About the French Drain Calculator
This French drain calculator works out the materials for a gravel drainage trench with a perforated pipe: cubic yards and tons of drain rock, feet of pipe, square feet of filter fabric to wrap the gravel, and how much the trench bottom must fall over its length. Add your local prices and it totals the material cost too.
It is meant for homeowners fixing a soggy yard or wet basement wall, landscapers quoting a drainage job, and anyone deciding between a 4-inch and 6-inch pipe or a narrow versus wide trench. A typical residential French drain is 6–12 inches wide and 18–24 inches deep, with a 4-inch perforated pipe bedded in washed ¾-inch to 1½-inch stone and wrapped in non-woven geotextile so soil does not clog it.
The gravel volume subtracts the space taken by the pipe and any topsoil or sod you plan to put back on top. Fabric is sized to wrap the whole gravel bed (both sides, bottom and top) with a 12-inch overlap, the common “burrito” method. Gravel weight uses about 1.4 tons per cubic yard, typical of washed crushed stone — check your supplier’s figure.
With the default inputs, the gravel needed is 2.15 cu yd. Change any value above to recalculate instantly.
How to use the french drain calculator
- 1Measure the length of the drain run from the high point to the outlet.
- 2Enter the trench width and depth and how much topsoil or sod will go back on top.
- 3Choose the perforated pipe size and any solid pipe needed to reach the outlet.
- 4Set the trench fall (at least 1%) and your gravel density and prices.
- 5Read the gravel, pipe, fabric and total cost, and order a little extra gravel.
Formula and method
The trench cross-section is its width times the depth filled with gravel — total depth minus any topsoil or sod laid back on top. The circular area of the pipe is subtracted because it displaces stone, and the result is multiplied by the trench length and divided by 27 to give cubic yards. Weight uses the gravel density you enter (about 1.4 tons per cubic yard for washed crushed stone, 1.3–1.5 for most drain rock).
Filter fabric wraps the gravel bed completely: up both sides, across the bottom and folded over the top, which is twice the width plus twice the gravel depth, plus a 1-ft overlap at the top seam. The perforated pipe runs the full trench length; any solid outlet pipe is added for the 10-ft section count. The fall is the slope percentage times the length, converted to inches.
- L
- Trench length (ft)
- W, D
- Trench width and depth (ft)
- C
- Topsoil cover depth (ft)
- d
- Pipe diameter (ft)
- ρ
- Gravel density (tons per cu yd)
Worked examples
50 ft yard drain, 12 × 18 in trench, 4-inch pipe
Gravel fills 15 in of the 18 in depth (3 in of topsoil on top): 1 × 1.25 = 1.25 sq ft of section minus 0.0873 sq ft for the pipe, × 50 ft = 58.1 cu ft, or 2.15 cu yd and about 3.0 tons. Fabric is 2 × (1 + 1.25) + 1 = 5.5 ft wide × 50 ft = 275 sq ft. At 1% fall the trench drops 6 in. Materials total about $253.
Foundation drain, 100 ft, 6-inch pipe, 2 ft deep
The section is 1 × 2 = 2 sq ft minus 0.196 sq ft for the 6-in pipe, so 100 ft needs 180.4 cu ft = 6.68 cu yd (9.35 tons). 120 ft of pipe is 12 sections, fabric is (2 × 3 + 1) × 100 = 700 sq ft, and the bottom drops 12 in. Cost: $467.61 gravel + $300 pipe + $175 fabric = $942.61.
Narrow gravel-only trench
A 6 × 12 in trench is 0.5 sq ft in section, so 30 ft needs 15 cu ft (0.56 cu yd) of stone with no pipe. Fabric is (2 × 1.5 + 1) × 30 = 120 sq ft.
Frequently asked questions
How much gravel do I need for a French drain?+
Multiply trench width × gravel depth × length in feet and divide by 27 for cubic yards, then subtract a little for the pipe. A 12 × 18 inch trench filled to the top uses 1.5 cu ft of gravel per foot before the pipe — about 1.39 cu yd, or roughly 1.9 tons, per 25 ft.
How deep and wide should a French drain be?+
Most residential French drains are 6–12 inches wide and 18–24 inches deep. Foundation (footing) drains go deeper, down to the footing level. Wider trenches hold more water and suit 6-inch pipe; narrow trenches are fine for light surface water.
How much slope does a French drain need?+
A minimum fall of about 1% — 1/8 inch per foot, or 1 inch every 8 feet — is the common rule, and 2% (1/4 inch per foot) drains faster. Over 50 ft, 1% is a 6-inch drop. The outlet must be lower than every point of the trench.
What size gravel is best for a French drain?+
Use clean, washed angular stone between about ¾ inch and 1½ inch, often sold as #57 stone or drain rock. Avoid pea gravel with fines or crushed stone dust, which packs down and clogs. Wrap the gravel in non-woven geotextile fabric to keep soil out.
Should the holes in a French drain pipe face up or down?+
Install perforated pipe with the holes facing down (at the 4 and 8 o’clock positions for two-row pipe). Water rises into the pipe from the gravel below as the trench fills, and downward holes are less likely to clog with sediment.