Drain Rock Calculator
Calculate tons of washed drain rock for a dry well or drainage trench using its 1.4 tons per cubic yard density, plus total cost.
How this calculator works
Drain rock is sold by weight but measured by volume, so this calculator converts your length, width, and depth into cubic yards, then multiplies by drain rock’s typical density of 1.4 tons per cubic yard to get the tons you need to order.
Formula: Cubic yards = (Length ft × Width ft × Depth ft) ÷ 27. Tons = Cubic yards × 1.4.
Worked example
A 20 ft long, 3 ft wide drainage trench filled 12 inches deep:
- Depth in feet: 12 ÷ 12 = 1.0 ft
- Cubic yards: (20 × 3 × 1.0) ÷ 27 ≈ 2.2222 yd³
- Tons: 2.2222 × 1.4 ≈ 3.11 tons
- At $45/ton, that’s $140.00 before waste.
Coverage reference
| Area | Tons needed (at 12 in) |
|---|---|
| 10 ft × 1.5 ft | 0.78 tons |
| 20 ft × 3 ft | 3.11 tons |
| 40 ft × 6 ft | 12.44 tons |
How to measure your area
Measure the length and width of the excavated drainage area or dry well footprint in feet, and use the full fill depth called for in your design, since drain rock isn’t compacted the way a base layer would be. If you’re backfilling around a perforated pipe or drain tile, measure the trench as dug rather than subtracting for the pipe — the difference is usually within a normal waste buffer.
Frequently asked questions
What is drain rock, and how is it different from crusher run?
Drain rock is washed, clean stone with the fines screened out, so water passes through it freely without being blocked or filtered by trapped dust and silt. Crusher run is the opposite — it's engineered to include fines so it compacts tight, which is exactly the property that makes it wrong for drainage.
What's the right size drain rock for a dry well or drainage trench?
3/4-inch to 1.5-inch washed stone (often sold as #57 or similar) is a common general-purpose drain rock size — large enough to leave generous void space for water flow, but not so large that it's hard to handle or wastes trench volume around a perforated pipe.
Why does drain rock need to be washed?
Unwashed stone carries rock dust and fine particles that gradually settle into the voids between larger pieces, reducing the open space water needs to move through and eventually clogging the drainage system. Washing removes those fines before the stone ever reaches your site.
Does drain rock need to be wrapped in fabric?
Usually yes — lining the excavation with landscape or filter fabric before placing drain rock keeps surrounding soil from migrating into the clean stone over time, which would otherwise slowly clog the same void space the rock is there to preserve.
What depth of drain rock is typical for a dry well?
12 inches is a common minimum fill depth for a residential dry well or drainage pit, though deeper pits are common where soil percolation is slow and more storage volume is needed to handle peak runoff.
Should I add waste when ordering drain rock?
Yes, a 10% buffer is reasonable for an irregular excavation shape and material lost while backfilling around pipe or backfilling the final few inches to grade.
Estimates only. Verify quantities with your supplier before purchasing.