French Drain Calculator

How to use this French drain calculator

1. Pick a drain size. The trench width, depth and pipe size fill in for you. You can change them.
2. Type in the length of your drain.
3. Pick your stone. Washed crushed stone is the most common choice.
4. Choose whether to include pipe, set the slope and fabric overlap, then review gravel, pipe sections and fabric.

What is a French drain?

A French drain is a trench filled with gravel, with a perforated pipe inside. Water soaks into the gravel, flows into the pipe, and runs away from your house. The fabric around the gravel keeps soil out, so the drain does not clog.

The French drain formulas

• Trench volume (cu ft) = length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
• Pipe space (cu ft) = π × (pipe diameter in inches ÷ 24)² × sloped run (ft). Use zero when pipe is off.
• Gravel (cu ft) = trench volume − pipe space (or the full trench volume with no pipe)
• Gravel (cu yd) = cubic feet ÷ 27, then add your selected extra allowance
• Gravel (tons) = cubic yards × weight per cubic yard
• Pipe needed = sloped run × (1 + pipe allowance). Round up to stock lengths to estimate how many sections or coils to buy.
• Fabric area = sloped run × (width + 2 × depth) × (1 + your overlap allowance)
• Slope drop (in) = drain length (ft) × 12 × slope (%) ÷ 100

For metric, use metres for length, cm for width and depth, and divide by 100 instead of 12.

Worked example

A drain that is 50 ft long, 8 in wide and 18 in deep, with a 4 in pipe, crushed stone (1.4 tons per cubic yard) and 10% extra:

1. Trench = 50 × 8 ÷ 12 × 18 ÷ 12 = 50 cu ft
2. Pipe space = 3.14 × 0.167 × 0.167 × 50 = 4.4 cu ft
3. Gravel = 50 − 4.4 = 45.6 cu ft
4. Cubic yards = 45.6 ÷ 27 = 1.69 cu yd
5. Add 10% extra = 1.69 × 1.10 = 1.86 cu yd
6. Tons = 1.86 × 1.4 = 2.60 tons
7. Pipe needed = 50 × 1.10 = 55 ft; with 10 ft sections, buy 6 sections (60 ft).
8. Fabric = 50 × (8 + 2 × 18) ÷ 12 × 1.20 = 220 sq ft.

Common trench sizes

Tips for a drain that works

• Set a steady slope. Aim it toward a safe outlet and use the slope required for your project. Enter that percentage above.
• Use fabric. Lay it in the trench, add gravel, then fold the fabric over the top before you cover it.
• Use clean stone. Washed stone has no dust, so water flows through it easily.
• Put the holes down. Most perforated pipe is laid with the holes facing down. Check the instructions on your pipe.
• Know where the water goes. The outlet should empty somewhere safe, away from your house and your neighbours.

For a related part of your project, use the Basement and Crawl Space Insulation Calculator.

Common questions

How much gravel do I need for a 50 ft French drain?

A standard drain (8 in wide, 18 in deep) needs about 2 cubic yards, or about 2.6 tons, of gravel.

Do I need landscape fabric?

Yes. Fabric stops soil from washing into the gravel. Without it, the drain can clog over time. Most people use non-woven fabric made for drains.

How wide and deep should the trench be?

Most home drains are 8 to 12 inches wide and 18 to 24 inches deep. Wetter ground needs a bigger drain.

Is this calculator exact?

No. It gives a good estimate. Check quantities with your supplier before you order.