How excavation volume is calculated
An excavation calculator works out how much soil you'll remove from a hole with straight, vertical sides: length × width × depth. Multiply by the number of identical holes for a set of footing pits. The result, in cubic meters and cubic feet, is the in-ground (bank) volume used to price digging, plan spoil removal and size backfill.
The formula
- Volume per cut = Length × Width × Depth
- Total volume = Volume per cut × Quantity
The same rectangular-prism formula covers all three excavation types in the selector:
| Type | Length | Width | Quantity |
|---|---|---|---|
| Open pit | Pit length | Pit width | Usually 1 |
| Trench | Total trench run | Trench width | Usually 1, or one per separate run of equal size |
| Footing | Pad length | Pad width | Number of identical pads |
Each dimension can be in mm, cm, m, ft or in. Results are rounded to three decimal places.
Worked examples
Strip footing trench: a trench 10 m long, 0.6 m wide and 1.2 m deep.
- Volume per cut: 10 × 0.6 × 1.2 = 7.200 m³.
- Quantity is 1, so the total is 7.200 m³, or 254.266 ft³.
Column footings: six pads, each 1.5 m × 1.5 m and 1.2 m deep.
- Volume per pad: 1.5 × 1.5 × 1.2 = 2.700 m³.
- Total: 2.7 × 6 = 16.200 m³, or 572.098 ft³.
Trenches with sloped sides
Deeper excavations often can't have vertical walls, because the sides have to be cut back to a safe angle. The calculator doesn't model slopes, but you can still use it. For a trench sloped equally on both sides, the cross-section is a trapezoid, and its area equals depth × average width:
Average width = Bottom width + (Slope ratio × Depth)
where the slope ratio is horizontal distance per unit of depth. In the US, OSHA's Appendix B to Subpart P sets maximum allowable slopes for excavations less than 20 ft deep:
| Soil or rock type | Maximum slope (H:V) | Angle from horizontal |
|---|---|---|
| Stable rock | Vertical | 90° |
| Type A | ¾ : 1 | 53° |
| Type B | 1 : 1 | 45° |
| Type C | 1½ : 1 | 34° |
Example: the 10 m trench above, 0.6 m wide at the bottom and 1.2 m deep, cut back at 1:1 on both sides. The top width is 0.6 + 2 × 1.2 = 3.0 m, and the average width is 0.6 + 1 × 1.2 = 1.8 m. Entering 10 × 1.8 × 1.2 gives 21.600 m³, three times the vertical-sided volume. That difference is why the slope matters when you price a job.
Steps to estimate an excavation
- Pick the excavation type so you remember what each dimension means.
- Measure the bottom dimensions of the cut, including any working space you need around a footing or pipe.
- Measure depth from the existing ground level, not from a finished level. On sloping ground, average several depth readings.
- If the sides will be sloped, replace the width with the average width as shown above.
- Enter the quantity of identical cuts and read off the volume.
What the result doesn't include
- Bulking. Dug soil loosens and occupies more volume than it did in the ground, and compacted fill occupies less. The figure here is the undisturbed volume.
- Benching, stepped sides or shoring. Trench boxes and shoring change the width you dig, so measure to the outside of the support system.
- Irregular shapes. Split them into rectangles and add the volumes.
Safety
This is a quantity estimate only. OSHA's trenching fact sheet says trenches 5 feet (1.5 m) or deeper need a protective system unless they're entirely in stable rock, and those 20 feet (6.1 m) or deeper need one designed or approved by a registered professional engineer. Follow your local regulations and have a competent person or engineer decide the support method for your site.
Related calculators
- Concrete calculator: concrete volume for the footings once they're dug.
- Brick foundation calculator: bricks and mortar for a masonry foundation in the trench.
- Floor concrete calculator: slab volume over the finished foundation.
- Volume converter: convert m³ and ft³ to cubic yards and more.
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