How to use the French drain gravel calculator
- Enter the trench length in feet, from the wettest point to the outlet.
- Enter trench width and depth in inches. A common residential French drain is about 12 inches wide and 18–24 inches deep.
- Enter the pipe diameter. A 4-inch perforated pipe is typical; enter 0 for a gravel-only drain.
- Choose the stone. Washed, angular stone about 3/4 to 1.5 inches with no fines is standard for drainage.
- Read the results: drain rock in cubic yards, tons and bags, plus the filter fabric area and pipe length.
The formula
Trench volume (cu ft) = length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
Pipe volume (cu ft) = π × (diameter ÷ 24)² × length
Gravel = (trench − pipe) × (1 + waste %)
Fabric width = 2 × depth + 2 × width + overlap
The fabric estimate assumes a full "burrito" wrap: fabric lines the bottom and both sides, then folds over the top of the gravel with an overlap. Fabric is sold in rolls of fixed width, so pick a roll at least as wide as the fabric width shown. Washed #57 stone is estimated at 100 lb/ft³ (1.35 tons per cubic yard); The Engineering ToolBox lists dry gravel at 105 lb/ft³, and washed single-size stone is slightly lighter because it has more voids — which is exactly what lets water through.
Worked example
A 50 ft French drain, 12 inches wide and 18 inches deep, with a 4-inch perforated pipe and 10% extra stone:
- Trench: 50 × 1 × 1.5 = 75 cu ft
- Pipe: π × (4 ÷ 24)² × 50 = 4.4 cu ft
- Net stone: 70.6 cu ft; with 10% = 77.7 cu ft = 2.88 yd³
- Weight at 100 lb/ft³: 7,770 lb = 3.89 tons
- Fabric: 2 × 18 + 2 × 12 + 12 = 72 in = 6 ft wide × 50 ft = 300 sq ft
- Pipe: 50 ft of 4-inch perforated pipe, plus fittings
Almost 4 tons is about 156 bags of 0.5 cu ft, so a bulk delivery is the practical choice.
French drain design tips
- Slope: the trench bottom should fall toward the outlet, commonly at least 1% (about 1 inch for every 8–10 ft). Check it with a line level or laser before adding stone.
- Bedding: put 2–3 inches of stone under the pipe so water can reach the perforations from below.
- Holes down: with single-wall perforated pipe, most guides recommend facing the holes downward so water rises into the pipe and sediment stays out.
- Use non-woven fabric for filtration. It lets water pass while keeping soil out of the stone.
- Avoid round stone and fines. Pea gravel works but packs tighter than angular stone; crusher run or stone dust will clog the drain.
- Call 811 (in the US) before you dig, to have buried utility lines marked.
- Top it nicely: finish the surface with decorative stone; the river rock calculator covers that layer.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply trench length by width by depth in feet, subtract the pipe volume, and add about 10%. A 50 ft trench 12 inches wide and 18 inches deep with a 4-inch pipe needs about 2.9 cubic yards, roughly 3.9 tons.
What size gravel is best for a French drain?
Washed, angular stone around 3/4 to 1.5 inches, such as #57 stone. It leaves large voids for water and does not contain fines that clog the pipe.
How wide and deep should a French drain be?
Typical residential drains are about 6–12 inches wide and 18–24 inches deep, with the bottom sloped toward the outlet.
How much does a 4-inch pipe reduce the gravel needed?
A 4-inch pipe displaces about 0.087 cubic feet per foot of trench, about 4.4 cu ft for every 50 feet.
Do I need fabric for a French drain?
Wrapping the stone in non-woven geotextile is strongly recommended in most soils. Without it, fine soil gradually migrates into the stone and reduces drainage.