French Drain Calculator
Planning a backyard drainage project? Enter your trench length and dimensions to estimate the exact amount of gravel, perforated drainage pipe, geotextile fabric, and total project costs (DIY vs. professional).
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How a French drain actually works
A French drain is a highly effective trench-style drainage system designed to redirect surface water and groundwater away from foundations and waterlogged yard areas. The system relies on a slope, drainage gravel, and usually a perforated pipe to evacuate water quickly.
Unlike standard solid pipes, a perforated drainage pipe has small holes or slots. Water enters the French drain from below. As groundwater rises in the trench, it filters up through the geotextile fabric and gravel, enters the pipe through the perforations, and flows rapidly down the sloped pipe to a safe discharge point (such as a dry well, street gutter, or pop-up emitter).
Formula: trench volume (cubic feet) = length × (width ÷ 12) × (depth ÷ 12)
We then subtract the volume occupied by the 4" pipe (approx. 0.087 cu ft per foot) to calculate the net volume of clean drainage gravel required. We add a 10% safety and settling margin to the totals.
The three golden rules of French drains
- Slope is mandatory: A French drain requires gravity to work. The trench must slope downward toward the exit point at a minimum rate of 1% grade (which is about 1 inch of drop for every 8 feet of length, or 1/8 inch per foot). Without a slope, water will sit in the trench and saturate the surrounding soil.
- Geotextile fabric is non-negotiable: Do not skip the fabric, and do not use cheap garden weed-barrier. You must line the trench with high-quality, heavy-duty non-woven geotextile fabric. The fabric acts as a filter: it allows water to pass through but blocks silt, clay, and soil particles. Skipping the fabric means soil will wash into the gravel, filling the air gaps and clogging the drain completely within 2–3 years.
- Holes go face down: If you use a perforated pipe, lay it with the holes pointing downward, not upward. This is the single most common installation error. If the holes point up, groundwater must fill the entire trench to the top of the pipe before it can enter and drain away. Pointing the holes down allows water to enter as soon as it reaches the bottom of the trench.
Why bulk drainage stone beats bags
A typical 50-foot French drain trench (12" wide and 18" deep) requires about 2 cubic yards of drainage stone after subtracting the pipe volume. If you buy bulk clean stone from an aggregate yard, it costs roughly $45 per cubic yard (or $35 per ton), bringing the gravel cost to around $90.
In contrast, standard bag aggregate is sold in 0.5 cubic foot bags. Because a cubic yard is 27 cubic feet, it takes 54 bags to make a single cubic yard! At $6 to $8 per bag, buying the gravel in bags would cost $320 to $430 per cubic yard — plus the grueling labor of carrying and tearing open more than 100 heavy plastic bags.
DIY vs. Professional installation costs
| Component | DIY Cost / ft | Professional Cost / ft |
|---|---|---|
| Excavation / Labor | $0.00 (Sweat equity) | $20.00 to $35.00 |
| 4" Perforated Pipe | $1.40 to $2.50 | Included in labor/quote |
| Geotextile Fabric | $0.45 to $0.60 | Included in labor/quote |
| Clean drainage stone | $1.80 to $2.50 (bulk) | Included in labor/quote |
| Fittings & Discharge | $1.50 to $3.00 | Included in labor/quote |
| Total per linear foot | $5.15 to $8.60 | $25.00 to $45.00 |
Building a French drain is labor-intensive because it requires digging a deep trench and moving tons of heavy stone. While professional installation will cost $1,250 to $2,250 for a 50-foot run, doing it yourself is very economical, costing under $450 in materials. For long runs, consider renting a dedicated walk-behind trenching machine or a mini-excavator to save your back.
Frequently asked questions
Do the holes in a French drain pipe face up or down?
Down. When groundwater rises in the trench, it filters through the fabric and enters the pipe from the bottom. If the holes face up, water must saturate the entire trench and rise to the top of the pipe before it can drain. Placing holes down prevents water from pooling in the bottom of the trench.
What is the best gravel for a French drain?
Washed clean drainage stone (usually 3/4" or 1" size). Do not use crusher run, road base, or any stone with "fines" (dust or sand) because they will pack together and clog the water flow. Washed stone has large air spaces between the rocks, allowing water to flow freely.
Do I really need filter fabric for a French drain?
Yes, absolutely. Geotextile filter fabric is non-negotiable. It keeps mud, sand, silt, and soil particles from entering the gravel trench and clogging the spaces between the stones. Without fabric, a French drain will typically clog and fail within 2 to 3 years.
How deep should a French drain be?
For surface/yard drainage, 12 to 18 inches deep is standard. If you are protecting a basement or crawlspace, the trench needs to be deeper — often down to the footing of the foundation wall, which may be 2 to 4 feet deep.
Where should a French drain discharge?
To a safe exit point where the water will not cause issues for you or your neighbors. Good options include a slope that drains to daylight, a pop-up emitter in the yard near a street gutter, or a dedicated underground dry well filled with gravel where the water can slowly percolate back into the ground. Never discharge water onto a neighbor's property.