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Excavation Calculator

Enter the shape, size, depth and side slope of the dig. Get the bank volume, the loose volume after swell, the number of truckloads to haul and how much compacted fill the spoil would make.

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How to use the excavation calculator

Digging out a basement, a pool, a foundation trench or a soakaway always produces more spoil than the hole seems to hold. The soil swells as it comes out of the ground, sloped sides add volume at the top of the dig, and the haulage contractor charges by the load. This calculator takes the dimensions of the dig and returns the volume in the ground, the volume in the truck and the number of loads.

  1. Choose units and a shape. Use a rectangular pit for basements and pools, a trench for footings and service runs (the ends are taken as vertical), or a round pit for soakaways and pier holes.
  2. Enter the bottom dimensions and the depth. Include working space around foundations in the bottom size.
  3. Pick the side slope. Choose vertical if the sides will be shored or protected by a trench box; otherwise choose the batter the soil and your local safety rules allow.
  4. Choose the soil type, which loads a typical swell and shrinkage, or enter your own figures from a site investigation.
  5. Enter the truck capacity to get the number of loads.

Excavation volume formulas

Top size = bottom size + 2 × slope × depth
Pit volume (prismoidal) = depth ÷ 6 × (Abottom + 4 × Amiddle + Atop)
Trench volume = length × depth × (bottom width + slope × depth)
Round pit volume = π × depth ÷ 3 × (rbottom² + rbottom × rtop + rtop²)
Loose volume = bank volume × (1 + swell) · Loads = ⌈loose volume ÷ truck capacity⌉
Compacted fill = bank volume × (1 − shrinkage)

The prismoidal formula is exact for a pit whose four sides slope at the same angle, which is why it is used instead of simply averaging the top and bottom areas. That shortcut slightly overstates the volume.

SoilSwell, bank to loose (estimate)Bank to compacted (estimate)
Sand and gravel10–15%About 5–10% smaller
Common earth / loam20–30%About 10–15% smaller
Clay30–40%About 10–20% smaller
Blasted rock50–65%Often 20–30% larger

These are typical planning ranges from earthmoving references. Real factors depend on moisture, grading and compaction effort; a geotechnical report gives better ones.

Worked examples

A basement dig with 1:1 sides

The bottom is 40 × 30 ft and the dig is 8 ft deep. With 1:1 slopes the top is 56 × 46 ft and the mid-depth section is 48 × 38 ft. Volume = 8 ÷ 6 × (1,200 + 4 × 1,824 + 2,576) = 14,763 ft³, or 547 yd³ in the bank. Common earth at 25% swell makes 683 yd³ loose, which is 69 loads in a 10 yd³ truck. The same spoil would make about 481 yd³ of compacted fill at 12% shrinkage.

A metric foundation trench

A strip footing trench 50 m long, 0.6 m wide and 1.2 m deep with vertical, shored sides holds 50 × 0.6 × 1.2 = 36 m³. In clay at 35% swell that becomes 48.6 m³ loose, or 7 loads in an 8 m³ tipper.

A round soakaway pit

A soakaway 1.5 m across and 1.5 m deep with vertical sides holds π × 0.75² × 1.5 = 2.65 m³. In sandy soil at 12% swell that is 2.97 m³ of loose spoil, which is a single small-truck load or a few skip loads.

Who this calculator helps

  • Architects and engineers putting early earthworks quantities on basements, extensions and site works.
  • Groundworks and excavation contractors pricing digs, haulage and disposal.
  • Quantity surveyors and estimators checking bank, loose and compacted volumes in a bill of quantities.
  • Pool, landscape and drainage installers sizing spoil removal for pools, ponds and soakaways.
  • Homeowners getting a feel for how many skips or loads a project will produce.

Practical tips

  • Price disposal on the loose volume and the weight, not the bank volume; tipping fees are often charged per tonne.
  • Work out the concrete for the footings once the dig is sized with the concrete calculator, and the bedding or drainage stone with the gravel calculator.
  • Check site falls and batter angles with the slope calculator.
  • Keep spoil heaps back from the edge of the excavation; OSHA requires at least 2 ft (0.6 m) in the US.
  • Call before you dig to locate buried services, and plan dewatering if the dig goes below the water table.

Want to show a client the finished garden or extension before the digger arrives? Visualise it on a photo of the site with Landscape.

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FAQ

Excavation Calculator FAQ

How do I calculate excavation volume?

For a pit with vertical sides, multiply length × width × depth. For a pit with sloped (battered) sides, the top is larger than the bottom, so use the prismoidal formula: depth ÷ 6 × (bottom area + 4 × mid-depth area + top area). A 40 × 30 ft basement dug 8 ft deep with 1:1 side slopes is about 14,760 ft³, or 547 yd³ of bank (in-place) soil.

What is the swell factor of soil?

Soil takes up more room once it is dug, because the particles loosen and trap air. Swell is the percentage increase from bank volume to loose volume. Typical textbook figures are roughly 10–15% for sand and gravel, 20–30% for common earth, 30–40% for clay and 50% or more for blasted rock. They are estimates: moisture and how the soil is handled change them.

How many truckloads will my excavation be?

Multiply the bank volume by (1 + swell) to get the loose volume, then divide by the truck's heaped capacity and round up. 547 yd³ of common earth at 25% swell becomes about 683 yd³ loose, or 69 loads in a 10 yd³ truck. Check the truck's weight limit as well, because wet clay and rock can hit the legal weight before the body is full.

What is the difference between bank, loose and compacted volume?

Bank volume is the soil in place before digging. Loose volume is the same soil in a truck or a spoil heap, which is bigger because of swell. Compacted volume is the soil after it has been placed and rolled as fill, which is usually smaller than bank volume for earth and clay (shrinkage), but can stay larger for rock.

What side slope should an excavation have?

In the US, OSHA 29 CFR 1926 Subpart P, Appendix B sets maximum allowable slopes for excavations under 20 ft deep: 3/4:1 (horizontal to vertical) in Type A soil, 1:1 in Type B and 1½:1 in Type C. Excavations 5 ft or deeper need a protective system such as sloping, benching, shoring or a trench box unless they are entirely in stable rock, and a competent person must classify the soil. Other countries have their own rules.

How much extra should I dig around a foundation?

Foundations and basement walls usually need working space around them for formwork, waterproofing and drainage, often around 1–2 ft (300–600 mm) beyond the outside face. Add that to the bottom length and width before calculating. The exact amount depends on the wall system and the contractor's method.

How do I work out cut and fill?

Compare the volume you need to dig (cut, in bank measure) with the volume you need to fill (compacted measure). Because earth usually shrinks when compacted, 100 m³ of bank cut typically makes only about 85–90 m³ of compacted fill. The calculator shows the estimated compacted volume from your cut using the shrinkage you enter, so you can see whether you will have spoil to haul away or need to import fill.

Does this replace a site survey or engineer's design?

No. It is for estimates and quantities on simple shapes. Uneven ground, rock, groundwater and changing levels across the site all affect the real volume, and deep or complex excavations need a survey-based earthworks model and a qualified engineer's design for support and safety.

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