How to Use This Excavation Calculator
Choose your unit system, then pick the shape that best matches the dig: Rectangular for basements and foundations, Trapezoidal for trenches and ponds with sloped walls, Circular Pit for dry wells and tanks, or Prismoidal when you know the cross-section areas at the top, middle and bottom. Enter the dimensions, soil type, truck capacity, haul cost, the share of soil you will keep on site, and a swell factor. Press Calculate to see bank volume, loose volume, truck loads, soil weight and haul cost. The diagram fills in your numbers. Labels and the truck-capacity field switch units when you change the unit system.
Why This Matters
Earthwork is paid for by the cubic yard, so a small volume error turns into real money fast. Take a 30 × 40 ft footprint dug 5 ft deep. That is 6,000 ft³, or 222.2 yd³ of bank (in-ground) soil. At 20% swell you haul 266.7 yd³, which is 27 loads in a 10 yd³ truck. At 30% swell it is 288.9 yd³, or 29 loads. Two extra loads at $300 each is $600 you did not budget, before dump fees.
Homeowners digging for a pool, footings or a drainage trench use the same math as contractors bidding a basement or utility run. Estimators use it to size equipment and crews. Landscapers use it to decide whether to haul soil away or regrade it on site. Weight matters too: a tandem dump truck loaded with wet clay can hit its legal weight limit before it is volume-full, so the tool reports the weight of one full load. Depth is also a safety issue. At 5 ft, OSHA rules call for sloping, shoring or a trench shield, and that changes how much you have to dig.
How It's Calculated
All shape volumes are computed in cubic feet or cubic metres first. Imperial results divide by 27 to give cubic yards.
The prismoidal formula is Simpson's rule applied along the depth. It is exact when the cross-section area changes as a quadratic or simpler function, as in pyramids, cones, frustums and wedges. Am is the area measured at mid-height, not the average of the end areas. Soil density does not change with swell, so weight is based on bank volume, while a full truck carries the bank density divided by (1 + swell). Densities are typical values and real soil can differ by 15% or more with moisture.
Truck loads and weights are always rounded up so you do not under-order. Safety messages use the OSHA 29 CFR 1926 Subpart P rules of thumb: a ladder or ramp within 25 ft of workers at 4 ft deep, a protective system at 5 ft, and an engineer-designed system beyond 20 ft. The maximum allowable wall slopes are 1½H:1V (Type C soil), 1H:1V (Type B) and ¾H:1V (Type A). Local rules vary.
Worked Example
A homeowner digs a 24 ft × 16 ft × 6 ft pit for a basement addition in loam. Settings: Rectangular tab, Imperial, Loam / topsoil, swell 25%, 10 yd³ truck, $300 per load, 20% of the soil kept on site.
| Step | Calculation | Tool shows |
|---|---|---|
| Bank volume | 24 × 16 × 6 = 2,304 ft³ ÷ 27 | 85.33 yd³ |
| Loose volume | 85.33 × 1.25 | 106.67 yd³ |
| Kept on site | 85.33 × 0.20 | 17.07 yd³ |
| Volume to haul | 106.67 × 0.80 | 85.33 yd³ |
| Truck loads | 85.33 ÷ 10 = 8.53, rounded up | 9 loads |
| Haul cost | 9 × $300 | $2,700 |
| Total soil weight | 2,304 ft³ × 112.37 lb/ft³ ÷ 2,000 | 129.46 US tons |
| Weight to haul | 80% of total | 103.57 US tons |
| One full load | 10 yd³ × 27 × 112.37 ÷ 1.25 ÷ 2,000 | 12.14 US tons |
Weights are rounded up to the next hundredth, so they can read 0.01 higher than ordinary rounding. At 6 ft deep the pit also triggers the tool's protective-system notice. If the soil swells 40% instead, the haul volume becomes 95.57 yd³, which is 10 loads and $3,000.
Tips & Common Mistakes
- Always include swell. Bank volume tells you how much hole you dig, but loose volume tells you how many trucks you need.
- Small depth errors add up. Digging 6 inches (0.5 ft) too deep over a 20 × 40 ft area adds 20 × 40 × 0.5 = 400 ft³, about 14.8 yd³. With swell that is roughly two extra truckloads.
- Add working room. Foundation and form crews usually need 1–2 ft of extra space around the footprint, so enter the dug size, not the finished size.
- Remember topsoil stripping and footing trenches. They are separate volumes. Run each one through the calculator and add the results.
- Check weight as well as volume. Wet clay and rock can overload a truck that is only partly full. Compare the full-load weight with your hauler's payload limit.
Frequently Asked Questions
What is swell factor and what number should I use?
Swell factor is the percentage soil expands when it is dug out and loosened. Sand and gravel swell about 10–15%, loam 20–30% and clay 30–40%. If you are unsure, pick the higher end so you do not run short of trucks.
Should I use bank volume or loose volume to count truck loads?
Use loose volume, because that is the space the soil takes up in the truck. Bank volume is the in-ground quantity you normally put on a bid. The calculator shows both and bases truck loads on loose volume.
What is the difference between rectangular and trapezoidal excavation?
A rectangular cut has vertical walls and works in stable rock or when shoring holds the sides. A trapezoidal cut is wider at the top than at the bottom because the walls are sloped back. Use the trapezoidal tab for open-cut trenches in soil.
Can I use metric units?
Yes. Switch the unit system to Metric and every label changes to metres and cubic metres, including the truck-capacity field. The default truck becomes 7.6 m³ (about 10 yd³). If you switch units, re-enter your dimensions.
How many cubic yards does a dump truck hold?
A tandem-axle dump truck typically carries 10–12 yd³, and a tri-axle about 14–16 yd³. Ratings are usually heaped capacity. Dense or wet soil often hits the weight limit before the truck is full, so confirm the payload with your hauler.
Does this tell me whether my trench is safe?
No. It flags depths and wall slopes that usually need sloping, shoring or a shield under OSHA Subpart P, but only a competent person on site can classify the soil and approve a protective system. Treat the notices as reminders, not approval.