Calculator
Tip: Use the same unit family for width/depth, and pick a gravel pricing basis that matches your supplier.
Example Data Table
| Input | Example Value | Why it matters |
|---|---|---|
| Trench length | 30 ft | Drives total volume and pipe run length. |
| Trench width / depth | 12 in / 18 in | Determines excavation and gravel backfill quantity. |
| Pipe diameter | 4 in perforated | Affects displacement, fittings, and flow capacity planning. |
| Gravel price basis | $55 per US ton | Suppliers often sell by weight; choose what matches your quote. |
| Waste factor | 8% | Covers compaction, spillage, and uneven trench dimensions. |
| Extras | 4 fittings + fabric wrap | Common add-ons that can change total cost noticeably. |
Formula Used
- Trench volume = Length × Width × Depth
- Gravel volume = Length × Width × Gravel Fill Depth
- Optional pipe displacement = π × (Pipe Radius²) × Pipe Length
- Waste adjusted gravel = Gravel Volume × (1 + Waste%/100)
- Gravel weight = Waste Volume × Density
- Pipe length with extra = Trench Length × Pipe Count × (1 + Extra%/100)
- Total = Materials + Labor + Delivery + Tax
The fabric wrap area uses an estimating shortcut: (2×(Width+Depth))×Length, plus overlap.
How to Use This Calculator
- Enter trench length, width, and depth using your preferred units.
- Set gravel fill depth (often equals trench depth for full backfill).
- Choose whether to subtract pipe displacement for tighter estimates.
- Add pipe diameter, pipe count, extra percentage, and unit cost.
- Select gravel pricing by weight or by volume, then enter the price.
- Include fittings, fabric wrap, and labor if you want a full project view.
- Add delivery and tax, then press Calculate.
- Use Download CSV or Download PDF for records and quotes.
Drainage Trenches: Performance and Cost Planning
A trench drain or French drain works when water can enter the gravel, move along the pipe, and discharge without clogging. This calculator converts measurements into gravel volume, pipe length, and a clear cost breakdown. It also helps you test “what‑if” choices before buying materials.
1) Site measurements that affect quantities
Length drives nearly every material line item, while width and depth set excavation and backfill volume. For many gardens, widths of 10–18 inches and depths of 12–24 inches are common, but sizing should match runoff, soil infiltration, and outlet elevation.
2) Gravel selection and drainage capacity
Clean, angular gravel (often 3/4 inch) resists compaction and preserves void space. A practical rule is that clean gravel can have about 30–40% voids, so much of the trench volume stays open for water movement.
3) Pipe diameter, slope, and fittings
Perforated 4‑inch pipe suits most yard drainage, while 6‑inch is used for higher inflows or long runs. Layout changes add fittings, so include elbows, couplers, tees, adapters, and end caps. Extra pipe percentage covers connections and trimming.
4) Why fabric wrap matters
Geotextile fabric slows fine soil migration into the gravel. In silty soils, fabric can extend service life by reducing clogging. The calculator estimates fabric using trench perimeter times length, then adds overlap for seams and wrap allowance.
5) Waste, settlement, and “real” volumes
Gravel settles after placement, and trench walls are rarely perfectly straight. A waste factor of 5–12% often covers spillage, rounding, and field adjustments. Subtracting pipe displacement can slightly reduce gravel on tight trenches.
6) Pricing by weight versus volume
Suppliers may quote by ton, cubic yard, or cubic meter. Because densities vary by stone type and moisture, conversions can drift. Enter a realistic density and match the pricing basis to your quote for consistent totals.
7) Cost drivers beyond materials
Delivery fees and excavation labor can exceed materials on small jobs. Use the labor fields to model per‑length rates or a flat project cost, then add tax if applicable to estimate your all‑in budget. Including disposal or haul‑off as delivery can keep estimates realistic.
8) Build checklist for fewer surprises
Confirm outlet location, mark utilities, and plan delivery access. Maintain continuous slope to the discharge point, keep pipe protected by gravel cover, and compact in lifts, and recheck slope after final backfilling. Save your quantities as CSV or PDF for records.
FAQs
How much gravel should cover the pipe?
Many installers aim for at least 2–4 inches of gravel below the pipe and 4–6 inches above it, depending on trench depth. Use local guidance and ensure the pipe stays fully surrounded by clean stone.
Should I price gravel by weight or volume?
Use the same basis your supplier quotes. Weight pricing needs a realistic density; volume pricing needs consistent yard or meter measures. This calculator lets you compare both so your estimate matches invoices.
What waste factor is reasonable?
Start with 8% for typical hand placement. Increase to 10–12% for uneven trenches, long carries, or tight access. Reduce slightly if you have precise excavation and mechanical placement.
Does subtracting pipe displacement matter?
It’s a small change for wide trenches, but it can help in narrow, shallow trenches where the pipe occupies noticeable space. Enable it when you want a tighter gravel estimate.
When is fabric wrap recommended?
Use fabric in silty or fine soils to reduce clogging and extend service life. In very clean sandy soils it may be optional, but fabric can still improve long‑term reliability for many gardens.
How do I estimate fittings accurately?
Count every change in direction and connection: elbows for turns, couplers for joining lengths, tees for branches, plus end caps or cleanouts. Add a small buffer if your layout is still flexible.
Can I include labor and delivery in the total?
Yes. Add a flat delivery fee and enable labor to price per length or as a flat cost. The results section combines materials, labor, delivery, and tax for an all‑in project number.