French Drain Gravel Calculator Guide
A French drain is only as good as its rock. Here's how to size the trench, pick the right stone, and calculate the exact volume you'll need.
A French drain is a trench filled with gravel — often around a perforated pipe — that captures and moves subsurface water. Simple in concept, unforgiving in execution. The wrong rock or a missing fabric wrap turns it into a mud pit within a year.
This guide covers trench sizing, rock selection, and the exact volume math so you order once and install right.
Trench sizing
Standard residential French drains:
| Use | Width | Depth |
|---|---|---|
| Foundation drainage | 12–18 inches | 24–36 inches or below footing |
| Yard / lawn drain | 8–12 inches | 12–24 inches |
| Downspout runoff | 6–8 inches | 12–18 inches |
| Slope interception | 12–18 inches | 18–30 inches |
The right gravel
- Size: 3/4-inch to 1.5-inch clean, washed drain rock. NOT crushed base or gravel with fines.
- Washed matters: unwashed rock has dust that clogs voids and defeats drainage.
- Angular vs rounded: either works; angular locks in place better on slopes.
Volume calculation
Trench volume = length × width × depth (all in feet). Convert to cubic yards by dividing by 27.
Worked example
A 40 ft drain, 12 inches wide, 24 inches deep with a 4-inch pipe:
- Trench volume = 40 × 1.0 × 2.0 = 80 cu ft.
- Pipe volume = 40 × π × (0.167)² ≈ 3.5 cu ft displaced by pipe.
- Net rock volume = 80 − 3.5 = 76.5 cu ft = 2.83 cu yd.
- Add 10% for settling and irregular walls = 3.1 cu yd.
- At 1.35 tons per cu yd, that's about 4.2 tons of drain rock.
Fabric wrap
A woven or non-woven geotextile wraps the rock and prevents soil from infiltrating and clogging voids. Two common approaches:
- Fabric-first: line the trench with fabric before rock, wrap over the top after backfill.
- Sock pipe: use a fabric-wrapped perforated pipe (drainage sock) instead of trench-lining fabric. Faster to install but the sock can clog with fine soil.
Layer-by-layer installation
- Excavate the trench to grade with a slight downhill slope (1% minimum — 1 inch drop per 10 ft).
- Lay geotextile fabric with 12 inches extra on each side.
- Add 2 inches of drain rock on the bottom.
- Lay perforated pipe (holes down for surface water; holes up for subsurface).
- Fill the trench with drain rock to within 4 inches of grade.
- Fold fabric over the top of the rock.
- Backfill the remaining 4 inches with soil, sod, or decorative rock.
Common mistakes
- No slope. A level trench doesn't move water — it's a puddle underground.
- Skipping the fabric. Silt infiltrates and clogs the rock in 2–3 seasons.
- Using wrong rock (fines-in). It compacts and stops draining.
- Not daylighting the outlet. A drain that terminates underground creates a hidden pool.
- Discharging onto a neighbor's property. Check local drainage regulations first.
Frequently asked questions
- What size gravel for a French drain?
- Clean, washed 3/4-inch to 1.5-inch drain rock. No fines.
- How much gravel do I need for a 50 ft French drain?
- For a 12" wide × 24" deep trench: about 3.7 cubic yards or 5 tons of clean drain rock, minus a small displacement for the pipe.
- Do I need a pipe in a French drain?
- Not always — rock-only drains work for short runs and light loads. For anything over 20 ft or foundation drainage, add a perforated pipe.
- How long does a French drain last?
- A properly installed drain with fabric wrap lasts 20–40 years. Skip the fabric or use the wrong rock and it fails in 2–5.
Summary
French drains work when the rock is clean, the slope is right, and the fabric is complete. Get those three details right and you have decades of quiet, effective drainage.
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These results are estimates only. Confirm quantities, compaction, waste, and delivery requirements with your supplier or project professional before ordering materials.