💧 French Drain Calculator

Reading the French drain estimate

Trench slope, pipe placement and fabric wrapping are drainage-design decisions that affect performance beyond this material quantity estimate.

• This calculator estimates gravel volume from trench and pipe geometry only; it does not account for landscape fabric (commonly wrapped around the gravel to prevent soil infiltration) or the pipe's own material.
• Effective French drains require a consistent downward slope along their length (commonly a minimum grade is recommended) to ensure water flows to the outlet — this is a design consideration separate from the material quantity calculated here.

How is French drain gravel volume calculated?

A French drain is a gravel-filled trench containing a perforated pipe that collects and redirects subsurface or surface water. Because the pipe itself occupies part of the trench's cross-section, the gravel volume needed is the trench volume minus the pipe's own volume — gravel surrounds the pipe rather than replacing it.

This calculator computes total excavation volume (the full trench), the pipe's displaced volume, and the net gravel volume needed to backfill around the pipe, using standard cylinder and rectangular-prism geometry.

How to use this French drain calculator

1. Enter the trench length in meters.
2. Enter the trench width in centimeters.
3. Enter the trench depth in centimeters.
4. Enter the perforated pipe diameter in centimeters.
5. Read the gravel volume and weight needed, the pipe length required, and the total excavation volume.

The formula behind the French drain estimate

Trench (excavation) volume equals trench length × trench width × trench depth (with width and depth converted from centimeters to meters). Pipe volume equals π × (pipe diameter ÷ 2)² × trench length, using the standard cylinder-volume formula. Gravel volume equals trench volume minus pipe volume, and gravel weight uses a typical crushed-stone density of 1,680 kg/m³.

Worked example: an 8 m trench, 30 cm (0.3 m) wide and 45 cm (0.45 m) deep, with a 10 cm (0.1 m) diameter pipe, gives a trench volume of 8 × 0.3 × 0.45 = 1.08 m³ and a pipe volume of π × (0.05)² × 8 ≈ 0.0628 m³, for a net gravel volume of about 1.017 m³, weighing roughly 1.017 × 1,680 ÷ 1,000 ≈ 1.71 tonnes.

Common mistakes

• Ignoring the pipe's own volume and ordering gravel for the full trench volume, which overstates the quantity needed.
• Digging a trench without adequate slope toward a drainage outlet, which can leave standing water in the pipe regardless of gravel backfill.
• Using non-perforated pipe where perforated pipe is needed for water to enter the drain along its length.
• Skipping landscape fabric around the gravel, allowing surrounding soil to infiltrate and clog the drain over time.

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参考文献

1. USDA Natural Resources Conservation Service (NRCS) — subsurface drainage design reference.
2. American Society of Civil Engineers (ASCE) — general drainage trench design guidance.
3. National Association of Home Builders (NAHB) — residential French drain and foundation drainage installation practice.