French Drain Rock Calculator

Q: What size rock is best for a French drain?

The industry standard rock for a French drain is 3/4-inch to 1-1/2-inch washed crushed aggregate (commonly designated as #57 washed stone or #4 rock). Clean, washed stone has high void space (approx. 40% void ratio) which allows water to flow rapidly into the perforated pipe. Avoid pea gravel or unwashed crusher run, as fine particles and stone dust quickly silt up the fabric and choke water infiltration.

Q: How much gravel do I need for a 100-foot French drain?

For a standard 100-foot long French drain that is 12 inches wide and 18 inches deep with a 4-inch perforated pipe deduction, you need approximately 5.2 net cubic yards (approx. 7.0 US tons or 280 fifty-pound bags) of washed #57 stone including a 10% safety buffer for trench irregularities.

Q: Should I use round river rock or crushed limestone in a French drain?

Both work exceptionally well provided they are thoroughly washed of dust and fines. Angular crushed limestone or granite (#57 stone) interlocks slightly better and supports heavy lawn traffic without settling unevenly. Smooth round river rock has a slightly higher void ratio and provides superior natural water flow, making it ideal if the drain terminates into an open decorative dry creek bed.

Q: How much pipe volume should be subtracted from trench gravel calculations?

A standard 4-inch outer diameter perforated corrugated or SDR-35 PVC pipe displaces approximately 0.087 cubic feet of volume per linear foot (approx. 8.7 cubic feet or 0.32 cubic yards per 100 feet). A larger 6-inch commercial drainage pipe displaces approximately 0.196 cubic feet per linear foot (approx. 19.6 cubic feet or 0.73 cubic yards per 100 feet). Subtracting pipe displacement prevents over-ordering expensive bulk rock.

Q: Why must drainage stone be washed before backfilling?

Drainage stone must be washed to eliminate quarry stone dust, clay, and rock fines. If unwashed aggregate (like road base or Crusher Run) is dumped into a French drain, the first heavy rain washes the stone dust into the non-woven geotextile fabric and pipe perforations, creating an impenetrable mud cake that clogs and ruins the drainage system within months.

Engineering Formula

📐 How Much Gravel Do I Need for a French Drain? (Trench Volume & Pipe Displacement Formula)

Calculating the exact volume of french drain gravel requires determining the three-dimensional cubic volume of the excavated trench, subtracting the physical volume displaced by the perforated drainage pipe, converting the net volume to cubic yards, and multiplying by the bulk density of washed stone to determine how many tons of rock for french drain backfilling are needed.

// French Drain Rock Calculator Formula (US Engineering Standard):

Gross Trench Volume (cu. ft.) = Length (ft) × [ Width (in) ÷ 12 ] × [ Depth (in) ÷ 12 ]

Pipe Displacement Volume (cu. ft.) = Length (ft) × 0.087 cu. ft./ft (for standard 4" pipe)

Net Gravel Volume (cu. ft.) = Gross Trench Volume - Pipe Displacement Volume

Total Cubic Yards = ( Net Volume ÷ 27 ) × 1.10 (Over-excavation & Settling Buffer)

Total US Tons = Total Cubic Yards × 1.35 Tons/yd³ (Washed #57 Stone Density)

How Much Gravel Do I Need for a 100 ft French Drain? (Step-by-Step Math)

Suppose you are installing a standard residential 100-foot French drain that is 12 inches wide and 18 inches deep, utilizing a standard 4-inch perforated drainage pipe:

Step 1: Find Gross Trench Volume

Multiply length by width in feet by depth in feet. For a 100-foot trench at 12" wide and 18" deep: 100 ft × (12 ÷ 12) × (18 ÷ 12) = 100 × 1.0 ft × 1.5 ft = 150.0 cubic feet.

Step 2: Calculate Pipe Displacement Volume

Deduct the volume occupied by the drainage pipe. Standard 4-inch perforated corrugated or PVC pipe displaces 0.087 cubic feet per linear foot: 100 ft × 0.087 = 8.7 cubic feet of displaced volume.

Step 3: Determine Net Gravel Volume

Subtract pipe displacement from gross trench volume: 150.0 cu. ft. - 8.7 cu. ft. = 141.3 net cubic feet.

Step 4: Convert to Net Cubic Yards

Divide total cubic feet by 27: 141.3 cu. ft. ÷ 27 = 5.23 net cubic yards.

Step 5: Apply a 10% Safety Buffer

Account for uneven trench excavation and rock settling by multiplying net yardage by 1.10: 5.23 cu. yd. × 1.10 = 5.75 gross cubic yards.

Step 6: Convert to US Tonnage & Truckloads

Multiply gross cubic yards by clean washed #57 stone density (1.35 tons/yd³): 5.75 cu. yd. × 1.35 = 7.76 US Tons (requiring 1 standard tandem dump truck load). To convert any drainage aggregate volume to weight at exact quarry densities, see our cubic yards to tons converter.

📊 Popular French Drain Dimensions: Quick Tonnage & Pipe Estimates

Contractor-calculated estimates for high-volume residential drainage trench configurations (calibrated with a 10% over-excavation buffer and 4" perforated pipe deduction using clean washed #57 stone):

25' Foundation Wall Run

1.9 Tons

1.4 yd³ • 76 Bags (12"×18")

Short foundation run: Diverts roof runoff away from basement walls. Can be transported with a heavy utility trailer or pickup truck bed.

50' Yard Swale Drain

3.9 Tons

2.9 yd³ • 155 Bags (12"×18")

50 foot french drain rock calculator: Perfect for soggy backyards and pooling lawn hollows. Fits comfortably in 1 single-axle dump truck.

100' Standard Yard Drain

7.8 Tons

5.8 yd³ • 310 Bags (12"×18")

How much gravel for 100 ft french drain: ~7.8 tons of washed stone. Order 1 full tandem dump truck delivery to avoid split-load freight charges.

150' Perimeter Property Run

11.6 Tons

8.6 yd³ • 465 Bags (12"×18")

Boundary curtain drain: Intercepts neighboring hillside groundwater runoff before it floods your turf and landscaped patio areas.

🪨 Choosing the Right French Drain Gravel (What Size Rock for French Drain?)

Selecting the proper aggregate size is critical. The rock must provide maximum void space for rapid subterranean water conveyance while preventing stone compaction and silt migration. Here is how drainage stone specifications compare:

Rock Classification	Sizing Spec	Void Space Ratio	Water Flow Rating	Best Drainage Application
#57 Washed Crushed Stone	3/4" – 1" (Angular)	~40% to 45%	Superior	#1 Industry Standard: Residential yard drains, pipe bedding, and foundation weeping tiles.
Washed Round River Rock	3/4" – 1-1/2" (Smooth)	~42% to 48%	Maximum	Open dry creek river rock beds, decorative swales, and daylight discharge basins.
#4 Washed Drainage Ballast	1-1/2" – 2-1/2" (Coarse)	~45% to 50%	High Volume	Deep hillside interceptor curtain drains, agricultural ditches, and muddy clay subgrades.
Pea Gravel (3/8" Rounded)	1/4" – 3/8" (Small)	~25% to 30%	Moderate / Fair	Caution: Only for very shallow swales. Packs too tightly; small stones can slip through pipe slots. Better suited for pea gravel patios.
Crusher Run / ABC / Road Base	3/4" with Stone Dust	< 10% (Dense)	Impermeable	NEVER USE: Stone dust and fines cement together, completely blocking water flow. For driveways, use our gravel driveway calculator or crushed stone calculator instead.

💧 Why Must Drainage Stone Be Washed Before Backfilling?

The number one cause of premature French drain failure within 12 to 24 months is the use of unwashed aggregate. Unwashed gravel directly from an unrinsed quarry screen contains up to 8% to 15% pulverised limestone dust and clay clods.

💧 Silt Cake Clogging

When rainwater percolates through unwashed stone, stone dust is carried downward directly into the geotextile filter fabric pores. Over several rain cycles, this dust consolidates into an impervious concrete-like mud cake, trapping water on top of the trench.

🌀 Pipe Slot Choking

Perforated pipes rely on precision laser slits or circular weep holes. Fine grit enters these openings and settles at the bottom invert of the pipe, reducing flow cross-section and providing a breeding ground for iron ochre bacteria and root intrusion.

🛡️ Void Ratio Preservation

Washed 3/4" #57 stone preserves its engineered 40%+ void ratio. This empty space acts as an underground retention chamber that holds hundreds of gallons of surge runoff while the pipe continuously discharges water downstream.

🛡️ The Filter Fabric Burrito Method: Why Non-Woven Geotextile Is Mandatory

A French drain is only as durable as its filter fabric envelope. Without proper geotextile encapsulation, the surrounding native soil will inevitably wash into the stone voids, completely clogging the drain within 3 to 5 years.

Key Rules for Fabric Installation (The Burrito Method):

Always Use Non-Woven Needle-Punched Fabric (4 oz to 6 oz): Non-woven fabric looks like dense felt. It allows water to pass through freely (90–120 GPM/sq. ft.) while stopping silt particles as small as 75 microns. Never use cheap woven black weed barrier fabric; woven plastic creates a slick barrier that blinds over and repels water.
Line the Entire Trench Perimeter: Drape the fabric into the open ditch so it rests flat on the bottom and runs up both side walls, leaving at least 12 inches of excess fabric folded back on the grass.
The 12-Inch Top Overlap & Topsoil Cover: Once the 2-inch rock bed, perforated pipe, and rock backfill are installed to within 4 inches of the surface, fold the left and right flaps over the top of the rock with a minimum 12-inch overlap. This creates a completely sealed "burrito" that prevents topsoil from ever mixing with the clean stone. Calculate clean loam needed to replant grass over your trench with our topsoil calculator.

💵 Estimated French Drain Material Costs & Contractor Pricing

Understanding the cost structure between self-installing (DIY) and hiring a professional drainage excavation contractor allows you to budget accurately:

DIY French Drain Cost per Foot

$5 – $8 / Lin. Ft.

Total for 100 ft: $500 – $800 (Materials only)

• Washed #57 bulk stone: ~$3.00 to $4.50 / ft
• 4" perforated corrugated pipe: ~$1.00 to $1.50 / ft
• 4oz non-woven geotextile fabric: ~$0.80 to $1.20 / ft
• Optional walk-behind trencher rental: ~$200 to $300 / day

Professional Contractor Turnkey Install

$25 – $45 / Lin. Ft.

Total for 100 ft: $2,500 – $4,500 (Turnkey)

• Includes laser-guided machine excavation & grading
• Bulk aggregate hauling, spreading, and pipe laying
• Native soil spoil hauling and off-site disposal
• Turf sod restoration and discharge pop-up emitter installation

❓ Frequently Asked French Drain Questions

Contractor answers to common rock sizing, pipe placement, and drainage volume calculation questions.

What size rock is best for a French drain?

The industry standard rock for a French drain is 3/4-inch to 1-1/2-inch washed crushed aggregate (commonly designated as #57 washed stone or #4 drain rock). Clean, washed stone has high void space (approx. 40% void ratio) which allows water to flow rapidly into the perforated pipe. Avoid pea gravel or unwashed crusher run, as fine particles and stone dust quickly silt up the fabric and choke water infiltration.

How much gravel do I need for a 100-foot French drain?

For a standard 100-foot long French drain that is 12 inches wide and 18 inches deep with a 4-inch perforated pipe deduction, you need approximately 5.2 to 5.8 net cubic yards (approx. 7.0 to 7.8 US tons or 280 to 310 fifty-pound bags) of washed #57 stone, including a 10% safety buffer for trench irregularities.

Should I use round river rock or crushed limestone in a French drain?

Both work exceptionally well provided they are thoroughly washed of dust and fines. Angular crushed limestone or granite (#57 stone) interlocks slightly better and supports heavy lawn traffic without settling unevenly. Smooth round river rock has a slightly higher void ratio and provides superior natural water flow, making it ideal if the drain terminates into an open decorative dry creek bed.

How much pipe volume should be subtracted from trench gravel calculations?

A standard 4-inch outer diameter perforated corrugated or SDR-35 PVC pipe displaces approximately 0.087 cubic feet of volume per linear foot (approx. 8.7 cubic feet or 0.32 cubic yards per 100 feet). A larger 6-inch commercial drainage pipe displaces approximately 0.196 cubic feet per linear foot (approx. 19.6 cubic feet or 0.73 cubic yards per 100 feet). Subtracting pipe displacement prevents over-ordering expensive bulk rock.

Why must drainage stone be washed before backfilling?

Drainage stone must be washed to eliminate quarry stone dust, clay, and rock fines. If unwashed aggregate (like road base or Crusher Run) is dumped into a French drain, the first heavy rain washes the stone dust into the non-woven geotextile fabric and pipe perforations, creating an impenetrable mud cake that clogs and ruins the drainage system within months.