How much drainage rock do I need?
Calculate the drain bed or trench volume, not the surrounding landscape surface area. For a 50 ft trench that is 12 inches wide with 18 inches of rock depth, the volume is about 2.78 cubic yards before waste.
Pipe and fabric are not included
This calculator estimates rock volume only. Pipe diameter, fabric wrap, trench shape, catch basins, cleanouts, outlets, and surface grading should be planned separately.
Use drainage-suitable rock
Decorative stone is not always the right material for drainage. Confirm aggregate size and drainage suitability with the supplier or project specification.
French drain rock example estimates
Examples treat trench length x trench width as area. Pipe, fabric, trench shape, and void space are not subtracted.
| Trench example | Length x width x rock depth | Cubic yards |
|---|---|---|
| Short drain run | 30 ft x 1 ft x 12 in | 1.11 cu yd |
| Medium drain run | 50 ft x 1 ft x 18 in | 2.78 cu yd |
| Wide trench | 50 ft x 1.5 ft x 18 in | 4.17 cu yd |
| Long drain run | 100 ft x 1 ft x 18 in | 5.56 cu yd |
Drain rock coverage by depth
Coverage assumes 1 cubic yard, which is 27 cubic feet. Waste, compaction, settling, and irregular grade are not included.
| Depth | Coverage from 1 cu yd | Coverage from 2 cu yd |
|---|---|---|
| 1 in | 324 sq ft | 648 sq ft |
| 2 in | 162 sq ft | 324 sq ft |
| 3 in | 108 sq ft | 216 sq ft |
| 4 in | 81 sq ft | 162 sq ft |
| 6 in | 54 sq ft | 108 sq ft |
| 12 in | 27 sq ft | 54 sq ft |
Before you calculate
- Measure drain length, width, and rock depth separately from decorative landscape areas.
- Keep pipe, fabric, basin, outlet, and cover-depth details visible as separate planning items.
- Calculate separate drain runs when width, depth, pipe detail, or aggregate type changes.
Common mistakes
- Treating a drainage trench like a shallow decorative rock bed.
- Ordering decorative rock without confirming drainage-suitable aggregate.
- Forgetting that trench shape, pipe, fabric, and uneven bottoms can change real usage.
Formula
units = ceil((area * (1 + wastePercent / 100)) / coveragePerUnit)
Assumptions
- This estimates material quantity only, not drainage system sizing or hydraulic capacity.
- Depth, compaction, fabric overlap, pipe layout, and supplier rounding can change real orders.
- Confirm local stormwater rules and site conditions before building drainage work.
Example
Estimated mat rolls needed: 4 mat rolls
How to calculate slope drainage mat mat rolls
- Measure the project area in square feet.
- Enter the coverage per mat rolls from the product label or supplier data.
- Add waste for cuts, overlaps, damaged pieces, or layout changes.
- Divide adjusted area by coverage per mat rolls and round up to a whole purchasable unit.
- Check accessories, trim, fasteners, seams, or prep materials separately.
Before you buy materials
- Confirm aggregate size and drainage suitability with the supplier or project specification.
- Round up for trench irregularity, placement loss, and delivery units.
FAQ
What is the example slope drainage mat mat rolls result?
Use the measured project inputs, product coverage, and waste, then calculate the planning result. In the default example, the result is 4 mat rolls.
How do I calculate drainage gravel?
Multiply drain length by width to get area, multiply by rock depth, divide by 27 to get cubic yards, then add waste.
Does this subtract pipe volume?
No. It estimates the trench rock volume as a planning number. Pipe, fabric, and void space can change real usage.
What depth should I use?
Use the rock depth planned for the trench, not the total landscape surface depth. Check the project specification before ordering.
Can I use decorative rock for a French drain?
Do not assume so. Use a drainage-suitable aggregate recommended for the project.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.