Why Calculate French Drain Materials Accurately?
French drains channel standing groundwater away from basement foundations, crawlspaces, low lawn spots, and retaining walls: - Prevent Soil Silt Clogging: Wrapping the trench in non-woven geotextile fabric keeps mud out of gravel voids, ensuring a 30+ year lifespan. - Deduct Pipe Displacement: Subtracting 4-inch pipe volume prevents over-ordering expensive washed gravel tons. - Ensure Proper Discharge: Calculating trench volume ensures sufficient gravel depth below and above the perforated pipe.
French Drain Governing Formulas
\[\text{Gross Trench Volume (cu ft)} = \text{Length (ft)} \times \left(\frac{\text{Width (in)}}{12}\right) \times \left(\frac{\text{Depth (in)}}{12}\right)\] \[\text{Pipe Displacement (cu ft)} = \pi \times \left(\frac{\text{Pipe Diameter (in)}}{2 \times 12}\right)^2 \times \text{Length (ft)}\] \[\text{Net Gravel Volume (cu yds)} = \frac{(\text{Gross Trench Volume} - \text{Pipe Displacement})}{27} \times 1.10 \quad \text{(10\% compaction allowance)}\] \[\text{Gravel Weight (Tons)} = \text{Net Gravel Volume (cu yds)} \times 1.35 \text{ tons/cu yd}\] \[\text{Filter Fabric Perimeter (ft)} = 2 \times \left(\frac{\text{Width (in)}}{12}\right) + 2 \times \left(\frac{\text{Depth (in)}}{12}\right) + 1.0 \text{ ft (Overlap)}\] \[\text{Filter Fabric Area (sq ft)} = \text{Fabric Perimeter (ft)} \times \text{Length (ft)}\]Drainage Material Requirements Table (12” Wide Trench, 4” Pipe)
| Trench Length | Trench Depth | Perforated Pipe | Washed Gravel (Cu Yds) | Gravel Weight (Tons) | Filter Fabric (Sq Ft) | Est. Material Cost |
|---|---|---|---|---|---|---|
| 30 ft Trench | 24 in | 32 ft | 2.01 cu yds | 2.71 tons | 210 sq ft (1 roll) | $271.00 |
| 50 ft Trench | 24 in | 53 ft | 3.35 cu yds | 4.52 tons | 350 sq ft (2 rolls) | $452.00 |
| 80 ft Trench | 24 in | 84 ft | 5.36 cu yds | 7.24 tons | 560 sq ft (2 rolls) | $723.00 |
| 100 ft Trench | 30 in | 105 ft | 8.52 cu yds | 11.50 tons | 800 sq ft (3 rolls) | $1,114.00 |
Step-by-Step French Drain Installation Guide
- Measure Drainage Trench Run: Input total linear feet of trench from high collection area to low gravity discharge point.
- Select Trench Dimensions: Choose 12” width and 24” depth for standard yard water control.
- Select Pipe Size: Choose 4-inch smooth-wall perforated pipe for optimal flow speed.
- Input Local Unit Prices: Enter cost per cubic yard of #57 stone, 300 sq ft fabric rolls, and pipe per foot.
- Review Drainage List: Order required perforated pipe, crushed gravel tons, and geotextile filter fabric rolls.
Frequently Asked Questions
What is the best gravel size for a french drain?
Clean washed #57 stone (3/4-inch to 1-inch crushed aggregate) is the industry standard for french drains. It leaves 45% open void space for rapid groundwater flow while supporting heavy soil loads.
Why is geotextile filter fabric required in a french drain?
Non-woven geotextile filter fabric wraps the entire gravel trench, acting as a sieve that lets groundwater enter while permanently blocking dirt silt, clay particles, and sand from clogging the gravel voids and pipe holes.
How much slope is needed for a french drain?
A french drain requires a minimum downward slope of 1 inch for every 10 feet of trench length (a 1% slope gradient) toward the gravity outlet.
Should perforated pipe holes face up or down in a french drain?
Perforated pipe holes must ALWAYS face DOWN toward the bottom of the trench. Water rises from the water table into the bottom of the gravel bed and flows into the pipe holes from underneath.
How many tons of gravel are in a cubic yard?
One cubic yard of clean washed #57 drainage gravel weighs approximately 1.35 tons (2,700 lbs).
How much does it cost to install a french drain per foot?
DIY materials cost $8 to $14 per linear foot for gravel, pipe, and fabric. Professional contractor installation costs $30 to $65 per linear foot depending on trench depth and hand vs machine digging.
Is my personal data saved when using this calculator?
No. All calculation logic operates locally in your web browser.