French Drain Calculator
Size every material in a French drain from five inputs: trench excavation volume, washed drainage stone in cubic yards and tons, perforated pipe length in 10-ft sections, non-woven filter fabric area including overlap, and the total elevation drop your chosen slope requires. Assumptions and formulas are documented below the tool.
Drain Dimensions
Materials List
Drainage stone to order
– tons
Assumes fabric-lined trench, stone bed under the pipe, and washed stone at ~2,400 lb/yd³ loose. Excavation spoil swells ~25% - plan disposal with the excavation calculator.
Anatomy of a French drain
The formulas this calculator uses
Gravel = (Trench − Pipe volume) × 1.05, Pipe volume = πr²L
Fabric width = 2W + 2D + 50% overlap · Drop (ft) = L × Slope%
Worked example: 50 ft drain, 12 × 18 in trench
- Trench: 50 × 1.0 × 1.5 = 75 ft³ (2.8 yd³ excavation)
- Pipe displacement (4 in): π × 0.167² × 50 = 4.4 ft³
- Gravel: (75 − 4.4) × 1.05 = 74.1 ft³ = 2.75 yd³ ≈ 3.3 tons
- Pipe: 6 sections of 10 ft (includes 5% extra)
- Fabric: (2 + 3 + 0.5) × 50 = 275 ft²
- Drop at 1%: 0.5 ft = 6 inches from inlet to outlet
Materials checklist
| Item | Specification | Notes |
|---|---|---|
| Drainage stone | 3/4–1.5 in washed, angular, no fines | Sold by the ton; ~1.2 tons/yd³ loose |
| Pipe | 4 in perforated PVC (SDR-35) or corrugated | PVC lasts longer and can be jetted clean |
| Filter fabric | Non-woven geotextile, 4–6 oz | Line the trench; overlap 6–12 in at top |
| Fittings | Couplers, end caps, cleanout riser | Add a cleanout every 50–100 ft |
| Outlet | Pop-up emitter, daylight grate, or dry well | Must sit lower than the entire pipe run |
Common mistakes to avoid
No fabric, or fabric-wrapped pipe only. Soil migrates into unlined stone and clogs the system in 2–5 years. Line the whole trench.
Wrong stone. Fines and screenings turn the trench into concrete. Washed stone only.
No positive outlet. A French drain without somewhere lower to discharge is just a buried bathtub. Confirm the outlet elevation before you dig - this is the first thing to survey, not the last.
Perforations up. Holes face down so water enters from the saturated stone bed below the pipe invert.
When to call a professional
Chronic basement seepage, water against a foundation wall, slopes showing movement, or drains deeper than 5 ft warrant a drainage contractor or geotechnic
Assumptions used in this calculator
- Washed drainage stone at 2,400 lb/yd³ loose, 5% waste included
- Pipe sold in 10 ft sections with 5% length allowance
- Fabric width = 2×trench width + 2×depth + 50% top overlap
- Elevation drop = length × slope %
Change the adjustable inputs above where your project differs. Supplier figures and local codes govern; see our methodology.
Built for planning, not blind ordering. Verify dimensions, supplier conversions, local codes, and site conditions before construction.
Frequently Asked Questions
What slope does a French drain need?
A minimum of 1% (1 foot of drop per 100 feet, or about 1/8 inch per foot) is the widely used standard. Flatter than 0.5% risks standing water and sediment buildup inside the pipe; steeper is fine if the outlet allows it.
How deep should a French drain be?
Typical yard drains run 18-24 inches deep and 12 inches wide. Drains protecting a basement footing must reach footing depth, and drains intercepting slope seepage may need to be deeper to catch the water table.
What gravel should I use in a French drain?
Washed, angular drainage stone between 3/4 and 1.5 inches, with no fines. Rounded pea gravel migrates and packs; crusher run and anything with fines will clog the system. Never use limestone screenings.
Does a French drain need fabric?
Yes - non-woven geotextile lining the trench (not wrapping the pipe alone) keeps soil out of the stone while letting water through. Skipping fabric is the single most common cause of clogged drains within a few years.
Where should a French drain discharge?
To daylight on a downhill slope, a storm drain where permitted, or a dry well. The outlet must be lower than the pipe along its entire run, stay open, and discharge away from your foundation and your neighbor's property - many municipalities regulate this.