French Drain Calculator (gravel, pipe & filter fabric for a trench drain)

Understanding the French drain calculator — concept and principle

This calculator estimates the gravel, perforated pipe and filter fabric needed for a French drain, and the drop from start to outlet. Use it to order materials for a drain that carries water away from a foundation, a wet lawn or the back of a retaining wall.

Trench minus the pipe: with the default inputs the result is 1.88 m³ gravel (≈ 3.01 t · 15 cm fall).

How it is calculated

Trench volume (m³) = length × width × depth (width and depth in cm ÷ 100).
Pipe volume = π × (outside diameter ÷ 2)² × length.
Gravel volume = trench volume − pipe volume; gravel weight = volume × bulk density (t/m³).
Filter fabric (m²) = (2 × depth + 2 × width) × length, so it lines the bottom and sides and folds over the top.
Drop = length × fall %.

The principle

A French drain is a gravel-filled trench, usually with a perforated pipe at the bottom. Water seeps through the soil into the open spaces between the stones, collects in the pipe and flows downhill to an outlet. The gravel volume is simply the trench minus the space the pipe occupies. Gravel is sold by weight, so volume is converted with the bulk density your supplier quotes, which already includes the air gaps between stones. The fabric keeps fine soil out of the gravel so it does not clog. A steady fall is what makes the water move, so the trench bottom must slope towards the outlet.

Worked example

Default inputs: 15 m long, 30 cm wide, 45 cm deep, 110 mm pipe, gravel 1.6 t/m³, fall 1%.

Trench = 15 × 0.30 × 0.45 = 2.02 m³ (2.025).
Pipe = π × 0.055² × 15 = 0.143 m³.
Gravel = 2.025 − 0.143 = 1.88 m³; weight = 1.8825 × 1.6 = 3.01 tonnes.
Pipe = 15 m; fabric = (0.90 + 0.60) × 15 = 22.5 m².
Drop = 15 m × 1% = 15 cm.

Each point is the calculator re-run with only “Trench depth (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Trench depth (cm)	Drainage gravel (m³)
11.2	0.36
22.5	0.87
33.8	1.38
45	1.88
56.2	2.39
67.5	2.89
90	3.91

How to read the result

Order a little more gravel than calculated, because trenches are never perfectly even and some stone settles. Buy fabric with enough overlap for joins. Check the drop against the ground: the outlet must be lower than the start, and it must discharge somewhere water is allowed to go, such as a soakaway or a legal drainage point.

Common mistakes

Laying the trench flat or with a dip that holds water.
Using fine gravel or sand that clogs.
Skipping the filter fabric in silty or clay soil.
Pointing the outlet towards a neighbour's property or a foundation.

Keep in mind: Drainage near foundations, retaining walls or boundaries can affect structures and neighbours. Check local rules on discharging water and call before you dig to locate buried services.

Frequently asked questions

How much gravel do I need for a French drain?

Trench volume minus the pipe volume; a 15 m × 30 cm × 45 cm trench with a 110 mm pipe needs about 1.9 m³.

What slope does a French drain need?

It needs a steady fall to the outlet; the calculator shows the drop for the percentage you enter.

Do I need filter fabric?

It is widely used to keep soil from clogging the gravel, especially in fine or clay soils.

Sources: ASTM C29/C29M, Standard Test Method for Bulk Density ("Unit Weight") and Voids in Aggregate

Checked 9 Nov 2026. Volume geometry (rectangular prism minus cylinder); bulk density is the supplier’s figure, measured by ASTM C29/C29M “Standard Test Method for Bulk Density (‘Unit Weight’) and Voids in Aggregate”.

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