French Drain Calculator
Estimate materials for French drain installation.
French Drain Materials by Length
12" wide × 18" deep trench, 4" pipe
| Drain Length | Gravel (tons) | Pipe (ft) | Fabric (sq ft) | Est. Cost |
|---|---|---|---|---|
| 25 ft | 0.9 | 25 | 88 | $41 |
| 50 ft | 1.8 | 50 | 175 | $81 |
| 75 ft | 2.6 | 75 | 263 | $117 |
| 100 ft | 3.5 | 100 | 350 | $158 |
| 150 ft | 5.3 | 150 | 525 | $239 |
| 200 ft | 7.0 | 200 | 700 | $315 |
How We Calculate This
This french drain calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
This calculator estimates French drain materials by computing trench volume, then calculating gravel fill (minus pipe displacement), fabric liner requirements, and perforated pipe length.
The basic rule:
- Trench volume (cu ft) = length × width × depth (all in feet)
- Gravel volume = trench volume minus pipe volume (pipe displaces ~10% of trench)
- Gravel weight: washed drainage gravel weighs approximately 1.4 tons per cubic yard
- Landscape fabric: enough to line the bottom and sides with 6" overlap at seams
- Minimum drain slope: 1% grade (1/8" per foot, or 1" per 8 feet)
Use clean, washed gravel (3/4" to 1.5" stone) — not crushed limestone or pea gravel, which can clog. The perforated pipe should sit on 2-3 inches of gravel, holes down (if using rigid pipe) or surrounded by gravel. Wrap the entire gravel bed in landscape fabric to prevent silt infiltration.
When Would You Use This Calculator?
This french drain calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
How deep should a French drain be?
Residential French drains are typically 18-24 inches deep. For footing drains, go to the depth of the footing (often 36-48 inches). The drain must be deep enough to be below the water problem — water flows down to the drain by gravity. Minimum practical depth is 12 inches.
What size gravel for a French drain?
Use 3/4" to 1.5" clean, washed gravel (river rock or granite). Avoid fine gravel, pea gravel (too small, clogs), crushed limestone (produces fines), or recycled concrete (clogs over time). The gravel should have void space for water to flow through.
How much does a French drain cost per foot?
DIY French drains cost $8-$15 per linear foot for materials. Professional installation runs $20-$50 per foot for exterior drains, or $40-$100+ per foot for interior basement drains (due to concrete cutting and waterproofing). A 50-foot DIY drain costs $400-$750 in materials.
Does a French drain need landscape fabric?
Yes, always use non-woven geotextile landscape fabric. It prevents soil from migrating into the gravel and clogging the drain over time. Line the entire trench before adding gravel, then fold the fabric over the top before backfilling with soil.
Should perforated pipe holes face up or down?
This is debated, but for most residential applications, holes face DOWN. Water rises into the pipe from below as the gravel bed saturates. The pipe then carries water away by gravity slope. Some manufacturers recommend holes up for intercepting surface water.
Where should a French drain discharge?
French drains should discharge to: a storm drain, a dry well, a rain garden, daylight (lower point on property), or a sump pump pit. Never discharge onto a neighbor's property or into a sanitary sewer. Check local codes for allowed discharge points.