French Drain Gravel Calculator

A French drain is a gravel-filled trench with a perforated pipe that redirects groundwater away from problem areas like foundations, yards, and basements. The key material is the gravel — it must be coarse enough to allow free water movement while preventing fine soil from clogging the system.

French Drain Construction

A standard French drain consists of:

1. A trench, typically 12–24 inches wide and 18–36 inches deep
2. A layer of landscape fabric (geotextile) lining the trench
3. A 3–6 inch bed of gravel at the bottom
4. A perforated pipe (4-inch diameter is most common)
5. Gravel filling the rest of the trench (leaving 2–3 inches for topsoil)
6. Landscape fabric folded over the top
7. Topsoil or sod to finish

Choosing the Right Gravel

• #57 crushed stone (¾ inch): The most common choice. Good void ratio, large enough to resist clogging.
• #4 crushed stone (1.5 inch): Better for high-flow areas; rarely clogs.
• Pea gravel (⅜ inch): Smooth; flows well but can compact over time.
• River rock: Aesthetically pleasing if used as a surface drain; heavier.

Never Use:

• Fine gravel or sand, clogs rapidly
• Limestone, can dissolve and compact
• Decomposed granite, settles into fine particles

Volume Calculation

The trench is a rectangular prism: Volume (ft³) = Length (ft) × Width (ft) × Depth (ft) Volume (yd³) = Volume (ft³) ÷ 27

Metric: Volume (m³) = Length (m) × Width (m) × Depth (m)

Gravel Weight

Crushed stone weighs approximately 2,700 lbs per cubic yard (1,600 kg per m³) when loose.

Note: Subtract the volume of the perforated pipe from your total if calculating precisely — for a 4-inch pipe in a long trench, this is typically 2–3% of total gravel volume.