French Drain Calculator
Size a complete French drain from its trench dimensions. Get the dig volume, the gravel to order (with a waste allowance), how many lengths of perforated pipe and how much filter fabric to buy, the fall over the run, and an optional total material cost.
Formula
Worked example
A 10 m run, 30 cm wide and 45 cm deep is 1.35 m³ of dig. A 100 mm pipe displaces about 0.08 m³, leaving 1.27 m³ of gravel; add 10% waste to 1.40 m³, about 2.10 t at 1.5 t/m³. Fabric ≈ (0.30 + 0.90 + 0.30) × 10 = 15 m², and the 10 m run needs four 10 ft pipe lengths.
How the French drain estimate works
A French drain is a gravel-filled trench with a perforated pipe at the bottom that collects groundwater and carries it to a lower outlet. The estimate begins with the trench: length times width times depth gives the volume of soil to excavate. Because dimensions are usually a mix of metres, centimetres and inches, every value is converted to metres first. The pipe runs the full length and occupies a cylinder of volume pi times its radius squared times the length, so the gravel needed is the trench volume minus that pipe volume, with a waste allowance added on top.
Gravel, pipe, fabric and fall
Gravel volume is converted to tonnes using a density of about 1.5 tonnes per cubic metre, since aggregate is sold by weight, and the calculator also reports cubic yards for US suppliers. The number of pipe lengths comes from dividing the run by the stock length you buy and rounding up. Geotextile filter fabric lines the bottom and both sides of the trench and overlaps at the top, so its area is roughly the trench width plus twice the depth plus a small overlap, multiplied by the run. The fall is the total drop over the run at your chosen slope; aim for at least one percent so water keeps moving.
Costing the job and good practice
Turn on the cost estimate to price the three main materials: gravel by the tonne, pipe by the length, and fabric by the square metre. The total is a planning figure, local prices and delivery vary. Order roughly ten percent more gravel than the bare volume to cover compaction and spillage (already included in the waste allowance), slope the trench toward the outlet, and wrap the gravel and pipe in fabric to keep silt out. These are estimates: confirm the trench size, pipe diameter and outlet arrangement against your site and any local drainage rules before you dig.
Typical French drain dimensions
| Element | Typical size | Notes |
|---|---|---|
| Trench width | 20-45 cm (8-18 in) | Wider for higher flow or larger pipe |
| Trench depth | 45-60 cm (18-24 in) | Deep enough to sit below the wet zone |
| Pipe diameter | 100 mm (4 in) | Standard perforated drain pipe |
| Slope (fall) | min. 1 cm per m (1%) | Keeps water flowing to the outlet |
| Gravel density | 1.4-1.7 t/m³ | Default 1.5 t/m³ used here |
Starting points for a residential drain; adjust to the water you need to move and local codes.
Frequently asked questions
How much gravel do I need for a French drain?
Take the trench volume (length times width times depth), subtract the volume of the pipe, then add a waste allowance of about 10%. Multiply that gravel volume by roughly 1.5 tonnes per cubic metre to get a tonnage to order. This calculator does all of that and also shows cubic yards.
How much filter fabric and pipe should I buy?
Filter fabric lines the bottom and both sides of the trench and overlaps at the top, so allow about the trench width plus twice its depth plus a small overlap, times the run. For pipe, divide the run by the length your supplier sells (commonly 10 ft or 6 m) and round up, then add a piece for the outlet connection.
What slope does a French drain need?
A minimum fall of about 1% (1 cm per metre, or 1 inch every 8 to 10 feet) keeps water flowing to the outlet. Steeper is fine. The calculator shows the total drop over your run so you can set the outlet depth correctly.