French Drain Calculator
Estimate the materials for a French drain by trench length, depth, and drain type. The calculator returns gravel by the yard and ton, pipe by the linear foot, fabric by the roll, and soil backfill by the bag — with the math shown.
What is a French drain?
A French drain is a sloped trench filled with gravel and a perforated pipe that moves surface or groundwater away from a problem area — a soggy lawn, a basement wall, or low ground that pools after rain. The water enters through the gravel, flows into the pipe, and exits by gravity at a lower point.
There are two common variants. A standard French drain wraps the trench in geotextile fabric, lays a perforated corrugated pipe in a bed of clean drainage gravel, then backfills with soil. A gravel-free drain uses a pipe pre-wrapped in fabric and rock-equivalent media — you skip the gravel and the liner, trading lower install effort for a smaller flow capacity.
Residential French drains are typically 12″ wide and 18″–24″ deep with a 4″ pipe. Foundation drains go deeper (3–6 ft) and often use a 6″ pipe. Either way, the trench needs at least 1% slope toward an outlet so water flows out by gravity.
How to use this calculator
- Pick a drain type. Standard drains surround the corrugated pipe with gravel; gravel-free drains use a wrapped pipe and skip the gravel layer.
- Enter the pipe diameter and total trench length in feet (along the centerline).
- Set the trench width and depth with the sliders. 18″–24″ deep is typical for residential drains.
- Decide on geotextile fabric. A liner reduces silting and is recommended for most installs.
- Adjust the three backfill layers — gravel, soil, and leave-room. The sliders rebalance so the sum equals the trench depth.
- Pick a surface material (sod, stone, or none) and the surface width past the trench edges.
- Read the materials breakdown — cubic yards, tons, bags, pipe sections, and the geotextile roll width — from the sticky results panel.
Tips for sizing yours
Slope toward daylight (or a sump)
A French drain only moves water if the pipe drops at least ⅛″ per foot toward an outlet. Add 1–2 inches of total fall per 10 feet when you plan.
Wrap the trench in geotextile fabric
Without fabric the surrounding soil migrates into the gravel and the pipe silts up in a year or two. The calculator includes overlap on each side at grade so the flaps protect the top.
Don't undersize the pipe
4″ is standard for residential lawn drainage. Go to 6″ for foundation drains or anywhere you'll have heavy runoff — a 6″ pipe carries roughly 2× the flow of a 4″.
Buy a little extra of everything
Gravel compacts, soil settles, and fabric tears. Order 10–15% more material than the calculator suggests if you're doing the work yourself.
Frequently asked questions
How deep should a French drain be?
18 to 24 inches is the typical residential range. Yard drains for surface runoff sit on the shallower side; foundation and footer drains go deeper (3–6 feet) so they intercept water before it reaches the basement wall.
How much gravel does a French drain need?
For a standard drain, gravel fills the trench from the bottom up to the soil cover — typically 6–12 inches of gravel above and around the pipe. A 100 ft × 12 in wide × 6 in gravel layer is roughly 1.5 cubic yards. Use the calculator for an exact number.
Standard vs. gravel-free French drain — which is better?
Standard drains move more water and last longer when sized right, but require excavation for the gravel and a fabric liner. Gravel-free uses a pre-wrapped pipe that goes in faster with less mess — good for retrofits, less ideal for heavy flow.
Do I need geotextile fabric?
For a standard drain, yes — without it, soil migrates into the gravel and clogs the system in a few years. For gravel-free drains the pipe is already wrapped, so a separate liner is usually unnecessary.
What slope does a French drain need?
At least ⅛″ per foot (about 1% grade) toward the outlet. The calculator doesn't compute slope directly — measure your trench fall with a string line and a level before backfilling.
Can I use the excavated soil as backfill?
Yes — set the soil slider to the depth you'll re-use and leave the remaining inches as 'leave room' for any top dressing. Loose excavated soil settles 10–20%, so plan for some topping-off over the first few weeks.
Common French drain questions
Calculate just the materials
If you only need to size a specific material — gravel for the bed, soil for the backfill — these material calculators handle it directly.