Plan clean drainage behind walls with accurate gravel quantities, season after season. Enter dimensions, pick material, add waste, and export results for your project.
Base backfill volume (rectangular approximation):
V_base = L × H × T
Drain trench volume (optional):
V_trench = L × W_trench × D_trench
Net volume: V_net = V_base + V_trench
Order volume with allowances:
V_order = V_net × (1 + compaction% + waste%)
Weight estimate: Mass = V_order × Density
| Wall Length (m) | Backfill Height (m) | Thickness (m) | Trench (W×D m) | Allowances (C+W) | Order Volume (m³) | Approx. Weight (tonnes) |
|---|---|---|---|---|---|---|
| 6.0 | 1.0 | 0.40 | 0.30×0.30 | 8% + 5% | ~2.98 | ~4.77 |
| 12.0 | 1.2 | 0.45 | 0.35×0.30 | 10% + 5% | ~8.72 | ~13.95 |
| 20.0 | 1.5 | 0.50 | No trench | 8% + 7% | ~17.25 | ~27.60 |
Example weights assume ~1600 kg/m³ density and rounded values.
This tool estimates a rectangular backfill zone behind a wall and an optional drain trench. The base volume uses wall length × backfill height × thickness. The trench adds length × trench width × trench depth, so you can budget gravel for both the drainage column and the outlet line.
For many garden retaining walls, a free‑draining gravel zone of 0.30–0.60 m (about 1–2 ft) is common. Taller walls, heavy clay, or poor surface grading often benefit from a wider zone. Use measured site conditions, not guesswork, to set the thickness value.
A simple trench along the wall base helps move water to an outlet. Typical trench widths fall around 0.20–0.45 m, with depths near 0.20–0.40 m, depending on pipe size and cover. The trench option adds this volume directly to your order total.
Even clean, angular stone settles as voids rearrange during placement. A compaction allowance of 5–12% is a practical planning range for small landscape builds. If you place in lifts and compact carefully, choose the lower end; irregular trenches usually need more allowance.
Real excavations are rarely perfect rectangles. Curves, stepped grades, over‑digging, and spillage increase consumption. A waste factor of 3–10% covers these realities. Increase waste when access is tight or when you expect reshaping after the first placement.
Bulk gravel density varies with gradation and moisture. Common values range roughly 1450–1900 kg/m³. The calculator converts your order volume into tonnes to help compare supplier quotes and plan vehicle capacity. Always confirm local supplier bulk density if available.
Suppliers may quote per tonne, per cubic yard, or per bag. Entering a price per tonne provides a consistent baseline cost estimate. For comparison, 1 m³ is about 1.308 yd³, so you can reconcile yard‑based quotes against weight‑based delivery pricing.
Verify the retained height, drainage outlet location, and that water can discharge away from structures. Confirm that fines are controlled with a suitable separator fabric where needed. For higher walls or uncertain soils, consult local requirements and qualified professionals for stability and drainage design.
1) Should the gravel backfill extend the full wall height?
In many garden walls, a full-height drainage column is used to reduce pressure. If you transition to soil, keep gravel at least where water collects and ensure a continuous path to the outlet.
2) What gravel type is best behind a retaining wall?
Clean, angular drain rock is commonly preferred because it maintains void space and drains well. Avoid material with many fines, which can clog, hold water, and reduce long-term drainage performance.
3) Do I always need a drain pipe in the trench?
Not always, but a perforated pipe with a proper outlet is widely used for reliability. If your site drains naturally, a pipe may be optional; however, poor drainage soils usually benefit from one.
4) How do I pick compaction and waste percentages?
Start with 8% compaction and 5% waste for typical small projects. Increase values for curved walls, uneven excavations, or limited access. Decrease slightly if your dimensions are precise and placement is controlled.
5) My backfill area is triangular or stepped—what should I do?
Break the wall into sections and calculate each section separately using average thickness and height. Summing smaller, realistic shapes usually gives a better estimate than forcing one large rectangle.
6) Why does density change the weight so much?
Density depends on stone size distribution, void ratio, and moisture. Two “gravel” loads can differ significantly in weight for the same volume. Use supplier-provided bulk density when possible for more accurate delivery planning.
7) Can I order by cubic yards using these results?
Yes. Use the displayed yd³ order volume for yard-based quotes. If you receive tonne pricing, compare both using the tool’s weight estimate to ensure the quote and delivery method align.
Build stronger walls by budgeting the right gravel today.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.