French Drain Calculator
Perforated pipe, filter fabric, gravel volume, daylighting, catch basin, and labor for foundation, yard low-spot, downspout, or retaining-wall drainage.
4-in perf: ~$2. 6-in: ~$3.50.
3/4-in clean stone delivered
Solid pipe to outlet
0 = no basin
Hand-dig: 8-12. Machine: 18-25.
Result
- Pipe cost
- $160
- Filter fabric
- 280 sf · $112
- Gravel
- 3.5 yd³ · $145
- Daylight pipe
- $45
- Fittings
- $12
- Catch basin
- —
- Material total
- $474
- Labor
- 6.7 hr · $400
- Total installed
- $874
- Effective $ / LF
- $11
This estimate is based on national average costs and may vary by region, project specifics, and market conditions. Use as a starting point for your bids.
French drain anatomy
Cross-section from top to bottom: 4 in topsoil + sod cap, filter fabric, 14 in clean 3/4-in gravel, 4-in perforated pipe (holes down), more gravel + fabric wrap. The whole thing sits in a trench 12-18 in deep with 1% slope toward the outlet.
Sizing the trench
| Application | Depth | Width |
|---|---|---|
| Yard low-spot | 12-18 in | 10-12 in |
| Perimeter foundation | At footing level | 10-12 in |
| Behind retaining wall | Base of wall | 8-12 in |
| Downspout extension | 10-14 in | 8-10 in |
Pricing benchmarks (installed)
- Yard French drain (4-in pipe, 18 in deep): $25-45 per LF
- Foundation perimeter drain: $40-80 per LF (deeper trench)
- Downspout extension: $20-35 per LF
- Add catch basin at inlet: $300-600 per basin
- Pop-up emitter at outlet: $40-80 per emitter
Critical do-not-skip items
- 1% slope minimum (1/8 in per LF) — water needs to move
- Filter fabric wrap (Mirafi 140N or equivalent) — prevents clogging
- Clean 3/4-in stone (no fines) — fines clog the system
- Daylight outlet OR sump pump — water has to GO somewhere
- Perforations facing DOWN (yes, down — water enters through gravel sides)
Frequently asked questions
How deep should a French drain be?
12-18 inches for residential yard drainage. Perimeter foundation drains run at the footing level (deeper, 24-48 in). Retaining-wall drains sit at the base of the wall. The trench needs at least 1% slope (1/8 in per LF) toward the outlet — without slope, water sits in the gravel and the drain doesn't drain.
4-in or 6-in perforated pipe?
4-in is the residential standard for most applications. 6-in handles 4× the flow capacity — use for downspout-fed systems with multiple downspouts feeding one drain, heavy-rain regions, or commercial applications. The pipe is the cheap part; the trench and gravel cost the same regardless.
How much gravel do I need?
Trench width × trench depth × length, in cubic yards. A 100 LF trench, 12 in wide × 18 in deep = 150 ft³ ÷ 27 = 5.6 yd³. At $42/yd³ delivered, that's $235 in gravel. Use clean 3/4-in stone (no fines). Pea gravel and crushed stone with fines clog the pipe and fabric.
Do I really need filter fabric?
Yes. Without it, fine soil migrates into the gravel and clogs the system within 3-5 years. Wrap the entire trench (sides + top) before backfilling. Non-woven geotextile (Mirafi 140N or equivalent) is the residential standard at $0.30-0.50 per sq ft. Skipping fabric is a $200 mistake that becomes a $3,000 re-do.
How much does a French drain cost installed?
$25-65 per linear foot installed in most US markets. Variance is in depth (foundation drains run higher), access (machine vs hand-dig), and outlet — daylighting to a low spot is cheap, piping to a city storm drain or pit can double the cost. Add $300-600 for a catch basin at the inlet.
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