# French Drain Calculator

Calculate French drain materials: gravel (cubic yards and tons), perforated pipe, and landscape fabric. Enter trench dimensions for material quantities.

## What this calculates

A French drain is one of the most effective ways to redirect groundwater away from your foundation, yard, or other low-lying areas. This calculator determines the gravel, perforated pipe, and landscape fabric needed for your French drain based on trench dimensions, helping you order materials accurately.

## Inputs

- **Drain Length** (ft) — min 5, max 500 — Total length of the French drain trench
- **Trench Width** (in) — min 6, max 24 — Standard French drain width is 12 inches (30 cm)
- **Trench Depth** (in) — min 8, max 48 — Standard depth is 18-24 inches (45-60 cm)
- **Pipe Diameter** (in) — min 3, max 6 — 4-inch perforated pipe is standard for residential
- **Gravel Cost** ($/ton) — min 0 — Local cost per ton of drainage gravel

## Outputs

- **Gravel Needed** (yd³) — Cubic yards of drainage gravel (3/4" washed)
- **Gravel Weight** (tons) — Gravel weight at typical density
- **Pipe Length** (ft) — Perforated pipe length including 10% for fittings
- **Landscape Fabric** (sq ft) — Filter fabric to line the trench
- **Estimated Material Cost** — formatted as currency — Gravel + pipe + fabric materials

## Details

A French drain consists of a trench filled with gravel surrounding a perforated pipe, all wrapped in landscape fabric to prevent soil from clogging the gravel. Water seeps through the gravel, enters the perforated pipe, and is carried away from the problem area by gravity.

Standard residential French drains use a 12-inch wide by 18-inch deep trench with a 4-inch perforated pipe. The pipe sits on 2-3 inches of gravel at the bottom, and the rest of the trench is filled with 3/4-inch washed drainage gravel. The trench should slope at least 1% (1 inch per 8 feet) toward the outlet.

Use washed, angular gravel (not round river rock) for best drainage. Landscape fabric is critical to prevent silt from migrating into the gravel and clogging the system. Wrap the entire trench before adding gravel, leaving enough overlap at the top to fold over the completed fill.

## Frequently Asked Questions

**Q: How deep should a French drain be?**

A: Standard French drains are 18-24 inches deep. For foundation drainage, the drain should be at or below the footing level. For yard drainage, 18 inches is typically sufficient. In cold climates, deeper drains (below the frost line) prevent freezing, which would block water flow.

**Q: What kind of gravel is best for a French drain?**

A: Use 3/4-inch washed, angular gravel (also called drainage stone or #57 stone). Avoid round river rock, which compacts less and drains slower. The gravel must be washed to remove fines (dirt, sand) that would clog the system over time.

**Q: How much slope does a French drain need?**

A: A French drain needs a minimum slope of 1% toward the outlet, which equals 1 inch of drop per 8 feet of run. A 0.5% slope can work in some cases but drains more slowly. Use a laser level or string line when digging to maintain consistent slope throughout the trench.

**Q: How long does a French drain last?**

A: A properly installed French drain with landscape fabric and washed gravel lasts 30-40 years. Without fabric, silt clogs the gravel within 5-10 years. Signs of failure include standing water reappearing, soggy soil near the drain, or water backing up at the inlet.

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Source: https://vastcalc.com/calculators/construction/french-drain
Category: Construction
Last updated: 2026-04-21