French Drain Gravel Calculator

How to use the French drain calculator

1. Measure the trench run. Enter the total length from the high end to the outlet (a daylight point, dry well or storm connection where permitted).
2. Enter the trench cross-section. Most residential French drains are 6–12 in wide and 18–24 in deep.
3. Choose the pipe. Rigid 4 in perforated PVC, 4 in or 6 in corrugated, or no pipe for a rock-only drain. The calculator subtracts the pipe’s outside volume from the trench.
4. Check the stone density. Washed ¾ in (#57) stone is about 1.4 tons per cubic yard.

You also get the width of filter fabric needed to “burrito wrap” the stone and how many 10 ft pipe sections to buy.

French drain gravel formula

The fabric lines both walls and the floor of the trench, then folds over the top of the stone. A 12 in overlap is typical, so a 12 × 18 in trench needs 2 × 18 + 2 × 12 + 12 = 72 in of fabric — exactly a 6 ft roll.

Pipe outside diameters used

Corrugated figures are the reference outside diameters in ADS’s single-wall pipe specification; other makers vary a little, and the difference changes the stone total by only a few percent.

Worked example

A 40 ft drain along a fence line, 12 in wide and 24 in deep, with 4 in corrugated pipe:

• Trench: 40 × 12 × 24 ÷ 144 = 80.0 cu ft
• Pipe: 40 × π × (4.6 ÷ 24)² = 4.6 cu ft
• Stone: 75.4 cu ft = 2.79 cubic yards ≈ 3.9 tons
• Fabric: 2 × 24 + 2 × 12 + 12 = 84 in wide (a 7 ft strip), 280 sq ft total

Order 4 tons of washed stone. In bags that would be about 150 half-cubic-foot bags — bulk delivery is the way to go.

The pipe only removes about 6% of the trench volume. Don’t skip it to save stone — the pipe is what moves water quickly to the outlet.

Buying and building tips

• Use clean, washed stone. ¾ to 1½ in angular stone (often sold as #57 or drain rock) leaves voids for water. Avoid crusher run or anything with fines — it packs tight and clogs.
• Slope the trench. A fall of at least 1% (about ⅛ in per foot, 1 in per 8 ft) keeps water moving. Many installers use 1–2%.
• Holes down. Lay perforated pipe with the holes facing down so water rises into the pipe from the stone bed and drains away.
• Use non-woven geotextile fabric. It lets water through but keeps soil out of the stone. Woven fabric is for stabilization, not drainage.
• Add 5–10% extra stone. Trench walls are never perfectly straight, and you’ll want some to top up after the first heavy rain settles things.
• Call 811 before you dig. It’s free in every US state and marks buried utility lines.

For a gravel path or pad on top of the drain, use the gravel calculator.

Stone needed per 10 ft of trench

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Stone volume = trench volume minus the space taken by the pipe, using the pipe's outside diameter.
• Corrugated pipe outside diameters come from the maker's specification (about 4.6 in for 4 in pipe, 7.0 in for 6 in); 4 in PVC drain pipe is about 4.215 in OD.
• Stone is converted to tons with a loose-aggregate density of about 1.4 tons per cubic yard unless you change it. Pipe sections, fabric and stone are rounded up.

Sources

1. Advanced Drainage Systems (ADS), ADS Single Wall Pipe Specification (Drainage Handbook). Reference pipe outside diameters: 4.6 in for 4 in, 7.0 in for 6 in.
2. Missouri Department of Transportation, Engineering Policy Guide 450, Estimate Factors (archived PDF). Gravel, crushed stone and chat estimated at 1.40 tons per cubic yard of loose aggregate.

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.