French Drain Calculator
Gravel volume, pipe length, and fabric coverage for perimeter and yard drains.
- Gravel cubic yards
- 2.22
- Gravel tons
- 3.11
- Perforated pipe (ft)
- 40
- Landscape fabric (sq ft)
- 200
At ~1.4 t/yd³ for #57 stone
One run of 4 inch pipe
Includes wrap and overlap
What a French drain actually does
A French drain is a gravel-filled trench with a perforated pipe at the bottom that collects and moves subsurface water to a lower discharge point. It solves saturated yards, wet crawl spaces, and water pooling against foundations. It does not solve surface water from a downspout dumping onto grade; that is a downspout extension or a catch basin problem.
The classic install: dig a trench with a fall of one percent minimum (one inch drop per eight feet of run), line the trench with woven landscape fabric, add two inches of gravel, place perforated pipe holes down, backfill with washed gravel to within four inches of the surface, wrap the fabric burrito-style, cover with sand and turf or decorative rock.
Trench sizing and gravel volume
The standard perimeter drain is a 12 inch wide trench, 18 to 24 inches deep, with 4 inch perforated pipe. For a 40 foot run at 12 by 18 inches, gravel volume is (40 x 1 x 1.5) / 27 = 2.22 cubic yards, or roughly 3.1 tons of #57 washed stone. Add ten percent for compaction and shaping.
Deeper trenches for foundation drainage (36 to 48 inch depth) triple the volume quickly. A 40 foot foundation drain at 12 inches wide and 36 inches deep is 4.44 cubic yards. Excavation labor and gravel cost scale with depth, not length.
Pipe selection: rigid vs corrugated, sock vs no sock
Rigid PVC perforated pipe (SDR-35 or Schedule 40) lasts 50+ years, resists crushing, and cleans out with a sewer machine when it clogs. Corrugated black pipe is cheaper and flexible but crushes under vehicle loads and cannot be cleaned effectively. For any drain that will get driven over or is more than two feet deep, use rigid PVC.
Sock-wrapped pipe (pipe pre-wrapped in filter fabric) is popular for quick installs but the sock clogs with silt within a few years in clay-heavy soils like most of North Texas. A better long-term system: line the trench with woven fabric and use unwrapped pipe. The fabric filters silt at the trench boundary where you can eventually excavate and replace if needed.
Discharge, cleanouts, and code
A French drain has to discharge somewhere. Options include daylight (open outlet at a lower grade), a dry well (buried gravel pit), a storm sewer connection (check code), or a sump pump that lifts the water to a discharge point. Never discharge onto a neighboring property; that is a lawsuit waiting to happen.
Install a cleanout at each end and at any bend over 45 degrees. A cleanout is a vertical riser with a threaded cap flush to grade. Without cleanouts, a clogged drain is a full excavation to fix. With cleanouts, it is a 20 minute sewer machine job.
For foundation repair and drainage contractors
French drain and yard drainage work is one of the highest-margin service categories in Texas because most homeowners call after they have already had water damage. Peak Local Leads builds and ranks drainage-focused sites for one contractor per Texas market. Free week of calls, flat monthly if the work is real.
Frequently asked.
- How deep should a French drain be?
- 18 to 24 inches for yard drainage. 36 to 48 inches for foundation drainage. The bottom of the pipe should sit below the level of the water you are trying to move.
- What kind of gravel goes in a French drain?
- Washed #57 stone (angular, roughly 1 inch) is standard. Avoid pea gravel (too small, migrates through fabric) and crushed concrete (fines clog the pipe).
- Do I need landscape fabric in a French drain?
- Yes. Woven fabric wrapping the trench prevents fine soil from migrating into the gravel and clogging the system. Skip fabric only in pure sand soils.
- How long does a French drain last?
- Properly built with rigid pipe, washed stone, woven fabric, and cleanouts, a French drain lasts 30 to 50 years. Cheaper corrugated-plus-sock installs typically clog within 5 to 10 years in clay soils.
Make the phone the busiest tool on the truck.
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