to buy.

It also settles disputes: when a contractor’s quote seems high, an independent volume estimate tells you whether the disagreement is about the dig or about the rate. Five minutes with the calculator is cheap insurance on a five-figure earthworks bill.

Worked Example: Comparing Vertical vs. Sloped Dig Costs

First: a 24 × 16 foot dig, 7 feet deep, in soil that needs sloped sides — top opening 32 × 24.

Vertical-wall volume would be 24 × 16 × 7 = 2,688 ft³ (99.56 yd³). The trapezoid reality is ((384 + 768) ÷ 2) × 7 = 4,032 ft³ (149.33 yd³).

Then: swollen spoil is 186.67 yd³ — 19 truckloads instead of 13.

The safer dig costs roughly 50 percent more to excavate and haul. That is the honest trade: safety and stability on one side, six extra truckloads on the other.

How to Interpret Your Result Correctly

Read the bank volume as the hole in the ground and the spoil volume as the pile beside it — they are different numbers and only the spoil matters for hauling. The truck count already rounds up, so treat it as a minimum order.

Remember the calculator models clean geometry. Real digs have uneven bottoms, overbreak at the edges, and obstructions like old concrete. Use the result as the engineered core of your estimate and add contingency for the messiness of actual dirt.

Safety Margins Worth Adding

For budgeting, add 10 percent to the spoil volume for overdig and spillage. For scheduling, add a day — trucks get delayed, and a half-dug pit cannot wait politely.

For safety, respect the slopes: the trapezoid math assumes stable angles, but wet or sandy soil may need gentler slopes than you planned, which grows the top opening and the volume with it. When soil conditions are uncertain, have a geotechnical engineer confirm before the excavator arrives.

Frequently Asked Questions

1. What is excavation volume?

It is the amount of earth removed from a dig, measured in cubic units. Contractors price digging and hauling from this number, so it drives the whole earthworks budget.

2. How do I calculate a rectangular pit?

Multiply length × width × depth, all in the same unit. A 20 × 10 × 6 ft pit is 20 × 10 × 6 = 1,200 cubic feet, or 44.44 cubic yards.

3. How do I calculate a sloped-side pit?

Average the bottom and top areas, then multiply by depth: ((bottom + top) ÷ 2) × depth. The calculator’s trapezoid option does exactly this.

4. How many cubic feet are in a cubic yard?

27. Divide cubic feet by 27 to get cubic yards — the unit dump trucks and disposal sites use for pricing.

5. What is the swell factor?

The increase in soil volume after digging, typically about 25%. The calculator multiplies bank volume by 1.25 to estimate the loose spoil you must haul.

6. How many cubic yards does a dump truck hold?

A common tandem-axle truck holds about 10 cubic yards. Smaller trucks hold 5–8 and large trailers up to 20; scale the calculator’s count to your hauler’s size.

7. Should I round truckloads up or down?

Always up. A fraction of a truckload still requires a whole truck, and the calculator rounds up automatically.

8. Does the calculator include overdig?

No. It computes clean geometry only. Add 5–10% for working room around footings, uneven digging, and spillage when you turn the result into an order.

9. Can I use meters for everything?

Yes. Select meters and enter all dimensions in meters; the calculator converts internally and still reports cubic yards for hauling quotes.

10. What if my pit is round?

Use the cylinder formula π × r² × depth for the bank volume, then apply the same 27-divisor and 1.25 swell factor by hand. This calculator covers rectangular and trapezoid shapes only.

11. Why is sloped excavation so much bigger?

Because every foot of depth adds a ring of extra soil around the full perimeter. On an 8-foot-deep dig the sloped version can easily exceed the vertical version by half.

12. How deep can I dig with vertical walls?

It depends on soil type and regulations — many safety rules require sloping or shoring beyond about 5 feet in unstable soil. Check local codes and get professional advice for deep digs.

13. What does “bank volume” mean?

The volume of soil in its undisturbed, in-the-ground state. It is the hole’s volume; the loose spoil after digging is larger because of swell.

14. How do I estimate fill to buy back?

Reverse the math: the bank volume is roughly what compacted fill will occupy. Order fill by the bank volume in cubic yards, plus a little extra for compaction losses.

15. Is this estimate enough for a contractor quote?

It is a solid starting point, but real quotes add mobilization, disposal fees, soil conditions, and access constraints. Use the calculator’s numbers to check a quote’s volume assumptions, not to replace the quote.