Landscape & Outdoor
Updated Jul 29, 2026

French Drain Calculator

Calculate gravel, perforated pipe, geotextile fabric, and slope required for a french drain.

How to Design a French Drain

A french drain is a gravel-filled trench with a perforated pipe at the bottom that collects subsurface water and moves it by gravity to a lower outlet. Common applications include yard drainage, foundation perimeter protection, retaining wall weep systems, and septic field subgrade. Done right, a french drain lasts 30 to 50 years. Done wrong - bad slope, dirty gravel, or no fabric - it clogs within 5 years and fails.

Gravel Volume Formula

Calculate trench volume, subtract the pipe displacement, and add 10 percent for compaction:

Trench Volcu ft=Lft×Win12×Din12
Gravelcu ft=(Trench Vol−Pipe Vol)×1.10

Slope Requirements

A french drain moves water by gravity alone. Without enough slope, water sits in the pipe and sediment settles, clogging the perforations:

Dropin=Lengthft×Slope%100×12
1%
1 in / 10 ft
Minimum - OK
2%
2.5 in / 10 ft
Ideal
3-5%
4-6 in / 10 ft
Self-scouring
Below 1% Slope Will Fail
Under 1 percent slope, water velocity drops below 1 foot per second. Suspended sediment settles in the pipe, clogs the perforations, and the drain fails within 2 to 5 years. If your site has less than 1 percent natural fall, install a sump basin at the low end and pump water mechanically - do not rely on gravity.

Pipe Size by Run Length

Larger pipe handles more flow and clogs less readily. Upsize if you are near capacity or have fine silty soil:

Pipe SizeMax RunFlow CapacityTypical Use
3 in / 75 mm30 ft~25 GPMFoundation weep, short runs
4 in / 100 mm100 ft~60 GPMStandard residential yard
6 in / 150 mm300 ft~150 GPMLong runs, heavy soil water
Use Slotted Pipe, Install Slots Down
Slotted drainage pipe (narrow machined slots) resists clogging far better than round-hole corrugated pipe. Install with slots or perforations facing downward so water enters from the gravel bed beneath rather than from above. This is counter-intuitive but correct - water pools at the trench bottom and rises into the pipe through the lowest openings.

The Burrito Wrap (Geotextile Fabric)

Wrap the entire trench - bottom, both sides, and top - with non-woven geotextile fabric before adding gravel. This burrito wrap prevents native soil from migrating into the gravel and fouling the pipe:

  • Fabric type: non-woven, 4-6 oz/sq yd (NEVER woven weed barrier)
  • Width needed: trench width + 2 x depth + 12 inches overlap
  • Overlap seams: minimum 6 inches
  • Top overlap: fold both sides in over the gravel before backfilling

Gravel Selection

The wrong gravel kills a french drain. Use:

MaterialVerdictReason
#57 washed stoneUseStandard. 1/2 to 1 inch angular, no fines.
#3 washed stoneUseLarger (1-2 in). Good for very wet soil.
Crusher runNeverContains fines - clogs the pipe.
Pea gravelNeverRounded, packs tight, poor flow.
River rockNeverRounded, inconsistent - same problem.

Trench Dimensions

  • Standard yard drain: 12 inches wide x 18-24 inches deep
  • Foundation drain: 12-18 inches wide, down to bottom of footing (36-48 inches)
  • Retaining wall weep: 6-12 inches wide behind the wall, full wall height
  • Driveway drain: 12-18 inches wide x 18 inches deep
Call 811 Before Digging
Call 811 (or your local utility locator) at least 3 business days before digging. Hitting a gas, water, fiber, or power line during trench excavation is expensive, dangerous, and in many jurisdictions illegal if you did not call first. The call is free.
Outlet Design
The drain pipe must daylight (exit to grade) at a point lower than the trench bottom. Use a rodent-proof grate on the outlet. If no daylight outlet is available, install a dry well - a pit filled with clean stone where water percolates into the soil. Size a dry well at 1 cubic yard per 100 sq ft of drainage area.

Frequently Asked Questions