French Drain Gravel Calculator: Pipe Bedding & Drainage Stone Estimator
Enter your trench dimensions and pipe size. This calculator subtracts the pipe's own volume automatically, so the stone total is the real amount you'll pour around it, not the raw trench volume.
Enter your trench dimensions and pipe size
Material
#57 Clean Stone (locked): 100 lb/ft³ / 1,600 kg/m³
Your results
Net stone needed
2.62 cubic yards
70.6 cu ft
Weight
3.53 tons
7,064 lbs
Pipe volume deducted
4.36 cu ft
Estimated cost
-
Add a price to see cost
Trench volume (gross)
75.0 cu ft
A 50 ft trench, 12" wide and 18" deep, needs 2.62 cubic yards of #57 clean stone, about 3.53 tons, after deducting the 4" pipe's volume.
Design
How a French Drain Is Built
A French drain is a gravel-filled trench with a perforated pipe running through it, designed to intercept groundwater or surface runoff and carry it away by gravity. Water enters through the gaps between the clean stone surrounding the pipe, seeps through the pipe's perforations, and flows along the pipe toward a lower outlet point, a daylight exit, a dry well, or a municipal storm drain, depending on the site.
The stone isn't just backfill. It's the mechanism. Because it's clean, single-sized #57 with no fine particles, water moves through it almost as freely as through open air, which is what lets the drain actually intercept and redirect water instead of just filling up like a wet ditch.
Sizing
Trench Sizing Guidelines
Most residential French drains run 18 to 24 inches deep and 12 inches wide, which is deep enough to sit below typical planting depths and, in cold climates, below the frost line where winter freezing could otherwise damage the pipe. Shallower trenches work for simple downspout diversion; deeper ones handle groundwater interception around a foundation.
Slope matters as much as depth. A French drain has no pump, so it relies entirely on gravity to move water toward the outlet. A consistent fall of about 1 inch per 8 feet of trench length is a common minimum; less than that and water can pool in low spots along the pipe instead of draining away.
Material spec
Why #57 Clean Stone Specifically
#57 stone is washed, angular, and sized between 3/4 and 1 inch with the fine particles screened out entirely. That combination is exactly what a drain needs. The angular shape keeps individual stones from packing tightly the way rounded pea gravel would, and the absence of fines means there's nothing to slowly clog the voids between stones. At 100 pounds per cubic foot, it's also dense enough to stay in place rather than floating or shifting as water moves through it.
A fines-heavy material like crusher run or Class 5 would be a mistake here. The same fine particles that make those materials compact so well for a driveway base are precisely what would choke a drain's ability to move water. If your project needs both a compactable base nearby and a drain, expect to order two different materials for two different reasons.
Common mistakes
Common French Drain Installation Mistakes
Skipping filter fabric or a filter sock around the pipe is the most common failure point. Without it, soil migrates into the stone and eventually into the pipe's perforations, gradually choking the drain from the inside over a few seasons. Using the wrong stone grade is the second most common mistake, for the reasons above. The third is inadequate slope, since even a well-built drain with the right stone won't move water without a consistent fall toward the outlet.
If your drainage project sits alongside a driveway build, the gravel driveway calculator sizes the base and surface layers for the driveway itself, so you can order both jobs' material in one trip. For any other crushed stone grade, including the same #57 used here, the crushed stone calculator adds an area and depth calculation on top of the weight conversion.
For the full site, every material this calculator covers, plus coverage charts and cost comparisons, see the gravel coverage calculator homepage.
Answers
Frequently Asked Questions
How much gravel do I need for a French drain?
Calculate your trench volume, length times width times depth, then subtract the volume the pipe itself takes up, since that space isn't filled with stone. A 50 ft trench, 12 inches wide and 18 inches deep, with a 4-inch pipe, needs about 2.7 cubic yards of #57 clean stone net of the pipe. This calculator does that subtraction automatically.
Why does French drain gravel need to be clean, with no fines?
A French drain works by giving water somewhere to move freely toward the pipe. Fine particles (dust, sand, stone screenings) fill the gaps between larger stones and clog the very voids the drain depends on, which is why #57 or a similarly clean, washed stone is specified rather than a fines-heavy material like crusher run or Class 5.
How deep should a French drain trench be?
A typical French drain trench runs 18 to 24 inches deep, with 2 to 4 inches of clean stone bedding beneath the pipe and 6 or more inches of stone covering it. Deeper trenches handle more water and sit below the frost line in cold climates, which matters if the drain needs to keep functioning through winter.
Do I need a filter sock or fabric around a French drain?
Yes, in almost every installation. A filter sock, a fabric sleeve around the perforated pipe, or a layer of filter fabric wrapping the entire stone envelope keeps soil from migrating into the stone and eventually the pipe itself. Skipping this is one of the most common reasons a French drain slowly stops working over a few years, as fine soil particles work their way in and clog the system from the inside.
What size pipe does a French drain need?
Residential French drains typically use 4-inch perforated pipe, which handles normal yard drainage and downspout runoff comfortably. Larger properties, steep grades, or drains handling water from multiple sources sometimes step up to 6-inch pipe. The pipe diameter matters for this calculator specifically because it's deducted from the trench volume. A bigger pipe means less stone needed for the same trench.
What are the most common French drain mistakes?
The most frequent mistakes are skipping the filter fabric or sock (letting soil clog the system over time), using a fines-heavy stone instead of clean #57 (defeating the drain's purpose from day one), and not giving the trench enough slope. A French drain relies on gravity, so even a gentle, consistent fall toward the outlet matters more than trench depth alone.