French Drain Calculator
ConstructionCalculate the cubic yards of gravel backfill needed for a French drain trench based on trench length, width, depth, and pipe diameter. Free tool for drainage projects.
Gravel Needed
2.62
Perforated Pipe Length
50
What is a French Drain?
A French drain calculator estimates the volume of gravel backfill needed for a drainage trench, accounting for the space taken up by the perforated pipe that runs through it. A French drain redirects water away from a foundation, yard, or low-lying area by combining a gravel-filled trench with a perforated pipe that channels water to a safe discharge point.
Because the pipe occupies real volume inside the trench, simply calculating trench volume alone overestimates how much gravel you actually need — especially with larger diameter pipe in a narrower trench. This calculator subtracts the pipe's cylindrical volume from the total trench volume to give an accurate gravel figure in cubic yards, the standard unit gravel is sold and delivered by.
This is a common step in yard drainage and foundation waterproofing projects, where getting the material order right the first time avoids either a mid-project supply run or paying for gravel you don't use. If you're using crushed limestone instead of standard gravel, the Limestone Calculator applies the same logic with a limestone-specific density.
How to use this French Drain calculator
- Enter your Trench Length in feet — the total straight-line run of your planned drain.
- Enter your Trench Width in inches — 12 inches is a common width for residential trenches.
- Enter your Trench Depth in inches — 18 inches is typical, though foundation drains may go deeper.
- Set your Pipe Diameter in inches — 4 inches is standard for most residential French drains.
- Read the Gravel Needed result at the top of the result card — this is your gravel order in cubic yards.
- Check the Perforated Pipe Length figure to confirm how much pipe to order alongside the gravel.
Formula & Methodology
Trench volume: Vt = (L × 12) × W × D Pipe volume: Vp = π × (d ÷ 2)² × (L × 12) Gravel volume: Vg = max(0, Vt − Vp) ÷ 46,656 Where L is trench length in feet, W is trench width in inches, D is trench depth in inches, d is pipe diameter in inches, and 46,656 converts cubic inches to cubic yards. Worked example: For a 50 ft trench, 12 in wide, 18 in deep, with a 4 in pipe: - Trench volume: (50 × 12) × 12 × 18 = 129,600 in³ - Pipe volume: π × (4 ÷ 2)² × 600 = 7,540 in³ - Gravel volume: (129,600 − 7,540) ÷ 46,656 ≈ 2.62 yd³
Frequently Asked Questions
The amount depends on your trench's length, width, and depth, minus the space taken up by the perforated pipe running through it. This calculator subtracts the pipe's cylindrical volume from the total trench volume, then converts the remaining space to cubic yards, the standard unit gravel is sold by. For a 50 ft trench that's 12 in wide and 18 in deep with a 4 in pipe, you'd need roughly 3.3 cubic yards of gravel.
A French drain is a gravel-filled trench containing a perforated pipe that redirects surface and groundwater away from a foundation, yard, or low-lying area. Water percolates down through the soil and gravel, enters the pipe through its perforations, and flows by gravity to a safe discharge point away from the problem area. The gravel serves two purposes: it prevents soil and debris from clogging the pipe, and it speeds up water infiltration into the drain.
Most residential French drains are dug 18 to 24 inches deep, though shallower trenches around 12 inches can work for simple surface water diversion. Deeper trenches are typically needed near foundations to intercept water before it reaches the footing. This calculator's Trench Depth input defaults to 18 inches but can be adjusted from 6 to 60 inches for different applications.
A 4 inch perforated pipe is the most common choice for residential French drains, offering enough capacity for typical yard drainage without being oversized for a standard trench. Larger 6 inch pipe is sometimes used for high-volume drainage areas or commercial applications. This calculator lets you enter any pipe diameter from 2 to 12 inches to match your specific design.
Yes, most French drain installations wrap the gravel and pipe in landscape fabric (geotextile fabric) to prevent surrounding soil from migrating into the gravel and clogging the drain over time. This calculator doesn't estimate fabric quantity directly, but you can measure the trench's total surface area (length times the sum of width and twice the depth) to estimate fabric needed for a full wrap.
Washed, angular gravel between 3/4 inch and 1.5 inches in size, often called drain rock or clean stone, is the standard choice for French drains. Washed gravel is free of fine particles and dust that could clog the pipe's perforations over time, unlike unwashed crushed stone. Avoid rounded pea gravel alone, since it compacts more easily and offers less pore space for water to move through.
Yes, a trench shallower than about 12 inches may not intercept enough subsurface water flow to be effective, especially in clay-heavy soils where water moves slowly downward. If your drainage problem is primarily surface water pooling, a shallower trench can still help, but foundation drainage generally needs greater depth to reach below the water-saturated zone near a structure.
DIY French drain installation typically costs $5 to $15 per linear foot for materials like pipe, gravel, and fabric, while professional installation can run $20 to $50 per linear foot including labor and excavation. Once you know your gravel volume from this calculator, multiply by your local price per cubic yard (often $40-$60 delivered) to estimate the gravel portion of your material budget.
No, this calculator subtracts the pipe's cylindrical volume from the total trench volume, since the pipe itself takes up space that gravel won't fill. Ignoring the pipe's volume, especially with larger diameter pipe in a narrow trench, can lead to over-ordering gravel by a meaningful margin on longer runs.
The [Gravel Calculator](/gravel-calculator/) is useful for general fill and base material estimates beyond drainage trenches, and the [Limestone Calculator](/limestone-calculator/) helps if you're using crushed limestone instead of standard gravel. For the surrounding graded area, the [Square Footage Calculator](/square-footage-calculator/) helps plan total yard coverage.
Yes, the same volume formula applies to any trench-style drain that combines gravel backfill with a perforated pipe, including curtain drains and overflow trenches from a dry well. Just enter the specific trench dimensions and pipe size for your design to get an accurate gravel volume estimate.
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