Calculate How Much Drainage Stone You Need
Drainage projects often require more stone than expected because the trench itself must be filled around the drainage pipe.
Use this free Drainage Stone Calculator to estimate the amount of gravel or drainage stone needed for French drains, drainage trenches and similar subsurface drainage systems.
Enter the trench length, width and depth, then add the drainage pipe diameter if applicable. The calculator estimates the stone volume after subtracting the approximate volume occupied by the pipe.
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Free Drainage Stone Calculator
Drainage Stone Calculator
Estimate the cubic yards and tons of drainage stone needed for a French drain or drainage trench.
How Much Gravel Do I Need for a French Drain?
The amount of drainage stone needed depends on:
- Trench length
- Trench width
- Trench depth
- Pipe diameter
- How much of the trench is actually filled with stone
- Extra material needed for grading and handling
A French drain trench may look relatively small, but long runs can require a significant amount of stone.
For example, a trench that is 100 feet long, 18 inches wide and 18 inches deep has a gross volume of approximately 8.3 cubic yards before subtracting the pipe and adding any extra-material allowance.
Use the calculator above for a more accurate project estimate.
Why Pipe Diameter Matters
A perforated drainage pipe takes up space inside the trench.
If you’re calculating drainage stone for a French drain, it makes sense to subtract the approximate volume displaced by the pipe rather than treating the entire trench as solid stone.
The calculator estimates the pipe as a cylinder and subtracts that volume from the gross trench volume.
For smaller projects, the difference may be modest. On long drainage runs using 4-inch or 6-inch pipe, it can become more meaningful.
How the Drainage Stone Calculator Works
First, calculate the gross trench volume:
Length × Width × Depth = Trench Volume
Widths and depths entered in inches are converted to feet.
Then calculate the approximate volume of the drainage pipe:
π × Radius² × Pipe Length
The pipe volume is subtracted from the trench volume:
Trench Volume − Pipe Volume = Stone Volume
The remaining volume is converted to cubic yards:
Cubic Feet ÷ 27 = Cubic Yards
Finally, the selected extra-material allowance is added.
If a material density is selected, the calculator also provides an estimated tonnage.
What Size Stone Is Used for French Drains?
French drains generally use clean, washed stone that allows water to move freely through the drainage zone.
Common materials may include:
- #57 stone
- #67 stone
- Washed gravel
- Other clean, open-graded aggregates
The exact stone size should depend on the drainage design, soil conditions, pipe type and locally available materials.
Avoid assuming that all gravel is suitable for drainage. Materials containing large amounts of fines can restrict water movement.
Should You Order Extra Drainage Stone?
Usually, yes.
Real trenches are rarely perfectly uniform. Width and depth can vary, some stone may be lost during handling, and additional stone may be needed around fittings, connections, basins or low areas.
A 5% to 10% allowance is a reasonable starting point for many straightforward projects.
Complex drainage systems may require more careful estimating.
Cubic Yards vs. Tons
Drainage stone may be sold by the ton or by the cubic yard.
The actual weight of a cubic yard varies depending on:
- Aggregate type
- Stone size
- Moisture content
- Supplier
- Void space
A commonly used estimating range for clean drainage stone is around 1.3 to 1.5 tons per cubic yard, but the supplier’s actual conversion should always be used whenever possible.
Drainage Stone Calculator FAQ
How much gravel do I need for a 100-foot French drain?
It depends on trench width, trench depth and pipe size. For example, a 100-foot trench that is 18 inches wide and 18 inches deep has a gross volume of about 8.3 cubic yards before subtracting pipe displacement and adding extra material.
How much stone goes around a French drain pipe?
That depends on the drainage design. In many systems, clean stone surrounds the perforated pipe and extends above it. The required thickness varies by project.
Does the calculator subtract the pipe volume?
Yes. If you enter a pipe diameter, the calculator estimates the cylindrical volume occupied by the pipe and subtracts it from the trench volume.
Should I use #57 stone for a French drain?
#57 washed stone is commonly used for French drains and other drainage applications because it contains relatively little fine material and allows water to move through the aggregate.
How many tons are in a cubic yard of drainage stone?
There is no universal conversion. A rough estimating range may be about 1.3 to 1.5 tons per cubic yard, but you should use your supplier’s actual conversion whenever available.
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