Landscaping · updated 2026
French Drain Calculator
A French drain is three material estimates in one: the drain rock that fills the trench, the perforated pipe that carries the water, and the landscape fabric that keeps soil out of the rock. This French drain calculator takes your trench's length, width, and depth and returns all three — rock in tons at 100 lbs per cubic foot, pipe in feet (sold in 10-ft sticks), and fabric in square feet to line the trench — with 2026 material costs. Slope the trench 1% toward daylight and the water handles itself.
1 ft wide is standard for a 4-in pipe with rock around it
18–24 in is typical; go deeper to intercept subsurface water
Show the math
Trench volume = 50 × 1 × 1.50 = 75 cu ft
Rock = 75 × 100 lbs/cu ft ÷ 2000 = 3.75 tons
Pipe = 50 ft = ⌈50 ÷ 10⌉ = 5 ten-ft sticks
Fabric = 50 × (1 + 2 × 1.50) = 200.0 sq ft (lines bottom and both sides)
Cost = 3.75 × $40–$60/ton + 50 × $0.80–$1.50/ft + 200.0 × $0.15–$0.30/sq ft
What the numbers mean
The calculator converts your measurements into a practical purchase quantity using a documented coverage or spacing assumption.
Any whole-unit quantity is rounded up so the result does not leave you short during the project.
Use the cost range as a planning baseline and confirm the final quantity, availability, and delivery charge with your local supplier.
2026 material prices
| Material | 2026 average price | Note |
|---|---|---|
| Drain rock / #57 stone, per ton | $40.00 – $60.00 / ton | Bulk delivered |
| 4-in perforated pipe, per ft | $0.80 – $1.50 / linear ft | Solid or sock-wrapped |
| Landscape fabric, per sq ft | $0.15 – $0.30 / sq ft | Non-woven; lets water through, blocks soil |
Material prices: Wave 1 Batch 2 2026 national-average guide, Sep 2026. Local prices vary. 2026 U.S. national averages. Prices vary by region and retailer — confirm locally before buying.
Pro tips
- Holes go DOWN — the pipe collects water that rises from below; perforations-up is the most common DIY mistake.
- Slope the trench 1% (1 in of fall per 8 ft) toward daylight or a dry well — a level French drain is just a buried pond.
- Wrap the trench in fabric burrito-style (overlap the top) or use sock-wrapped pipe — soil infiltration is what kills drains.
- Never use round pea gravel; angular #57 stone locks together and won't migrate into the pipe slots.
- Daylight the outlet where you can see it — a buried-to-nowhere drain fails silently and you’ll never know.
- A 50-ft trench (1 ft × 1.5 ft) needs 3.8 tons of rock, 50 ft of pipe, and 200 sq ft of fabric — roughly $220–$360 in materials.
Common questions
Frequently asked questions
How much rock for a 50-ft French drain (1 ft wide, 1.5 ft deep)?
75 cu ft × 100 lbs = 7,500 lbs = 3.8 tons, plus 50 ft of pipe (5 sticks) and 200 sq ft of fabric.
How about a 100-ft drain?
Double it: 7.5 tons of rock, 100 ft of pipe (10 sticks), 400 sq ft of fabric.
How deep should a French drain be?
18–24 in for surface water; deeper (2–4 ft) to intercept subsurface flow or a high water table.
What slope does a French drain need?
1% minimum — 1 in of fall per 8 ft of run, toward daylight or a dry well.
Do the pipe holes face up or down?
Down — groundwater rises into the pipe from below; holes-up only catches water already above the pipe.
Pipe sock or fabric-lined trench?
Either works; sock-wrapped pipe is simpler, a full fabric burrito gives more protection in silty soil.
Related calculators
Planning estimates only. Prices are 2026 U.S. national averages and vary by region — confirm with your local supplier before buying.