A swale ditch quantity starts with the open channel as measured in the field. Enter the top width, the bottom width, the vertical depth, and the run length, keep the bank volume visible, and add a loose planning volume only when you have a project-justified swell percentage.
How much soil does a swale ditch hold?
Enter the four field measurements and the number of identical runs. The calculator first works out the trapezoid section area from the two measured widths, then the bank volume, then a separate loose volume from the swell percentage you enter.
Record the bank volume first, the loose planning volume second, and the swell percentage used third. Each record answers a different project question, and the third record is what makes the first two checkable later.
| Result | Calculation | What you must confirm |
|---|---|---|
| Cross-section area | (Top width + bottom width) / 2 x vertical depth | Both widths measured, depth vertical |
| Bank volume, one run | Section area x run length | Uniform section along the run |
| Total bank volume | Bank volume per run x identical runs | Run count |
| Loose planning volume | Total bank volume x (1 + swell % / 100) | Reason and evidence for the swell percentage |
When should you use this swale ditch calculator?
Use it for one uniform open ditch run where both widths can be measured directly. Run the calculator separately when the shape, width, depth, product state, or material changes.
| Project condition | Calculator method | Record to keep |
|---|---|---|
| Uniform trapezoid ditch | Enter top width, bottom width, depth, and length | Field dimensions and depth source |
| V-shape ditch with no flat floor | Enter 0 as the bottom width | Note that the section is a V-shape, not a flat floor |
| Rectangular ditch | Enter the same value for top and bottom width | Note that the sides are vertical in this limit case |
| Several identical runs | Enter the run count | What makes each run identical |
| Width or depth changes along the run | Calculate each reasonably uniform section separately and add unrounded volumes | Section limits and measurements |
| Rounded or parabolic bottom | Do not use this calculator for the rounded section | Surveyed method used instead |
| Buried trench with a slope ratio design | Use the sloped-trench calculator for that task | Slope ratio and soil classification record |
Measure the ditch first with the swale ditch measurement guide. Choose the method that matches the measured shape with the swale ditch calculation method guide.
Which measurements do you need?
You need four field numbers for one uniform run, plus a run count and an optional swell percentage. Measure in the units printed beside the fields and record where each dimension came from.
Top width
Measure across the surface opening from edge to edge, at the level the depth is measured from. A tape stretched across a grassed edge or a slumped bank gives a top width that does not match the excavated section, so use the accepted surface edge.
Bottom width
Measure across the ditch floor. Enter 0 only when the section truly comes to a point at the floor. A blank bottom width is rejected by the calculator, because a missing floor measurement is not the same as a V-shape.
Vertical depth
Measure straight down from the surface line to the floor. A depth read along the sloped side is longer than the vertical depth and overstates the section area, so it must not be entered here.
Run length and identical runs
Measure the length of the uniform section only. The run count multiplies a truly identical section. A longer run that changes width or depth is a new section, not another identical run.
Swell percentage
Enter a swell percentage only when a project record supports it. The field starts at 0%, so the first result is a checkable bank volume rather than a result changed by a general extra percentage. Soil, moisture, handling, and the project specification can all change the correct swell value, so the calculator publishes no universal figure.
What formulas does the calculator use?
The calculator finds the trapezoid section area, multiplies it by length and run count, and applies swell only to the loose result. It keeps full JavaScript precision during the calculation and displays up to 3 decimal places.
Section area = (top width + bottom width) / 2 x vertical depth
Bank volume, one run = section area x run length
Total bank volume = bank volume per run x identical runs
Loose planning volume = total bank volume x (1 + swell % / 100)
| Displayed conversion | Relationship | Calculator use |
|---|---|---|
| Cubic feet to cubic yards | 27 ft³ = 1 yd³ | Imperial bank and loose volume |
| Cubic metres to litres | 1 m³ = 1,000 L | Metric loose volume equivalent |
| Metres to feet | 1 m = 3.280839895 ft | Unit switching for lengths |
| Cubic yards to cubic metres | 1 yd³ = 0.764554857984 m³ | Checking a supplier unit conversion |
NIST Handbook 44 Appendix C supplies the cited volume relationships. Unit switching converts populated lengths, leaves runs and swell unchanged, clears the displayed result, and requires a fresh calculation in the selected units.
Worked metric example
A uniform swale ditch measures 2.4 m across the top, 0.8 m across the floor, 0.6 m deep vertically, and 75 m long. The measurements describe one uniform run with no swell entered.
- Section area: (2.4 + 0.8) / 2 x 0.6 = 0.96 m².
- Bank volume: 0.96 x 75 = 72.0 m³.
- Loose volume at 0% swell: 72.0 m³, or 72,000 L.
Measured result: 72.0 m³ bank volume. A second arithmetic path checks the area: a rectangle 0.8 x 0.6 gives 0.48 m², plus two side triangles of 2 x (0.8 x 0.6 / 2) gives 0.48 m², for the same 0.96 m² total.
Worked imperial V-shape example
A V-shape ditch measures 5 ft across the top, 0 ft across the floor, 1.75 ft deep vertically, and 180 ft long.
- Section area: 5 / 2 x 1.75 = 4.375 ft².
- Bank volume: 4.375 x 180 = 787.5 ft³.
- Bank volume in cubic yards: 787.5 / 27 = 29.1667 yd³.
Measured result: 29.167 yd³ and 787.5 ft³. A user-entered 20% swell on a separate run would give a loose volume of 787.5 x 1.20 = 945 ft³, or 35 yd³, kept as a separate planning figure.
How do you handle a changing section?
Split the run at every point where the top width, bottom width, or depth changes by an amount that matters to the quantity. Calculate each segment at its own measurements and add the unrounded bank volumes.
Example: a 40 m segment at 2.0 m top, 0.5 m bottom, and 0.5 m deep gives a section area of 0.625 m² and 25.0 m³. A following 35 m segment at 2.4 m top, 0.5 m bottom, and 0.6 m deep gives 0.87 m² and 30.45 m³. The combined bank volume is 55.45 m³, shown as 55.5 m³. One average section across both segments would hide the change and give a different total.
A rounded bottom does not fit this trapezoid model. The straight-side area can differ from the true rounded area by an amount that depends on the curve, so measure a rounded section by a surveyed polygon or an approved section method instead of forcing it into this calculator.
How should bank, loose, and haulage figures be kept apart?
Bank volume is the volume in the ground. Loose volume is the larger volume the same soil occupies after excavation. Haulage needs a usable vehicle volume and a legal payload on top of the loose volume, and cost needs a current quote for the exact work and location.
Keep three records separate: the measured bank volume, the loose planning volume with its swell reason, and the haulage or order record with its own vehicle and supplier figures. The bank and loose volume guide explains the state change, and the swale ditch haulage guide covers the ordering step.
Common swale ditch calculation mistakes
- Entering a depth measured along the sloped side instead of the vertical depth.
- Leaving the bottom width blank and letting a tool read it as a V-shape.
- Entering a bottom width wider than the top width.
- Using one average width or depth across an abrupt change.
- Applying swell before preserving the bank volume.
- Using a generic swell percentage from an unrelated soil or project.
- Rounding every section before adding the total.
- Treating a loose volume as a truck count or a legal payload.
- Using this volume result as a hydraulic design for the swale.
- Forcing a rounded bottom into the trapezoid formula.
The swale ditch mistakes guide works through these errors with its own examples.
Questions about swale ditch volume
Can this calculator size a swale for drainage?
It calculates excavation volume only. Channel size, flow, slope along the run, outlet, and erosion control are design decisions that need the project drawings and the responsible designer.
Does a V-shape ditch use the same formula?
Yes, with a bottom width of 0. The trapezoid formula then gives top width / 2 x depth, which is the triangle area.
What if the two sides have different slopes?
The measured top and bottom widths already capture the combined effect of both sides, so the trapezoid area still applies to that section. Record each side slope separately in the project record when the design needs it, because this calculator does not store side slopes.
Does the calculator count truck loads?
It does not. Truck loads need a usable body volume and a legal payload, which vary by vehicle, material mass, and route. Use the loose volume as the input to that separate check.
Does the swell field calculate compaction?
It applies one user-entered percentage to the bank volume to give a loose planning volume. A compacted placed volume needs a separate documented material-state calculation with its own project basis.
Can the bank volume set an excavation price?
It gives a quantity. Price needs a current quote for the exact work, access, disposal or reuse, and site conditions.
Sources and scope
- NIST Handbook 44 (2026), Appendix C: cubic-foot, cubic-yard, litre, and cubic-metre relationships; accessed 5 November 2026.
- NIST Guide to the SI, Appendix B: metric and US customary conversion relationships; accessed 5 November 2026.
- OSHA, Trenching and Excavation: US excavation safety context; accessed 5 November 2026.
- OSHA, 29 CFR 1926.651, Specific Excavation Requirements: excavation safety requirements context; accessed 5 November 2026.
- How to Measure a Swale Ditch Before Excavation: field measurement method for top width, bottom width, vertical depth, and length; published 18 October 2026.
- Choosing the Right Calculation Method for a Swale Ditch: method selection and the same trapezoid fixtures; published 18 October 2026.
Source scope: NIST supports only the stated conversions. OSHA supports only the stated safety context. Geometry supports the trapezoid, triangle, rectangle, and prism formulas. The measurement and method guides support the field steps and fixture checks. Project drawings, specifications, soil records, supplier and hauler records, and responsible people control the swell value, protective system, haulage, price, and work plan.
Review note: Calculation logic and independent automated fixtures passed internal review. Saleem Sial is the research and editorial owner. Waseem Sial's external review remains ongoing. The published page is indexable.
