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Drainage Calculator

Calculate gravel, pipe, and fabric for French drain projects

Gravel Needed

2.62 yd³

Gravel Weight

3.53 tons

Total Drop

6.3 in

Total Cost

$242

Units

Standard: 1/8" per foot (0.125). Min recommended: 1/16" per foot.

Recommended to prevent soil from clogging the drain

$
$
Gravel (yd³)

2.62

Gravel Weight

3.53 tons

Total Drop

6.3 in

Total Cost

$242

Materials Needed

Gravel2.62 yd³ (3.53 tons)
Drain Pipe50 ft (5 sections)
Landscape Fabric200 sq ft (1 roll)
Total Drop6.3 in

Cost Breakdown

Gravel$92
Pipe$100
Fabric$30
Fittings$20
Gravel$92
Pipe$100
Fabric$30
Fittings (est.)$20
Total Cost$242

Example Calculations

1Standard French Drain (50 ft, 12" wide, 18" deep)

Inputs

Drain Length50 ft
Trench Width12 inches
Trench Depth18 inches
Pipe Diameter4 inches
Gravel Type3/4" Washed
Landscape FabricYes
Slope1/8" per foot

Result

Gravel Needed2.72 yd³
Gravel Weight4.07 tons
Fabric Area200 sq ft (1 roll)
Total Drop6.3 inches
Total Cost$265

Trench = 50 × (12/12) × (18/12) = 75 cu ft. Pipe = π × (4/24)² × 50 = 1.45 cu ft. Gravel = 73.55 cu ft = 2.72 yd³. Fabric = 50 × (12+36)/12 = 200 sq ft. Drop = 50 × 0.125 = 6.25".

2Foundation Drain (80 ft, 12" wide, 24" deep)

Inputs

Drain Length80 ft
Trench Width12 inches
Trench Depth24 inches
Pipe Diameter6 inches
Gravel Type3/4" Washed
Landscape FabricYes
Slope1/8" per foot

Result

Gravel Needed5.77 yd³
Gravel Weight8.63 tons
Fabric Area400 sq ft (2 rolls)
Total Drop10.0 inches
Total Cost$473

Trench = 80 × 1 × 2 = 160 cu ft. Pipe = π × (6/24)² × 80 = 3.93 cu ft. Gravel = 156.07 cu ft = 5.78 yd³. Fabric = 80 × (12+48)/12 = 400 sq ft. Drop = 80 × 0.125 = 10.0".

Frequently Asked Questions

Q

How much gravel do I need for a French drain?

Calculate the trench volume (length × width × depth) and subtract the pipe volume. For a 50 ft drain with a 12" wide, 18" deep trench and 4" pipe, you need about 2.7 cubic yards of gravel.

  • Formula: (L × W × D in feet) – pipe volume = gravel in cubic feet
  • Convert to cubic yards: divide cubic feet by 27
  • Gravel weighs ~1.4–1.5 tons per cubic yard (order by ton for bulk)
  • Use 3/4" washed/clean gravel – NOT crushed limestone (it packs tight)
  • Budget $30–$50 per ton for gravel, delivered
Trench Size (W × D)Gravel per 10 ftGravel per 50 ftWeight (50 ft)
9" × 18"0.40 yd³2.0 yd³~3.0 tons
12" × 18"0.55 yd³2.7 yd³~4.1 tons
12" × 24"0.73 yd³3.6 yd³~5.4 tons
18" × 24"1.10 yd³5.5 yd³~8.3 tons
Q

What slope does a French drain need?

A French drain needs a minimum slope of 1/8 inch per foot (about 1% grade). This means over a 50-foot run, the drain should drop at least 6.25 inches from start to end.

  • Minimum: 1/8" per foot (1% grade) – gravity does the work
  • Ideal: 1/4" per foot (2% grade) – faster drainage, less clogging
  • Use a string line or laser level to verify slope during trenching
  • No slope = standing water in pipe = mosquitoes + eventual failure
  • For 50 ft run at 1/8"/ft: inlet must be 6.25" higher than outlet
Slope RateDrop per 10 ftDrop per 50 ftUse Case
1/8" per ft (min)1.25"6.25"Flat yards with limited fall
1/4" per ft (ideal)2.5"12.5"Standard French drains
1/2" per ft (steep)5.0"25.0"Short runs, fast drainage
Q

What size gravel is best for a French drain?

3/4-inch washed gravel is the most common choice for French drains. It provides good drainage flow while preventing pipe clogging. Avoid fine gravel or crushed stone that can pack together and slow water movement.

  • 3/4" washed/river gravel is the gold standard – smooth, round, drains fast
  • NEVER use crushed limestone or pea gravel – they compact and clog
  • Washed means no fines/dust that would silt up the pipe over time
  • Layer 2–3" of gravel below pipe, fill remaining trench above pipe
  • Cost: $30–$50/ton; 1 ton covers roughly 10–12 linear ft of 12"×18" trench
Q

Do I need landscape fabric for a French drain?

Landscape fabric is recommended to wrap the gravel and prevent soil from migrating into the drain and clogging it over time. Without fabric, the drain may need replacement in 5-10 years instead of lasting 20+ years.

  • Use non-woven geotextile fabric (4–6 oz weight) – NOT the cheap woven weed barrier
  • Wrap fabric around ALL sides of the gravel, overlapping on top by 6"
  • Fabric formula: Length × (Width + 2 × Depth + 12") for overlap
  • Standard roll: 3 ft or 6 ft wide × 100–300 ft – $50–$150 per roll
  • Clay soil = fabric is essential; sandy soil = fabric still recommended
FactorWith FabricWithout Fabric
Lifespan20–30+ years5–10 years
Clogging riskLowHigh (soil migration)
Added cost$0.15–$0.50/sq ft$0
Installation time+15–20 minutesNone
Q

How deep should a French drain be?

French drains are typically 18-24 inches deep and 9-12 inches wide. For surface water drainage, 18 inches is sufficient. For foundation drainage or intercepting groundwater, dig 24 inches or deeper.

  • 18" deep: Surface water, soggy lawn, downspout drainage
  • 24" deep: Foundation perimeter drains, groundwater interception
  • 36"+ deep: Footer drains, high water table areas
  • Width: 9–12" for most residential; 18" for heavy-flow areas
  • Call 811 before digging – utility lines can be as shallow as 12"
Drain PurposeDepthWidthPipe Size
Yard drainage (surface)18"9–12"4" perforated
Foundation perimeter24"12"4–6" perforated
Groundwater intercept24–36"12–18"6" perforated
Footer/footing drainAt footer depth12–18"4–6" perforated

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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