French Drain Gravel Calculator
A French drain is a gravel-filled trench carrying a perforated pipe. To estimate the gravel, compute the trench volume as length times width times depth, then subtract the cylindrical volume of the pipe, which is pi times its radius squared times its length. The difference is the gravel fill. This calculator reports that fill in cubic feet, cubic yards, and tons, using a supplier-provided density rather than a guessed weight so the tonnage reflects your actual drainage rock.
French drain gravel formula
Trench volume = length * width * depth
Pipe radius (feet) = (pipe diameter inches / 12) / 2
Pipe volume = pi * radius^2 * length
Gravel fill = trench volume - pipe volume
Cubic yards = gravel fill / 27
Tons = cubic yards * density
The pipe is modeled as a cylinder running the trench length. There are 27 cubic feet per cubic yard. The density you enter converts cubic yards to tons for your specific drainage rock.
French drain notes
- A French drain is a sloped, gravel-filled trench with a perforated pipe that carries water away.
- Typical trenches are 6 to 12 inches wide and 18 to 24 inches deep with at least 1 percent slope.
- The pipe volume is subtracted because the pipe occupies space gravel cannot fill.
- One cubic yard equals 27 cubic feet; multiply cubic yards by density to convert to tons.
- Wrap the trench in filter fabric to keep fines out of the gravel and maintain drainage over time.
French drain gravel: frequently asked questions
How much gravel does a French drain need?
The gravel volume is the trench volume minus the volume taken up by the drain pipe. Trench volume equals length times width times depth. The pipe is a cylinder: pi times its radius squared times its length. Subtract the pipe volume from the trench volume to get the gravel fill in cubic feet, then convert to cubic yards or tons.
Why subtract the pipe volume?
A French drain is a gravel-filled trench with a perforated pipe running through it. The pipe occupies space that gravel does not fill, so subtracting the pipe's cylindrical volume from the trench volume gives a more accurate gravel quantity. For a 4 inch pipe in a typical trench the pipe is a small fraction, but it is worth accounting for on long runs.
What size trench does a French drain use?
A common French drain trench is about 6 to 12 inches wide and 18 to 24 inches deep, sloped at least 1 percent so water flows. Width and depth depend on the volume of water and the pipe size. Enter your planned trench width and depth in feet, or convert from inches by dividing by 12.
How do I convert gravel volume to tons?
Multiply the gravel volume in cubic yards by the gravel's density in tons per cubic yard, a figure your supplier publishes for the specific drainage rock. Clean drainage gravel commonly weighs around 1.3 to 1.5 tons per cubic yard, but enter your supplier's value so the tonnage is accurate.
Should I add extra for compaction or waste?
Drainage gravel for a French drain is placed loose around the pipe and not heavily compacted, so settlement is modest. Order a small surplus, often around 10 percent, to allow for uneven trench walls and topping up. This calculator reports the geometric fill volume; add your own allowance on top when ordering.
Official sources
- U.S. Department of Agriculture Natural Resources Conservation Service: Subsurface drainage practice standards.
- U.S. National Institute of Standards and Technology: Unit conversion (cubic yard, ton).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.