French Drain Calculator
This french drain calculator gives the stone, pipe and filter fabric a trench needs, with the pipe displacement taken out, and checks the fall is enough to drain.
Trench Materials
Fall and Grade
Whether the trench actually drains. A drain with no fall is a trench full of water.
French Drain Cost
How to use the French Drain Calculator
Enter the trench dimensions and the pipe size. The stone figure has the pipe volume taken out of it, which most estimates forget, a 4 inch pipe in a 12 inch trench is about a tenth of the volume.
Then check the Fall tab. A french drain works on gravity alone, so the trench has to fall continuously from the high end to wherever it discharges. One percent is the practical minimum. If your drain is 60 feet long, that is 7 inches of drop, and the far end has to be somewhere the water can actually go.
French Drain Cost prices the three materials on separate lines, stone, pipe and filter fabric. The fabric is the one most estimates leave out and then buy in a hurry on the day.
French Drain formula
trench volume = length × width × depthpipe volume = π × (diameter ÷ 2)² × lengthstone = trench volume − pipe volumefabric = length × (width + 2 × depth + overlap)fall = length × grade%
Fabric is the one that surprises people. It lines the trench, so it wraps up both sides and folds over the top, a 12 inch wide, 18 inch deep trench needs about 5 feet of fabric width per foot of length, not 1 foot.
| Grade | Fall per 100 ft | Verdict |
|---|---|---|
| 0.5% | 6 in | Absolute minimum, needs careful work |
| 1% | 12 in | The practical standard |
| 2% | 24 in | Drains briskly, digs deep at the far end |
Worked example
A 60 ft drain, 12 in wide, 18 in deep, 4 in pipe, #57 stone, 10% waste:
- Trench: 60 × 1 × 1.5 = 90 ft³
- Pipe: π × (2÷12)² × 60 = 5.24 ft³
- Stone: 90 − 5.24 = 84.8 ft³, plus 10% = 93.2 ft³ = 3.45 yd³
- At 2,700 lb per yard: 4.66 tons
- Fabric width: 1 + 2(1.5) + 1 overlap = 5 ft, so 60 × 5 = 300 sq ft
- Fall at 1%: 7.2 in over the run, so the outlet end is 25 in deep
What the result means
Fall is the whole design. Stone and pipe are just plumbing; if the trench does not fall continuously to a lower outlet, the drain fills and stays full. Check the outlet before you dig, not after.
Use washed stone, not crusher run. The point is void space for water to move through. Anything with fines in it packs solid and defeats the drain, this is the opposite of the driveway advice.
Fabric goes round the stone, not under the pipe. It keeps soil fines out of the void space. Wrapping only the pipe (a sock) helps a little; lining the whole trench is what makes a drain last decades rather than years.
Common mistakes
- No fall. The most common failure by a wide margin. Level trenches hold water.
- Nowhere to discharge. A drain has to end somewhere lower and open. A drain into a buried box is a soakaway, and it needs sizing for the soil.
- Crushed run instead of clean stone. Fines seal the voids. Use #57 or similar washed material.
- Fabric under the stone only. Line the whole trench and lap the fabric over the top before backfilling.
- Perforations facing up. Slots go down, so water enters at the bottom of the pipe and the pipe drains fully.
- Forgetting the pipe volume. Over-orders stone by roughly a tenth on a standard trench.
Frequently asked questions
How much gravel do I need for a french drain?
Trench volume less the pipe. A 60 foot drain 12 inches wide and 18 inches deep is 90 cubic feet of trench, less about 5.2 cubic feet of 4 inch pipe, which is roughly 3.4 cubic yards or about 4.6 tons of washed stone with ten percent waste.
What slope does a french drain need?
At least one percent, which is an inch of fall for every eight feet, or a foot per hundred feet. Half a percent can work with very careful grading. Anything flatter holds water and the drain stops functioning.
How deep should a french drain be?
Eighteen to twenty-four inches for surface and yard drainage, and down beside the footing for a foundation drain. Remember the trench gets deeper along its length as it falls, so the outlet end is the deepest point.
Do the pipe holes face up or down?
Down. Water enters the pipe at the lowest point in the trench and the pipe drains completely between storms. Slots facing up means water has to rise most of the pipe diameter before it can get in.
What kind of gravel goes in a french drain?
Washed, clean, angular stone, #57 is the usual choice. The void space between the stones is what carries the water, so anything with dust or fines in it, like crusher run or #411, will pack solid and defeat the purpose.
How much landscape fabric do I need for a french drain?
More than the trench length suggests, because the fabric lines the sides as well as the bottom and laps over the top. For a 12 inch wide, 18 inch deep trench allow about 5 feet of fabric width per foot of drain, 300 square feet for a 60 foot run.
Sources and assumptions
Stone volume is trench volume less the pipe displacement, computed as a cylinder at the nominal pipe diameter. Corrugated pipe outside diameter is slightly larger than nominal, so this is marginally conservative.
Stone density is taken at 2,700 lb per cubic yard for washed #57, consistent with the gravel and limestone calculators on this site. Loose density varies.
Fabric width allows the trench width plus both sides plus a one foot overlap at the top. Manufacturers recommend laps of 6 to 12 inches; check the product.
Minimum grade of 1% reflects common drainage practice. Foundation drains, septic fields and anything tied into a municipal storm system are governed by code and are outside this calculation.