French Drain Calculator

For a standard 50 ft French drain with an 18-inch wide × 24-inch deep trench: volume = 50 × 1.5 × 2 = 150 cu ft = 5.6 cubic yards. At ~1.5 tons per cubic yard for crushed stone, that's about 8.3 tons. This calculator computes the exact gravel volume accounting for the pipe's volume displacement and converts to tons for ordering.

Use clean, washed crushed stone (drain rock) — no fines or dust. The best sizes are 3/4-inch to 1.5-inch clean crushed limestone or granite. Avoid pea gravel (too small, tends to migrate) and gravel with fines (dust blocks drainage over time). Landscape fabric wrapped around the gravel keeps soil out of the drain system. Do not use sand — it will clog immediately.

For surface water drainage (yard flooding): 18–24 inches deep is usually sufficient. For foundation drainage (keeping basements dry): 24–48 inches, placed at or below the foundation footing level. The drain must slope at least 1% (1 inch per 8 feet) toward the discharge point. More slope is better — 1–2% is ideal for consistent drainage. Call 811 before digging to locate underground utilities.

Landscape fabric (filter fabric / geotextile) wrapped around the gravel-filled trench prevents soil migration into the drain over time. Without it, fine soil particles gradually fill the voids between the gravel and reduce drainage capacity within 5–10 years. Use a non-woven geotextile fabric (not woven landscape fabric, which can clog more easily) — lay it in the trench, add gravel, then fold the fabric over the top before backfilling.