Which direction should French drain pipe holes face?

Holes facing down (6 o'clock position). A French drain collects groundwater that is rising from below — placing holes at the bottom allows water to enter the pipe as it rises through the gravel layer. Holes facing up collect only surface water that has already penetrated the full gravel depth. Holes facing down also reduces the accumulation of debris inside the pipe since gravity keeps particulates away from the openings. This is the most common installation error — always check hole orientation before covering the pipe.

What is the difference between a French drain and a trench drain?

A French drain collects subsurface groundwater through a perforated pipe buried in gravel — water enters through the gravel and pipe holes as it rises from below or infiltrates from surrounding soil. A trench drain (channel drain) collects surface water through a grated top opening — it handles sheet flow and concentrated surface runoff. Both use a pipe to carry collected water to an outlet. For yard flooding from surface water, a trench drain at the low point is often more effective. For soggy ground from a high water table or hillside seepage, a French drain is the correct solution.

How do I know if I need a French drain?

Signs you need a French drain: water pools in the yard for more than 24 hours after rain, soggy or permanently wet patches even in dry weather (indicating a high water table), basement dampness or water infiltration along the foundation, erosion channels where water flows across the yard in a consistent path during rain, or dead grass in areas where water stress is evident. A French drain is a solution for excess groundwater and subsurface water migration. For surface runoff that does not infiltrate, regrading, berms, or surface channel drains may be more effective.

French Drain Calculator — Gravel, Pipe, Fabric & Cost 2026

Q: How do you calculate gravel for a French drain?

Three-step formula: 1) Trench volume: Length (ft) × Width (ft) × Depth (ft) = cubic feet. 2) Subtract pipe volume: π × (pipe radius in ft)² × Length = pipe cubic feet. 3) Net gravel volume: (Trench volume − Pipe volume) ÷ 27 = cubic yards. Add 10 percent for waste. Example: 50 ft trench, 12-inch wide, 18-inch deep, 4-inch pipe. Trench = 50 × 1 × 1.5 = 75 cu ft. Pipe = 3.14159 × (0.167)² × 50 = 4.38 cu ft. Net = (75 − 4.38) ÷ 27 × 1.10 = 2.88 cubic yards.

Q: What type of gravel is best for a French drain?

Clean angular #57 crushed stone (3/4-inch nominal) is the best gravel for French drains. It is angular, so void spaces stay open and do not compact together over time. It is clean — no fines or dust — so drainage pathways remain unobstructed. Alternatives: #3 stone (1 to 2 inch) for high-volume drains, pea gravel (3/8 inch) for shallow decorative drains. Never use crusher run, stone dust, or any material with fines — the fine particles migrate into and clog drainage voids within 2 to 5 years. Density for calculation: #57 stone 1.40 tons per cubic yard.

Q: How deep should a French drain be?

Depth by application: surface water drainage (yard flooding, low spots): 18 to 24 inches. Foundation drainage (keeping basements dry): 24 to 48 inches, placed at or below the foundation footing elevation. Curtain drain (intercepting water migrating across a slope): 18 to 36 inches depending on water table depth. Minimum practical depth is 18 inches — shallower than this provides insufficient gravel above the pipe for effective infiltration and risks frost heave of the pipe in cold climates.

Q: What slope does a French drain need?

Minimum slope: 1 percent (1 inch of fall per 8.3 feet of run). This is the absolute minimum for gravity drainage. Recommended slope: 1 to 2 percent for most residential installations. Steeper is always better if the site allows — 2 percent (1 inch fall per 50 inches of run) provides faster drainage and better flushing of fine particles from the pipe. The required fall distance in inches = Trench length (ft) × (slope percent ÷ 100) × 12.

Q: How much landscape fabric for a French drain?

Fabric square footage = Trench length × (Width + Depth × 2) × 1.20. The 1.20 factor adds 20 percent for overlap and the top fold-over. Example: 50 ft trench, 12 inches wide, 18 inches deep: 50 × (1 + 1.5 × 2) × 1.20 = 50 × 4 × 1.20 = 240 sq ft of fabric. Use non-woven geotextile fabric (not woven) — non-woven fabric allows water to pass through in all directions while filtering soil particles. Woven fabric in a French drain can restrict flow across the fabric surface.

Q: How much does a French drain cost?

DIY French drain cost per linear foot: gravel $0.60 to $1.20, perforated pipe $0.40 to $0.80, landscape fabric $0.20 to $0.40, total materials $1.20 to $2.40 per linear foot. A 50-foot French drain DIY materials: $60 to $120. A 100-foot drain: $120 to $240. Professional installation adds labour at $10 to $30 per linear foot, bringing a 50-foot drain to $560 to $1,620 installed. Costs vary significantly by trench depth — deeper drains require more excavation time and equipment.

Q: Should I use a sock on my French drain pipe?

No — the fabric sock pre-fitted on many perforated pipes clogs with silt within 3 to 7 years in most soil conditions. The sock has a fine mesh that captures small soil particles efficiently — eventually so efficiently that it blocks water entry. The better method is the burrito wrap: lay non-woven geotextile fabric in the trench, fill with gravel, lay the pipe with holes facing down, cover with more gravel, then fold the fabric over the top of the gravel and overlap it before backfilling. This wraps the entire gravel column in fabric, filtering soil at a larger surface area that resists clogging far longer.

Q: Can you use pea gravel in a French drain?

Pea gravel can be used in shallow, lightly-loaded French drains (yard surface drains, decorative drainage features). It is not the ideal choice because its rounded shape means the stones pack together more tightly than angular stone, reducing void space and drainage capacity. #57 crushed stone has better hydraulic conductivity than pea gravel of the same nominal size. For foundation drainage, curtain drains, and any high-volume application, use #57 or #3 angular crushed stone rather than pea gravel. For a shallow 12-inch backyard drain handling light surface runoff, pea gravel is acceptable.

Q: How long does a French drain last?

A properly installed French drain lasts 30 to 40 years with the burrito wrap fabric method and clean angular stone. A drain installed with a fabric sock on the pipe lasts 7 to 15 years before sock clogging significantly reduces performance. Signs a French drain needs replacement: water is no longer draining away, the outlet produces significantly less flow than it used to, or the area above the drain becomes saturated again after previously draining well. Drain flushing with a high-pressure hose through a cleanout can extend life by clearing accumulated fine particles.

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Pipe displacement formula from civil engineering practice · Slope minimum (1%) from drainage engineering standards · Fabric calculation from geotextile industry specification · Methodology · Updated June 2026

French Drain Calculator

Trench Length (feet)

Trench Width (inches)

Custom Width (inches)

Trench Depth (inches)

Custom Depth (inches)

Pipe Diameter (inches)

Slope

Gravel Type

Price (optional)

Gravel Price ($)

Please enter a valid trench length greater than zero.

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Gravel yd³

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Gravel tons

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Pipe (lin ft)

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Fabric (sq ft)

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Fall needed (in)

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Gravel cost

Gravel includes 10% waste. Pipe includes 10% for connections and outlet. Fabric includes 20% for overlap and top fold. Fall distance is minimum required at selected slope.

Formula: Gravel = (Trench volume − Pipe volume) ÷ 27 × 1.10. Pipe volume = π × (pipe radius in ft)² × length. Fabric = length × (width + depth × 2) × 1.20. Slope fall = length × (slope% ÷ 100) × 12 inches. Use #57 angular stone — not pea gravel for most applications.

Formula Explained — Pipe Displacement

Most French drain calculators give you the full trench volume as gravel needed. Which overestimates the material because it ignores the space the pipe itself occupies. The correct formula subtracts pipe volume from trench volume before converting to cubic yards.

Correct French drain gravel formula: Trench volume (cu ft) = Length × Width (ft) × Depth (ft)
Pipe volume (cu ft) = π × (pipe radius in ft)² × Length
Net gravel (cu ft) = Trench volume − Pipe volume
Cubic yards to order = Net gravel ÷ 27 × 1.10

For a standard 4-inch pipe: radius = 2 inches = 0.167 feet. Pipe volume per linear foot = π × (0.167)² × 1 = 0.0875 cubic feet per foot. A 50-foot drain has 0.0875 × 50 = 4.38 cubic feet of pipe displacement. Not insignificant on a short drain but manageable to calculate with the calculator above.

Why it matters: On a 50-foot standard drain (12 in × 18 in trench), the pipe displacement is about 6 percent of the total trench volume. On a long drain with a small pipe this is minor. On a short drain with a large 6-inch pipe it can be 10 to 15 percent. Enough to meaningfully overorder if ignored.

Which Gravel to Use

Gravel type	Size	Drainage rating	Use case
#57 crushed stone	3/4 inch nominal	Excellent	Standard for all French drain applications
#3 crushed stone	1–2 inches	Excellent	High-volume drains, curtain drains
Pea gravel	3/8–5/8 inch	Good	Shallow decorative drains, light surface runoff only
Drain rock / clean round stone	3/4–1.5 inch	Good	Acceptable alternative to #57 where angular unavailable
Crusher run / DGA	Mixed sizes to dust	Poor — avoid	NOT suitable — fines clog drainage voids
Pea gravel (fine, 1/4 in)	Under 3/8 inch	Poor — avoid	Too fine — migrates through fabric and clogs system

#57 crushed stone is the standard for residential French drains because its angular shape creates and maintains larger void spaces between stones compared to rounded stone. The angular surface also locks stones together under load, preventing void collapse over time. Always use clean (washed) stone. Stone with dust or fines reduces hydraulic conductivity immediately on installation.

Depth Guide by Application

Application	Minimum depth	Standard depth	Pipe position
Surface yard drainage (standing water)	12 inches	18–24 inches	6–8 inches below surface
Curtain drain (slope seepage)	18 inches	24–36 inches	At or below seepage level
Foundation drainage (exterior)	24 inches	36–48 inches	At footing elevation
Basement footing drain	36 inches	48 inches	At or below footing base
Decorative / landscape drain	12 inches	12–18 inches	Bedded in gravel, no pipe needed

Slope Requirements

A French drain relies entirely on gravity to move water from the inlet to the outlet. Without adequate slope, water sits in the pipe and the system does not drain. The slope requirement is simple but the math to verify it is often skipped on DIY projects.

Slope	Fall per 100 feet	Performance	When to use
0.5% (below minimum)	6 inches	Poor — standing water likely	Avoid
1.0% (minimum)	12 inches	Adequate	Flat sites where more slope is not achievable
1.5%	18 inches	Good	Standard residential installation
2.0%	24 inches	Excellent	Preferred specification where site allows
3.0%+	36+ inches	Excellent	Sloped sites — no special measures needed

Before digging, verify the outlet elevation is achievable. Measure the elevation difference between the planned inlet (highest point) and the outlet (lowest point). Divide the elevation drop in inches by the trench length in feet. This is your available slope percentage × 12. If the result is less than 12 (less than 1 percent slope), either move the inlet uphill or the outlet further downhill, or add a sump pump at the outlet to lift the collected water.

Fabric — Burrito Wrap vs Pipe Sock

The landscape fabric method determines how long the French drain performs before becoming clogged with fine soil particles. Most French drain failures are fabric failures, not gravel failures.

Burrito wrap method (recommended):

1. Lay non-woven geotextile fabric in the trench. Allow enough to fold over the top.

2. Add 2 to 3 inches of gravel on the fabric floor.

3. Lay the perforated pipe with holes facing down on the gravel bed.

4. Fill with gravel to 3 to 4 inches above the pipe.

5. Fold the fabric over the top of the gravel, overlapping the edges by 6 inches. Backfill with soil above.

The burrito wrap wraps the entire gravel column in fabric. The effective filter area is the full perimeter of the trench times the length. This large surface area means the filter loads slowly and the drain retains high performance for 25 to 40 years. The pipe sock, by contrast, has only the surface area of the pipe itself. It loads quickly in any soil with silt content and typically begins restricting flow within 3 to 7 years.

Fabric type for French drains: Use non-woven geotextile (spunbond), not woven. Non-woven fabric allows water to pass through in all directions across its surface. Woven fabric restricts lateral flow. Water can only enter through the fabric at approximately right angles to the surface, limiting infiltration capacity in a trench application.

Pipe Hole Direction

Perforated pipe holes must face down (6 o'clock position). A French drain collects groundwater rising from below. The holes at the bottom allow water to enter as it rises through the gravel. Holes facing up only collect water that has already risen to near the top of the gravel column. By which point the soil above may already be saturated.

This is the most common French drain installation error and it significantly reduces system performance without causing any visible problem during installation. Always mark the top of the pipe with a paint line or chalk before placing it in the trench so hole orientation can be verified before covering.

Pre-Calculated Quantities

All figures use standard 12-inch wide × 18-inch deep trench with 4-inch pipe, 1% slope, and 10% waste on gravel. #57 stone at 1.40 t/yd³.

Trench length	Gravel yd³	Gravel tons	Pipe (lin ft)	Fabric (sq ft)	Fall needed (in)
25 feet	0.77	1.07	28	125	3.0
50 feet	1.53	2.15	55	250	6.0
75 feet	2.30	3.22	83	375	9.0
100 feet	3.07	4.30	110	500	12.0
150 feet	4.60	6.44	165	750	18.0
200 feet	6.14	8.59	220	1,000	24.0

Cost Guide 2026

Material	Unit cost	50 ft drain	100 ft drain
#57 crushed stone (bulk)	$28–$45/ton	$60–$97	$120–$194
Perforated PVC pipe (4 in)	$0.40–$0.80/lin ft	$22–$44	$44–$88
Non-woven geotextile fabric	$0.10–$0.20/sq ft	$25–$50	$50–$100
Delivery fee	$60–$120/load	$60–$120	$60–$120
DIY total (materials only)	—	$167–$311	$274–$502
Professional installation	$10–$30/lin ft	$500–$1,500	$1,000–$3,000

Frequently Asked Questions

How do you calculate gravel for a French drain?

Three steps: 1) Trench volume = Length × Width (ft) × Depth (ft). 2) Pipe volume = π × (pipe radius ft)² × Length. 3) Net gravel = (Trench − Pipe) ÷ 27 × 1.10 = cubic yards. Standard 50 ft × 12 in × 18 in trench with 4-in pipe = 1.53 yd³.

What type of gravel is best for a French drain?

#57 clean angular crushed stone (3/4-inch nominal). Angular shape maintains void spaces better than rounded stone. Must be clean — no fines. Pea gravel is acceptable for shallow decorative drains only. Never use crusher run — fines clog drainage voids immediately.

How deep should a French drain be?

Yard drainage (standing water): 18–24 inches. Foundation drainage: 24–48 inches at footing elevation. Curtain drain (slope seepage): 18–36 inches. Minimum practical depth: 18 inches. Shallower risks frost heave of the pipe in cold climates.

What slope does a French drain need?

Minimum 1% (1 inch fall per 8.3 feet of run). Recommended 1.5 to 2%. Required fall = Length (ft) × (slope% ÷ 100) × 12 inches. Verify outlet elevation before digging — if 1% is not achievable, a sump pump at the outlet is needed.

How much landscape fabric for a French drain?

Length × (Width + Depth × 2) × 1.20 = square feet. For 50 ft × 12 in × 18 in trench: 50 × 4 × 1.20 = 240 sq ft. Use non-woven (spunbond) geotextile — not woven. Burrito wrap the entire gravel column rather than using a pipe sock.

Should I use a sock on my French drain pipe?

No. Pipe socks clog with silt within 3–7 years. The burrito wrap method is better — lay non-woven fabric in the trench, add gravel, lay pipe holes down, cover with gravel, fold fabric over the top. The larger filter surface area of the full trench wrap resists clogging far longer.

Which direction should pipe holes face?

Holes down (6 o'clock position). French drains collect groundwater rising from below — holes at the bottom allow water to enter as it rises. Holes facing up only collect water near the top of the gravel. Mark the pipe top with paint before placing to verify orientation.

Can you use pea gravel in a French drain?

Acceptable for shallow decorative drains handling light surface runoff. Not ideal — rounded shape packs tighter than angular stone, reducing drainage capacity. For foundation, curtain, or high-volume drains use #57 or #3 angular crushed stone. Never use fine pea gravel under 3/8 inch in any French drain.

How long does a French drain last?

Burrito wrap method: 30–40 years. Pipe sock method: 7–15 years before clogging reduces performance significantly. Signs of failure: area re-saturates, outlet flow reduces noticeably. High-pressure flush through a cleanout can extend life before full replacement is needed.

How much does a French drain cost?

DIY materials: $1.20–$2.40 per linear foot. 50 ft: $167–$311. 100 ft: $274–$502. Professional installation: $10–$30 per linear foot (50 ft: $500–$1,500). Main DIY costs: gravel, perforated pipe, non-woven fabric, delivery.

Do I need a permit for a French drain?

Usually no permit for residential drains discharging to own property. Permits may apply near property lines or connecting to municipal storm drains. Always call 811 (Dig Safe) before digging — required by law in every US state regardless of permit status.

Related Calculators

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Cubic Yards to Tons

Convert crushed stone and drainage gravel between cubic yards and short tons. Covers 18 material types.

Sources & Methodology

USGS — Natural Aggregates Statistics — aggregate density values (#57 stone 1.40 t/yd³)
OSHA — Trenching and Excavation Safety — call 811 requirement and excavation safety standards

Formula: Net gravel = (Trench volume − Pipe volume) ÷ 27 × 1.10. Pipe volume = π × r² × L where r = pipe radius in feet. Fabric = L × (W + D × 2) × 1.20. Slope fall = L × (slope% ÷ 100) × 12 inches. Minimum slope: 1% from civil engineering drainage practice. Full methodology

Last reviewed: June 2026