A swale ditch volume comes from four field numbers: top width, bottom width, vertical depth, and run length. The examples below turn those numbers into a checked excavation volume in metres and in feet, then show how to handle a section that changes along the run. Check the same numbers in the Swale Ditch Volume Calculator before ordering or digging.
How do you work out the excavation volume of a swale ditch?
Average the top and bottom widths, multiply by the vertical depth to get the cross-section area, and multiply that area by the run length. For a V-shaped swale with no flat bottom, the area is half the top width times the depth.
Section area = (top width + bottom width) / 2 × depth
V-shaped section area = top width / 2 × depth
Swale volume = section area × run length
Metric example: trapezoidal swale
A measured swale has a top width of 2.4 m, a bottom width of 0.8 m, a vertical depth of 0.6 m, and a run length of 75 m. These figures are bank (in-place) measurements from the swale measurement guide.
- Average the widths: (2.4 + 0.8) / 2 = 1.6 m.
- Multiply by the depth: 1.6 × 0.6 = 0.96 m² section area.
- Multiply by the run length: 0.96 × 75 = 72.0 m³ bank volume.
Result: 72.0 m³ bank. Check the arithmetic a second way: split the section into a 0.8 m × 0.6 m rectangle (0.48 m²) and two side triangles, each 0.8 m wide and 0.6 m deep (0.24 m² each). The sum is 0.48 + 0.24 + 0.24 = 0.96 m², which matches the average-width method.
Imperial example: V-shaped swale
A V-shaped swale has a top width of 5 ft, no flat bottom, a vertical depth of 1.75 ft, and a run length of 180 ft.
- Halve the top width: 5 / 2 = 2.5 ft.
- Multiply by the depth: 2.5 × 1.75 = 4.375 ft² section area.
- Multiply by the run length: 4.375 × 180 = 787.5 ft³ bank volume.
- Convert to cubic yards: 787.5 / 27 = 29.1667, reported as 29.2 yd³ bank.
Result: 787.5 ft³ bank, which is 29.2 yd³ bank. The section area checks independently as a triangle: 5 × 1.75 / 2 = 4.375 ft². The yard conversion uses 1 yd³ = 27 ft³ (NIST Handbook 44, 2026, Appendix C).
| Conversion | Factor | Direction |
|---|---|---|
| Feet to cubic yards | 1 yd³ = 27 ft³ | Divide ft³ by 27 |
| Cubic metres to cubic feet | 1 m³ = 35.3147 ft³ | Multiply m³ by 35.3147 |
| Cubic metres to cubic yards | 1 m³ = 1.30795 yd³ | Multiply m³ by 1.30795 |
What if the section changes along the run?
Split the run into segments, calculate each segment's volume with its own section, and add them. Do not average the sections first, because the segment lengths differ.
Segment A: top width 2.0 m, bottom width 0.5 m, depth 0.5 m, length 40 m. Section area = 1.25 × 0.5 = 0.625 m². Volume = 0.625 × 40 = 25.0 m³ bank.
Segment B: top width 2.4 m, bottom width 0.5 m, depth 0.6 m, length 35 m. Section area = 1.45 × 0.6 = 0.87 m². Volume = 0.87 × 35 = 30.45 m³ bank.
Result: 25.0 + 30.45 = 55.45 m³ bank, reported as 55.5 m³ bank.
How do you check the answer before it reaches the order?
- Confirm the depth was read vertically, not along the side slope. A slope reading inflates the area.
- Confirm the bottom width is 0 only for a true V-shape; a flat floor needs its own measurement.
- Re-run the area by a second path, such as the rectangle-plus-triangles split used above.
- Label the state beside every number: 72.0 m³ bank is not the loose spoil volume.
- Round to one decimal place for ordering and keep the unrounded intermediate beside it.
Bank volume becomes loose spoil once it is dug. As a labelled fixture only, 72.0 m³ bank × 1.18 (a stated 18% swell) = 84.96, reported as 85.0 m³ loose. The 18% is a calculation fixture, not a recommended factor; use a project-supported factor and keep bank and loose records separate as the bank vs loose volume guide shows. The swale ordering guide turns the loose figure into haul and disposal plans, and the common swale ditch mistakes page collects the errors that most often reach the order.
Sources and scope
- NIST Handbook 44, 2026 Appendix C: cubic-foot, cubic-yard, litre, and cubic-metre relationships used for the unit conversions.
- U.S. Office of Surface Mining Reclamation and Enforcement, Directive TSR-1 Appendix A: bank, loose, and compacted volume definitions used for the state labels.
- OSHA Trenching and Excavation and 29 CFR 1926.651: U.S. occupational excavation-safety context.
Scope: the fixtures demonstrate the calculation method with stated inputs. Field measurements, the applicable safety plan, and the project quantity-control process govern real quantities. The swell percentage above is an illustration, not a project factor.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer and Engineer, remains listed for ongoing external review; no completed external-review date is claimed.
Next, use the swale measurement guide to take the field dimensions, or return to the Earthwork & Drainage Calculators hub.
