French Drain Calculator
How much gravel do I need for a french drain? What pipe size handles my runoff? This free 3-in-1 french drain calculator answers both — plus landscape fabric and a material cost estimate — with real drainage engineering (Manning flow capacity, rational-method runoff) instead of one thin gravel box.
Gravel results
Pipe displacement uses SDR35 PVC outside diameters (3″=3.25, 4″=4.22, 6″=6.28). Corrugated HDPE runs slightly larger, so this estimate is conservatively high for it. 1 yd³ of #57 washed stone ≈ 1.35 tons.
Pipe sizing results
Capacities are full-pipe Manning flow at your slope. The recommendation is the smallest standard size that carries the design storm with margin. Always verify against your local stormwater code.
Fabric & cost results
Fabric estimate wraps the trench burrito-style (up both walls + 12 in overlap) with non-woven geotextile. Cost is materials only — DIY all-in typically runs $10–$15/ft; a contractor $20–$30/ft.
What Is a French Drain and How Does It Work?
A french drain is a gravel-filled trench with a perforated pipe at the bottom that collects groundwater and surface runoff, then carries it by gravity to a safe discharge point. Despite the name, it is not French — it is named after Henry Flagg French, who described the system in 1859.
Water always follows the path of least resistance. The loose, washed gravel around the pipe has far more void space than the surrounding soil, so water migrates into the trench, drops through the gravel, enters the pipe through its perforations, and flows downhill to daylight, a dry well, or a storm drain. Landscape fabric lining the trench keeps soil fines from migrating into the gravel and clogging the system — the single most common cause of french drain failure.
Typical components
| Component | Standard choice | Why it matters |
|---|---|---|
| Perforated pipe | 4-in SDR35 PVC or corrugated HDPE, holes down | Carries collected water; 4 in handles most residential runoff |
| Gravel | ¾-in washed stone (#57), no fines | High void space = fast drainage; fines would clog it |
| Landscape fabric | Non-woven geotextile, 4–8 oz | Filters soil out of the gravel bed; extends life to 30–40+ years |
| Trench slope | Minimum 1% (⅛ in drop per foot) | Water must flow by gravity — flat drains pond and fail |
| Outlet | Daylight, dry well, or storm connection | A drain with nowhere to go is just a buried pond |
Trench Dimensions: How Deep and How Wide?
For yard drainage, a 12-in-wide, 18-in-deep trench is the residential standard — wide enough to work around the pipe, deep enough to intercept shallow groundwater. Foundation (footing) drains go deeper: to the bottom of the footing, typically 24–36 in for most homes.
| Application | Width | Depth | Gravel bed under pipe |
|---|---|---|---|
| Yard / surface water | 8–12 in | 12–18 in | 2 in |
| Downspout extension | 8–12 in | 12–18 in | 2 in |
| Foundation / footing drain | 12–18 in | 24–36 in (to footing) | 2–4 in |
| Retaining wall base drain | 12 in | Below first course | 2 in |
Always slope the trench bottom at least 1% — that is 1 in of drop every 8 ft (⅛ in per foot). Use a line level or laser level; eyeballing the slope is the most common DIY mistake.
French Drain Pipe Size: Flow Capacity Table
Pipe capacity is computed with the Manning equation for full-pipe flow. Smooth PVC (n = 0.010) carries roughly twice what corrugated HDPE (n = 0.022) carries at the same diameter and slope. Use Tab 2 above to match your roof or yard runoff to the smallest pipe that carries it.
| Pipe (ID) | 0.5% slope | 1% slope | 2% slope |
|---|---|---|---|
| 3 in smooth PVC | 36.46 GPM | 51.56 GPM | 72.92 GPM |
| 4 in smooth PVC | 78.52 GPM | 111.04 GPM | 157.04 GPM |
| 6 in smooth PVC | 231.50 GPM | 327.39 GPM | 463.01 GPM |
A 4-in smooth pipe at 1% slope (111 GPM) handles a typical 1,000 sq ft roof in a 4 in/hr storm (~39 GPM) with wide margin. Corrugated 4-in at 1% carries ~50.5 GPM — still enough for that roof, but with less margin.
How much water does your runoff produce?
Engineers use the rational method: Q = 1.008 × C × i × A, where C is the surface runoff coefficient, i is rainfall intensity (in/hr), and A is area in acres. A 1,000 sq ft roof (C = 0.95) in a 4 in/hr storm produces about 39.5 GPM — Tab 2 runs this exact math for your numbers.
Worked Example: 50-ft French Drain
A 50-ft trench, 12 in wide, 18 in deep, with 4-in SDR35 perforated pipe (OD 4.22 in) and gravel filled to the surface:
| Step | Math | Result |
|---|---|---|
| Excavation | 50 × 1 × 1.5 ft | 75 ft³ = 2.78 yd³ |
| Pipe displacement | π/4 × (4.22/12)² × 50 | 4.86 ft³ |
| Gravel needed | 75 − 4.86 ft³ | 70.14 ft³ = 2.60 yd³ |
| Order with 10% waste | 2.60 × 1.10 | 2.86 yd³ |
| Weight (#57 stone ≈ 1.35 t/yd³) | 2.60 × 1.35 | 3.51 tons (3.86 with waste) |
| 50-lb bags | 3.86 × 2000 / 50 | 155 bags |
| Fabric (burrito wrap + 12 in overlap) | 6 ft perimeter × 50 ft | 300 sq ft = one 3×100 ft roll |
Materials at typical US prices: 2.86 yd³ gravel × $65 = $185.90, five 10-ft pipe sticks × $22 = $110, one fabric roll $45 → $340.90 total, or $6.82/ft in materials. DIY all-in projects typically land at $10–$15/ft; hiring a contractor runs $20–$30/ft.
What Gravel Is Best for a French Drain?
Use ¾-in washed crushed stone (ASTM #57). It is angular, so it locks together and holds its void space for decades, and "washed" means the stone dust (fines) has been rinsed off — fines are what clog drains. Buy it by the cubic yard from a landscape supply yard; it is far cheaper than bags for any trench over ~10 ft.
Avoid: pea gravel (too small, migrates and clogs), rounded river rock (shifts and compacts), crusher run or any stone with fines (the dust fills the voids you are paying for), and limestone dust / screenings.
Do I need landscape fabric?
Yes — line the trench with non-woven geotextile (4–8 oz) before adding gravel, then fold it over the top like a burrito with about 12 in of overlap. Woven "weed barrier" fabric is too tight and slows water; the cheap thin stuff tears on the gravel. A fabric-wrapped drain lasts 30–40+ years; an unwrapped one can clog in under 10.
Interior vs Exterior French Drain
| Exterior (yard / footing) | Interior (basement perimeter) | |
|---|---|---|
| Where | Trench dug outside, along foundation or across yard | Channel cut in basement slab, inside the footing |
| Best for | Surface water, soggy yard, downspouts, new construction | Existing basement with water seepage, high water table |
| Discharge | Gravity to daylight (needs slope) | Sump pump pit (works on flat lots) |
| DIY difficulty | Moderate — digging is the hard part | Hard — concrete cutting, dust, pump wiring |
This calculator covers exterior french drains. For a wet basement with no exterior access, an interior perimeter drain to a sump pit is usually the right call — see our basement size calculator when planning that project.
7 Mistakes That Kill French Drains
- No slope (or negative slope). Verify 1% minimum with a level, not your eye.
- Pipe holes up. Perforations go down so groundwater rises into the pipe; holes-up only catches water falling from above.
- No fabric, or woven weed barrier. Soil migrates into the gravel and the drain dies young. Use non-woven geotextile.
- Wrong gravel. Pea gravel and stone with fines clog; ¾-in washed #57 does not.
- No outlet plan. The pipe must daylight downhill, hit a dry well, or tie into storm drainage — check local code first.
- Crushed pipe. Thin-wall pipe under a driveway crushes; use SDR35 or schedule 40 where vehicles cross.
- Discharging onto the neighbor. Keep the outlet on your property and away from the foundation — yours and theirs.
Related tools
French Drain FAQs
French Drain Calculator — free DIY drainage planner: gravel volume, perforated pipe sizing, landscape fabric, and material cost. Estimates only; verify pipe sizing and discharge rules against your local stormwater code.
Free tool by NRTechStudio