How Much Gravel for a French Drain

Calculating Gravel for a French Drain

A French drain is one of the most effective solutions for managing water drainage around your home, directing groundwater away from foundations and problem areas. A typical French drain trench runs 10 metres long, 300mm wide, and 300mm deep, creating a channel that can handle significant water flow. At these dimensions, you'll need approximately 0.9 cubic metres of blue metal gravel to fill the trench around the drainage pipe.

Just under one cubic metre of blue metal gravel weighs approximately 1.35 tonnes. This quantity is easily delivered in a single bulk load and is manageable to distribute along the trench using a wheelbarrow. The narrow width of drain trenches means spreading is relatively straightforward compared to larger area coverage.

French Drain Construction

The gravel in a French drain serves as both filter and conductor, allowing water to flow freely while preventing soil from clogging the drainage pipe. Dig your trench with a slight fall towards the outlet point, typically a minimum of 1% grade. Line the trench with geotextile fabric to prevent soil migration, lay your slotted drainage pipe on a bed of gravel, then fill around and over the pipe with more gravel. The geotextile can be folded over the top before backfilling with soil or leaving the gravel exposed.

Choosing the Right Gravel Size

For French drains, 20mm blue metal is the standard choice. This size allows good water flow between the stones while being heavy enough to stay in place. Avoid using fine gravels or crusher dust, as these compact too tightly and restrict water movement. The angular nature of blue metal also helps it lock together and resist shifting, maintaining the drain's function over many years.

Building a longer or differently sized drain? Use our main gravel calculator to enter your specific trench dimensions.