| Method | Base length | Slope | Turns | Offsets | Effective trench length | Pipe length (with waste) |
|---|---|---|---|---|---|---|
| Straight trench | 18.00 m | 1.00% | 2 | 0.50 m + 0.50 m | 19.60 m | 20.97 m |
| Perimeter rectangle | 2 × (12 + 8) = 40.00 m | 0.75% | 4 | 0.25 m + 0.25 m | 41.70 m | 44.62 m |
| Custom segments | 6 + 4.5 + 3.5 = 14.00 m | 1.50% | 1 | 0.40 m + 0.40 m | 15.10 m | 16.16 m |
- Base length depends on the selected method.
- Effective trench length: Leff = Lbase + Lstart + Lend + (Nturn × Aturn)
- Pipe length with waste: Lpipe = Leff × (1 + W/100)
- Elevation drop from slope: Drop = Leff × (S/100)
- Trench excavation volume: Vtrench = Leff × Width × Depth
- Gravel fill volume (planning factor): Vgravel = Vtrench × VoidFactor
- Fabric area (wrap and overlap): Area = Leff × (Width + 2×Overlap) (plus waste allowance)
- Cleanout count (if spacing > 0): N = floor(Leff / Spacing) + 1
- Select a layout method that matches your drain path.
- Choose meters or feet before entering any lengths.
- Enter the base path dimensions, then add offsets.
- Count turns and set a realistic allowance per turn.
- Set slope and cleanout spacing for maintainability.
- Optionally enter trench width and depth for volumes.
- Press Calculate to see results above the form.
- Download CSV or PDF for estimating and sharing.
Site assessment and water sources
Start by mapping where water collects after irrigation or rainfall. Note downspouts, hardscape edges, and low garden pockets. Measure the planned intake point, then mark the safest discharge point that will not erode beds. Check soil texture, because clay needs more gravel volume and longer trench runs to move water consistently.
Choosing a practical drain layout
Use the straight method when you have a direct path to daylight or a sump basin. Select the rectangle perimeter method for raised beds, patios, or lawn borders where water enters from multiple sides. For irregular gardens, break the route into segments and sum them. Add offsets for catch basins and outlet transitions.
Slope, elevation drop, and outlet control
The calculator converts slope percent into expected elevation drop over the effective trench length. A modest slope helps flow without scouring the trench. Confirm the outlet is lower than the inlet by at least the computed drop, allowing extra margin for settling. If the outlet cannot be lowered, consider a sump and pump or a shorter run.
Quantity planning for pipe, gravel, and fabric
Effective trench length drives pipe ordering, while the waste factor covers overlaps, cuts, and fittings. Excavation volume uses trench width and depth, supporting equipment planning and spoil management. Gravel volume applies a void factor to approximate compacted fill. Fabric area estimates wrap coverage with side overlap so fines stay out of the gravel.
Verification, cleanouts, and maintenance strategy
Cleanouts improve long term performance and simplify jetting. The cleanout count uses spacing along the run, with at least one point for access. Before digging, call utility marking services and verify local drainage rules. After installation, flush the line, watch for standing water, and correct any flat spots to keep flow reliable. Record the final measured length in your maintenance log. Recheck outlets after heavy storms and seasonal mulch changes. If early sediment appears, open cleanouts, rinse the line, and replace fabric at inlets to prevent clogging later.
1) What length should I enter for a curved trench?
Measure along the centerline with a tape, wheel, or string, then enter that distance as the base length. Add turn allowances if the route includes distinct bends and fittings.
2) How do I choose a slope percent?
For most gardens, 0.5% to 2.0% works well. Use the smallest slope that still provides a reliable drop to the outlet, and confirm elevations with a level or laser.
3) Why does the calculator add offsets and turn allowances?
Inlet boxes, outlets, and sweeping bends add real length that is easy to overlook. Including these allowances reduces under-ordering and improves planning for fittings and transitions.
4) What does waste or overlap percent represent?
Waste accounts for cutting losses, fabric overlaps, and adjustments during installation. A typical planning range is 5% to 10%, higher when the site has many turns.
5) Are the trench and gravel volumes exact?
They are planning estimates based on your entered trench dimensions. Actual quantities vary with side sloughing, compaction, and rock grading. Use the results to budget, then refine after test digging.
6) How many cleanouts do I need?
If you use spacing, the tool estimates cleanouts along the effective length. Add one at key direction changes or near the inlet. More access points make maintenance easier over the years.