French Drain Calculator
Calculate pipe, gravel, and landscape fabric needed for a French drain. Get material estimates with cost breakdowns.
How to Calculate French Drain Materials
To estimate materials for a French drain, start with the drain length in feet. Add 2 feet for pipe connections and overlap. Calculate the trench volume by multiplying the length, width, and depth (converting inches to feet). Subtract the volume occupied by the perforated pipe to find the net gravel needed. Convert cubic feet to cubic yards by dividing by 27, then multiply by 1.4 to convert to tons. For landscape fabric, plan on a length equal to the drain length plus 4 feet for overlap at the ends.
When You Need a French Drain
- Standing water collects in your yard after rain, especially near the foundation.
- Your basement or crawl space has moisture problems or water intrusion.
- Water pools at the base of a slope, hill, or retaining wall.
- Downspout runoff has nowhere to go and saturates the soil near your home.
- Your driveway or walkway floods during heavy rain.
- You notice soil erosion along the side of your house or yard.
Trench Dimensions Guide
| Dimension | Standard | Notes |
|---|---|---|
| Width | 12 inches | Minimum for 4-inch pipe; 14–16 inches for 6-inch pipe |
| Depth | 18–24 inches | Deeper in areas with heavy clay soil or deep water tables |
| Slope | 1% grade (1 inch per 8 ft) | Ensures water flows to the outlet; steeper is fine |
| Pipe Size | 4 inches | Use 6-inch for heavy drainage or long runs over 100 ft |
Gravel Selection Tips
- Use washed crushed stone (3/4-inch or #57 stone) for the best drainage and structural support.
- Avoid using fine gravel or pea gravel, which can clog the perforated pipe over time.
- River rock works well but costs more and is heavier to transport and handle.
- Always wrap gravel in landscape fabric to prevent soil from migrating into the drain and reducing flow.
- Place at least 2–3 inches of gravel below the pipe and fill to within 3–4 inches of the surface.
Common Mistakes to Avoid
- Not maintaining a consistent downhill slope — the drain will not work without gravity.
- Skipping landscape fabric, which causes the gravel to clog with silt within a few years.
- Using the wrong gravel size — too fine clogs the pipe, too large leaves gaps that fill with dirt.
- Forgetting to call 811 (utility locator) before digging the trench.
- Not providing an outlet point — the water must have somewhere to discharge (daylight outlet, dry well, or storm drain).
- Placing the drain too close to the foundation — maintain at least 2 feet of distance from footing walls.
If your French drain runs alongside a retaining wall, be sure to install it behind the wall at the base to relieve hydrostatic pressure. For projects involving gravel driveways or paths, our gravel calculator can help estimate those materials separately.
Frequently Asked Questions
A French drain should be 18 to 24 inches deep for most residential applications. For drains intended to protect a basement or foundation, dig to the depth of the footing — typically 24 inches or more. The trench must maintain a consistent downhill slope of at least 1 inch per 8 feet so water flows by gravity to the outlet point.
For a typical French drain that is 50 feet long, 12 inches wide, and 18 inches deep with a 4-inch pipe, you need approximately 1.5 to 2 tons of gravel. The exact amount depends on your trench dimensions and pipe size. Use 3/4-inch washed crushed stone for the best drainage performance. Our calculator above accounts for the pipe volume and gives you an accurate gravel estimate in tons.
Yes, French drains are one of the most effective drainage solutions for residential properties when installed correctly. They work by collecting groundwater and surface water through perforated pipe surrounded by gravel, then directing it away from the problem area using gravity. The keys to a working French drain are proper slope, correct gravel selection, landscape fabric to prevent clogging, and a clear outlet point for the water to discharge.
Yes, a French drain is a manageable DIY project for most homeowners. The main challenges are the physical labor of digging the trench and ensuring a consistent slope. You will need a shovel or trencher, a level, perforated pipe, gravel, and landscape fabric. Before digging, call 811 to have utility lines marked. A typical 50-foot French drain takes one to two days for a DIY installation. Consider renting a trenching machine for longer runs to save time and effort.
A DIY French drain costs roughly $3 to $8 per linear foot for materials, including pipe, gravel, fabric, and fittings. Professional installation typically runs $10 to $30 per linear foot, depending on depth, accessibility, and soil conditions. A 50-foot DIY French drain costs approximately $150 to $400 in materials, while professional installation for the same length ranges from $500 to $1,500. Interior basement French drains cost significantly more due to concrete removal and sump pump installation.