Read the trench before ordering
Measure the rock envelope, not just the excavation
If soil or sod will cover the drain, that top layer is not drain rock. Enter the full excavated depth and the planned cover separately. The calculator takes the pipe's outside volume out of the remaining stone zone. A trench with no pipe can use a zero pipe diameter.
The sketch shows the default inputs. The gravel-colored zone includes the gaps between stones. You buy the whole loose rock volume; the gaps are not a second deduction.
The takeoff method
From trench dimensions to a supplier order
- Find the stone height.Excavated depth − non-rock top cover. For the example, 18 − 4 = 14 inches.
- Calculate the envelope.Length × width × stone height after converting inches to feet. Here: 50 × 1 × 14/12 = 58.33 cubic feet.
- Remove the pipe.For a 4-inch outside diameter, length × π × (4/24)² = 4.36 cubic feet. The finished stone fill is 53.97 cubic feet, or 2.00 cubic yards.
- Plan the purchase.Apply any chosen site allowance to the stone volume. At 10% extra and 2,500 lb per cubic yard, the example is 2.20 cubic yards or 2.75 US short tons.
A 4-inch nominal pipe can have a different outside diameter by product. Replace the default with the manufacturer's actual dimension when it matters. The estimate assumes the entire pipe cross-section is within the rock zone.
Choose the actual product
Clean stone, gradation, and supplier weight
“Drain rock” describes a job, not one universal rock type. A yard may sell washed, rounded gravel or clean, angular crushed stone under the same name. #57 stone is one common graded option, but check the product's stated size and whether it includes fines. A dense crusher run base includes fines and serves a different purpose.
| Supplier product | Published loose weight | Why it matters |
|---|---|---|
| South Bay 3/4-inch washed round drain rock | 2,500 lb/cu yd | The calculator's editable starting value. |
| Three Z #57 washed rounded gravel | About 2,800 lb/cu yd | The same volume produces a higher ton estimate. |
Ask whether your yard sells by cubic yard or by scale ton, and use its weight for that specific product. If you are comparing with other rock weights, keep pounds per cubic yard and US short tons separate from metric tonnes.
Beyond the material count
Check the outlet, soil, and fabric plan
Follow the water
A subsurface drain needs a route to an appropriate outlet. Check the site grade and where water can discharge before fixing the trench size. The stone calculator cannot establish drainage capacity or an allowable discharge location.
Keep soil out of the stone
A filter fabric or specified drain envelope can separate soil from the aggregate. The tool estimates one full-wrap cut width: twice the trench width, twice the stone height, plus your selected top overlap. It does not select a fabric or count rolls; soil and project specifications control that choice.
Match the installation
Foundation, retaining-wall, and engineered stormwater drains may call for a particular stone gradation, pipe, filter, and outlet. Follow the project detail. For a simple garden drain, still check nearby utilities before digging and avoid directing water onto another property.
Drain rock questions
How much drain rock do I need for a 50-foot French drain?
A 50-foot trench that is 12 inches wide and 18 inches deep needs about 2.00 cubic yards of installed stone if 4 inches at the top are reserved for cover and a 4-inch outside-diameter pipe runs the full length. A 10% optional allowance brings the example order to 2.20 cubic yards. Change the dimensions to match the part of your trench actually filled with rock.
Should I subtract the pipe and the voids between the rocks?
Subtract the outside volume of a pipe that lies within the rock-filled zone. Do not subtract the air spaces between stones: a supplier cubic yard of loose drain rock already includes those spaces. Void space matters to drainage storage and flow design, which this quantity estimator does not calculate.
What is the best rock for a French drain?
Look for a clean, appropriately graded, free-draining product specified for your trench and pipe. Washed angular stone and washed rounded drain gravel are both sold for this use. Confirm size, fines content, and any project specification with the supplier; crusher run with fines is a different product.
How much does a cubic yard of drain rock weigh?
There is no single weight for every drain-rock product. For example, South Bay Materials lists 2,500 pounds per cubic yard for its 3/4-inch drain rock, while Three Z lists about 1.4 US short tons, or 2,800 pounds, per cubic yard for its #57 washed gravel. Use the weight quoted for the stone you will buy.