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How Much Gravel for a 100 ft French Drain (18" × 24" Trench)?

Instant Pre-Solved Contractor Answer (US Standards)

You need 15.03 tons of #57 washed drainage gravel (10.74 cu yd net) with 88" wide non-woven geotextile trench fabric.

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💧 French Drain Trench Dimensions

100 ft
10 ft 150 ft 300 ft
ℹ️ Pipe Displacement Accounted For: A 4" corrugated pipe displaces ~10.085 cu ft of gravel. We subtract this volume to prevent you from overpaying for stone.
Net Drainage Stone Needed

7.25 tons (#57 Washed Gravel)

Net Gravel Volume

5.18 cu yd

Gross: 150 cu ft

Pipe Displacement: -10.085 cu ft Fabric Width: 64" wide roll Total Fabric: ~586.7 sq ft

French Drain Cross-Section (Burrito Wrap)

12"W × 18"D
Lawn Grade Lawn Grade Geotextile Overlap (64" roll) 4" Pipe 2" Stone Bedding 12" Trench Width 18" Trench Depth

🛒 Drainage Material Shopping List

#57 Washed Drainage Gravel 1.4 tons/yd³ bulk density
7.25 tons (5.18 cu yd net)
4" Corrugated Perforated Pipe Slotted drain pipe with sock or bare
1 coil(s) (100') Check Home Depot ›
Non-Woven Geotextile Fabric 4 oz/yd² needle-punched (no slit woven)
64" wide roll Check Amazon ›
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Digging a 100' trench by hand takes 18+ hours.

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📐 Exact US Engineering Formula Breakdown

// 1. Gross Excavated Trench Volume Gross Volume = Length (100') × [Width (18") ÷ 12] × [Depth (24") ÷ 12] Gross Volume = 300 cu ft // 2. Corrugated Drain Pipe Displacement (OD = 4.3") Pipe Displacement = π × (2.15" ÷ 12)² × 100' = 10.08 cu ft // 3. Net Washed #57 Gravel Weight Net Cubic Yards = (300 - 10.08) ÷ 27 = 10.74 cu yd Gravel Weight (Tons) = 10.74 cu yd × 1.4 tons/yd³ = 15.03 Tons // 4. Non-Woven Geotextile Fabric Width (Burrito Wrap) Fabric Width = Width (18") + [Depth (24") × 2] + 4" Overlap = 88"

Cost Comparison: DIY Materials vs. Turnkey Professional Installation

2026 US Contractor Averages
Option / Method Quantity / Specs Estimated Cost Labor Hours
DIY: Bulk #57 Stone + Pipe + Fabric 15.03 tons stone + 100' pipe $931 13 hrs
Professional Drainage Contractor (Turnkey) Laser grading, trenching, gravel, turf repair $3,200 1 day

Contractor Installation & Engineering Guide: How Much Gravel for a 100 ft French Drain (18" × 24" Trench)?

💡 Jobsite Recommendation

For a 100 ft French drain in a 18"×24" trench, you need 15.03 tons of #57 washed drainage stone. A 4" corrugated pipe displaces 10.1 cu ft of void space. Line the trench with 88" wide 4 oz non-woven geotextile fabric to prevent clay silt clogging.

1. Engineering Specifications & Material Requirements

When executing a project of this scale, relying on standard generic rules of thumb leads to premature cracking, settling, or water backup. For structural pours and hardscaping, every cubic yard weighs approximately 4,000 lbs, exerting substantial compressive dead loads onto the sub-grade. Ensuring appropriate compressive strength (standard 4,000 PSI at 28-day cure for outdoor slabs, or 3,000 PSI for post hole collars) guarantees structural integrity against vehicle loading, frost heaves, and seasonal thermal expansion.

2. Sub-Base Excavation & Compaction Protocol

Over 80% of outdoor construction failures originate beneath the material layer rather than within the concrete or timber itself. Remove all organic topsoil, sod, and loose clay until reaching solid undisturbed subsoil. For flatwork and paver bases, install a minimum 4-inch lift of #57 crushed limestone or dense grade aggregate (#411 crusher run). Compact the base aggregate in maximum 2-inch lifts using a mechanical gas-powered vibratory plate compactor, maintaining a steady 1/4-inch per foot slope away from building foundations for positive water runoff.

3. Step-by-Step Installation & Application Sequence

  • Layout & Staking: Establish perimeter lines with mason's string and batter boards. Check diagonal squareness using the 3-4-5 Pythagorean theorem before driving form stakes.
  • Forming & Reinforcement: Secure 2×4 or 2×6 dimensional lumber forms with duplex nails. Elevate rebar grid (#4 1/2" deformed steel) on 2-inch plastic chairs to embed steel in the bottom third of the tension zone.
  • Pouring & Consolidation: Deposit material continuously to prevent cold joints. Consolidate with a mechanical pencil vibrator or edge screed to eliminate internal honeycombing and air pockets.
  • Finishing & Texture: Screed flush with forms, bull float immediately to bring paste to the surface, and delay final brooming or edging until all bleed water has evaporated.

4. US Building Code, Permitting & Local Inspections

Under the US International Residential Code (IRC) and local municipal building ordinances, projects exceeding specific square footage or depth thresholds frequently require zoning permits and inspection sign-offs:

  • Frost Line Depth: Post footings and structural foundations must extend below local frost depth (typically 36" to 48" in northern states, 12" to 24" in southern states) to avoid frost-jack displacement.
  • Permit Thresholds: Flatwork within 5 feet of property lines, accessory buildings exceeding 120 sq ft, or electrical mini-split branch circuits typically require municipal permits and licensed trade sign-offs.
  • Call Before You Dig: Dial 811 at least 48 to 72 business hours before excavating to locate underground gas, power, fiber, and water mains free of charge.

5. Pro Contractor Tips to Prevent Costly DIY Pitfalls

Never add excess water to speed up concrete mixing or epoxy pouring—each additional gallon of water per yard drops concrete strength by approximately 500 PSI and causes severe surface scaling. When working in temperatures above 85°F, use shade tarps or morning pour schedules to prevent flash-setting. Apply a curing compound or damp burlap blanket for at least 7 continuous days to maximize full design strength.

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❓ Frequently Asked Questions: How Much Gravel for a 100 ft French Drain (18" × 24" Trench)?

You need 15.03 tons of #57 crushed stone (10.74 cu yd net) after subtracting the volume displaced by the 4" corrugated drain pipe.
You need 88" wide non-woven geotextile fabric (calculated as 18" bottom + (48") sides + 4" top overlap).
Always install perforated French drain pipe with the holes facing DOWN at the 4 o'clock and 8 o'clock positions. This allows rising groundwater to enter the pipe and flow away before saturating the lawn.
Maintain a minimum slope of 1% (1 inch of fall per 8 feet of pipe run, or approximately 12.5 inches total drop over 100 feet).