About French Drain Calculator (Gravel, Fabric & Pipe)
The french drain calculator turns a trench plan — width, depth, and run length — into a shopping list: washed drainage gravel by volume and weight, non-woven filter fabric by the wrapped girth, and perforated pipe in 10-foot sticks. A french drain is a gravel-filled trench with a perforated pipe near the bottom that intercepts groundwater or surface water and carries it away from a foundation, a soggy lawn, or a retaining wall. The gravel quantity subtracts the volume the 4-inch pipe displaces, so you are not buying stone the pipe already fills, and the fabric area uses the "burrito wrap" girth — down one trench wall, across the bottom, up the other wall, back across the top of the stone, plus a 12-inch lap — so silt cannot migrate into the stone from any direction.
The slope check compares your available elevation fall to the 1% practice minimum (about 1/8 inch per foot): water in a perforated pipe needs continuous fall to keep moving, and 1% is the figure most installers work to. Some references accept 0.5% for smooth-wall (rigid PVC) pipe, which carries water more efficiently than corrugated pipe, but flatter grades leave no margin for installation error. All quantities here are representative trade practice rather than code requirements: the 1.4 tons-per-cubic-yard density is typical for washed stone (use the gravel & aggregate calculator for other materials), pallet-style fabric rolls vary by supplier, and a drain is only as good as its outlet — plan where the water daylights before you dig.
How It Works
- Pick a trench width — 6, 9, or 12 inches are the common presets — or enter a custom width from 6 to 24 inches (a 4-in pipe with fabric will not fit anything narrower), then enter the trench depth (6–48 in) and the run length in feet.
- The trench volume is width × depth × length; the calculator subtracts the volume displaced by the standard 4-inch perforated pipe and reports the remaining gravel need in cubic feet, cubic yards, and tons at a representative 1.4 tons per cubic yard for washed stone.
- Filter fabric is figured from the wrap girth — 2 × width + 2 × depth + 12 inches of lap, covering both walls, the bottom, and a full pass across the top of the stone — times the run length, then converted to 3 ft × 100 ft rolls counted at 90% effective coverage for end laps and seams.
- Perforated pipe is the run length plus a 5% allowance for fittings, couplings, and offcuts, rounded up to whole 10-ft sticks.
- Enter the elevation fall over the run (optional) to check the grade against the 1% practice minimum; leave it blank and the calculator reports the fall you need instead.
Worked Example
A 50-ft drain in a 12-inch-wide, 24-inch-deep trench: the trench is 1 ft × 2 ft × 50 ft = 100 ft³. The 4-inch pipe displaces π × (2/12 ft)² × 50 = 4.36 ft³, leaving 95.64 ft³ of gravel — 3.54 yd³, or about 4.96 tons of washed stone. The fabric wrap girth is (2 × 12 + 2 × 24 + 12)/12 = 7 ft — bottom, both walls, a full top pass, and the 12-inch lap — so the run needs 350 ft² of fabric: 2 rolls at 270 effective ft² each. Pipe is 50 × 1.05 = 52.5 LF, which rounds up to 6 ten-foot sticks. With 6 inches of available fall, the slope is 6/(50 × 12) = exactly 1.0% — right at the practice minimum, so it passes.
Formulas
- Gravel volume
gravel_ft3 = (w/12) x (d/12) x L - pi x (2/12)^2 x L- Gravel weight
tons = gravel_ft3 / 27 x 1.4- Filter fabric
fabric_ft2 = (2w + 2d + 12)/12 x L; rolls = ceil(fabric_ft2 / 270)- Perforated pipe
pipe_LF = L x 1.05; sticks = ceil(pipe_LF / 10)- Slope check
slope_% = fall_in / (L x 12) x 100 >= 1% (required fall_in = L x 12 x 0.01)
Standards & References
- All material factors are representative drainage-trade practice, not code: the 4-in pipe standard, 1.4 tons/yd³ washed-stone density, 90% fabric roll efficiency, 5% pipe fittings allowance, and the 1% minimum slope are common published installer figures — confirm quantities against your supplier and site
- The 1% slope minimum (about 1/8 in/ft) is the common practice figure; some references accept 0.5% for smooth-wall (rigid) pipe — flatter grades leave no margin for installation error
- Where a drain protects a foundation, grading and drainage around the structure are governed by IRC R401.3 — see the grading & drainage slope calculator; discharge must not create a hazard or drain onto neighboring property
Frequently Asked Questions
How deep and wide should a french drain be?
For surface water and soggy-lawn problems, a common trench is 9 to 12 inches wide and 18 to 24 inches deep — deep enough for a few inches of bedding stone, the 4-inch pipe, and at least 12 inches of gravel above it. Drains intercepting groundwater at a foundation footing go deeper (to the footing base), and curtain drains on slopes are sized to cut across the flow path. Narrower trenches (6 in) work for short runs with trencher equipment but leave little stone around the pipe; this calculator accepts 6 to 24 inches of width (a 4-in pipe — actual OD about 4.5 in — plus fabric will not fit anything narrower) and 6 to 48 inches of depth so you can price the option you are actually able to dig.
Why does a french drain need filter fabric?
Silt is the number-one killer of french drains. Fine soil particles carried by the water migrate into the gravel, fill the voids between stones, and eventually clog the pipe perforations — a fabric-less drain in silty soil can fail in a few years. Non-woven geotextile fabric lines the trench and wraps over the top of the stone (the "burrito wrap" this calculator sizes, with a 12-inch overlap flap), letting water pass while keeping fines out. Use non-woven fabric, not the woven weed-barrier sold for landscaping: woven fabric blinds over with silt much faster.
Do the pipe perforations face up or down?
Down. It seems backwards, but water in the gravel rises from the bottom of the trench, and holes at the bottom of the pipe let it enter at the lowest level — so the drain starts collecting before the water table in the trench rises past the pipe invert. Holes facing up would let the trench fill several inches before any water enters the pipe. With two-row perforated pipe, the rows go at roughly 4 and 8 o’clock. Bed the pipe on a couple of inches of stone rather than soil so the perforations stay clear.
Where should a french drain discharge?
The outlet matters as much as the trench. The best option is daylight — the pipe exits on a downhill slope face or bank so water leaves by gravity, protected by a rodent grate. Without a daylight point, discharge to a dry well sized for your soil percolation, a storm-sewer connection where the municipality allows it, or a sump pump as a last resort. Never discharge onto a neighboring property or a walkway (ice hazard), and keep the outlet at least 10 ft from any foundation. If the run cannot achieve about 1% of continuous fall to the outlet, rethink the routing before you dig.
How much slope does a french drain need?
The common practice minimum is 1% — about 1/8 inch of fall per foot of run, or 6 inches over a 50-ft drain. Water in a perforated pipe moves slowly, and a continuous positive grade keeps silt suspended and moving toward the outlet rather than settling in the pipe. Some references accept 0.5% for smooth-wall rigid pipe, which flows better than corrugated pipe, but a flatter grade leaves no margin for a low spot introduced during backfill. Enter your available elevation fall and the calculator checks it; leave it blank and it tells you the fall the run needs.
What kind of gravel goes in a french drain?
Washed, angular drainage stone in the 3/4-inch to 1-1/2-inch range is the standard fill: washed so fines do not clog the voids, and large enough that water moves freely between stones. Pea gravel packs tighter and drains worse; crushed stone with fines (like road base or crusher run) is the wrong material entirely — it compacts nearly watertight. The 1.4 tons-per-cubic-yard figure here is representative for washed stone; densities run roughly 1.2 to 1.5 t/yd³ by size and source, so use the gravel & aggregate calculator for a material-specific weight and check your quarry’s conversion before ordering by the ton.