Article — French Drain Calculator
French drain calculator: gravel, perforated pipe, fabric and slope
A french drain calculator returns gravel volume, perforated pipe length, filter fabric area and slope drop from four inputs (drain length, trench width, trench depth, slope percent). A 50 ft drain at 12 in wide and 24 in deep needs about 3.5 cubic yards of 3/4 in washed gravel, 50 ft of 4 in perforated PVC, 300 sq ft of non-woven geotextile, and a minimum 6 in vertical drop along the run.
The french drain is the workhorse residential drainage system — a gravel-filled trench around a perforated pipe that collects subsurface water and routes it away from foundations, lawns and patios. The math is volume-by-area-by-depth; the engineering is in the slope, pipe orientation and filter fabric.
What the french drain calculator does
The tool above accepts drain length, trench width, trench depth, pipe diameter, and slope percentage. It returns gravel volume in cubic yards (or meters), perforated pipe length, filter fabric area, slope drop in inches, and a slope-check warning if you fall below 1 percent.
The calculator handles both imperial (length in feet, width and depth in inches, output in cubic yards) and metric (length in meters, width and depth in centimeters, output in cubic meters). Toggle the unit at the top.
The french drain volume formula
Trench volume equals length times width times depth. Gravel volume equals trench volume minus the pipe volume (the cylinder displaced by the pipe). Filter fabric area equals length times (trench width + 2 × depth + 12 in overlap).
Trench V = L × W × DPipe V = π × r² × LGravel V = Trench V − Pipe VFabric A = L × (W + 2D + 12 in)Slope drop = L × slope/100Convert cubic feet to cubic yards by dividing by 27 (the bulk-gravel delivery unit at most US landscape supply yards). A 50 ft drain at 12 in by 24 in is 100 cubic feet (3.7 yd³). Order 4 yd³ to allow for compaction loss and a small overfill above the pipe.
French drain slope (1% minimum)
Minimum slope is 1 percent (1 ft drop per 100 ft of run), recommended is 2-3 percent. Below 1 percent water sits in the pipe and silts the perforations within a few years. Above 5 percent the water accelerates too fast to filter, washing fines through the gravel.
Measure slope with a string level or laser level along the trench bottom. Mark the inlet and outlet elevations before digging; cut the trench bottom on a continuous downward grade. Avoid “belly” spots where the trench dips and water collects.
French drain pipe size
4 inch perforated PVC is the residential workhorse, handling 20-30 gallons per minute at 2 percent slope. 6 inch handles 70-100 GPM and is the choice for clay soils, big roof areas, or any drain receiving downspout connections. 3 inch only for short downspout extensions to grade; 8 inch for commercial installations.
Pipe perforations face down so water filters up through the gravel and enters the pipe from beneath. Pipe with perforations facing up traps gravel above the pipe and lets water pool. NDS EZ-Drain wraps the perforated pipe in gravel and fabric pre-installed, eliminating the orientation question.
French drain gravel selection
Use 3/4 inch washed angular gravel (also sold as “clean stone” or “No. 57 stone”). Washed means the fines are screened out; angular means crushed and interlocking. Round pea gravel compacts too tight and traps fines. Bank-run or pit-run gravel has too many fines and silts up.
The permeability of clean 3/4 in washed gravel is 100-300 meters per day — meaning a column of water can pass through 100-300 meters of gravel in 24 hours. Compare with clay at 0.001-0.01 m/day. The french drain works because the gravel is 10,000 to 100,000 times more permeable than the surrounding soil; water rushes to the path of least resistance.
Gravel layers in a properly built french drain: 6 inches below the pipe (drainage and bedding), pipe diameter, then 4-6 inches above the pipe to grade. Wrap the entire gravel envelope in non-woven geotextile filter fabric to prevent soil migration.
French drain filter fabric
Non-woven geotextile, 4-6 ounces per square yard, lines the trench (sides and bottom) and overlaps 12 inches at the top after gravel is placed. The fabric stops soil fines from migrating into the gravel and clogging the system.
Without fabric, soil fines wash into the gravel with every rain, slowly filling the voids that water needs to flow through. Permeability drops 90 percent in 3-5 years; the drain stops working. Fabric adds maybe $50 to a $1,500 installation and triples the service life. Always install it.
Choose non-woven (felt-like) over woven (canvas-like) geotextile. Non-woven has higher flow rates and better filtration. Common brands are Mirafi 140N, US Fabrics US 115NW, and SKAPS GT135. The home center generic equivalent is fine for residential work.
French drain outlet and termination
The drain ends at one of three places: daylight to grade (cheapest, most common), a dry well (buried perforated barrel for low-flow disposal), or a storm sewer (where local code permits). Never connect to the sanitary sewer — the inflow load overwhelms wastewater treatment and most US municipalities prohibit it.
- Daylight outlet simplest, free, terminates downhill
- Dry well $100-300, hides outlet, slow disposal
- Pop-up emitter small dome valve at grade
- Storm sewer permit required, local code varies
- Sump pit and pump for sites with no downhill option
- Splash pad 2-3 ft of stone at the outlet to prevent erosion
French drain maintenance and life
A properly built french drain (washed gravel, non-woven fabric, 2-3 percent slope) lasts 20-30 years. Failure modes are fines migration (slow clog), root intrusion (especially willows and poplars within 30 ft), and outlet blockage (debris at the daylight terminus).
Annual maintenance: clear the outlet, inspect for ponding above the drain after rain, check cleanouts for silt accumulation. Every 5-10 years flush the drain with a garden hose from the upstream cleanout to dislodge any silt buildup.
Install a cleanout at the upstream end and at any direction change. A cleanout is a 4 in PVC tee with a threaded cap brought to grade. It costs $20 in parts and saves hours of guesswork when the drain backs up. The upstream cleanout doubles as a flush port for hosing out silt.