Drain Rock Calculator
A drain rock calculator finds cubic yards (L × W × depth ÷ 27) and tons at 1.4 t/yd³. A 20 × 3 ft trench 12 in deep needs 3.11 tons, 3.42 with 10% extra.
What your order looks like
One bulk delivery is usually easier than hauling it yourself.
2.22 yd³ · 3.1 tons
30 wheelbarrow trips (2 cu ft each)
5 pickup loads (0.75 ton each)
1 small dump truck load (6 tons each)
Rules of thumb: A wheelbarrow trip is 2 cubic feet of heavy material, about 200 lb, a load most people can push. A pickup load is 0.75 ton, the payload of a full-size half-ton pickup (1,500 lb). A small single-axle dump truck carries about 6 tons.
How much drain rock do I need?
A drain rock calculator converts the size of a trench or pit into the tons of washed stone to order. Cubic yards = length × width × (depth in inches ÷ 12) ÷ 27. Tons = cubic yards × 1.4. A drainage trench 20 ft long, 3 ft wide, and filled 12 inches deep holds 2.22 cubic yards, which is 3.11 tons, or 3.42 tons with the 10% waste box ticked. At $45 per ton that is $140.00, or $154.00 with waste.
Drain rock is clean stone used where water has to move: French drains, dry wells, footing drains, and the backfill behind retaining walls.
The formula in plain words
- Cubic feet = length × width × fill depth in feet.
- Cubic yards = cubic feet ÷ 27.
- Tons = cubic yards × 1.4.
- With waste = tons × 1.10.
- Cost = tons × price per ton.
Use the depth of stone, not the depth of the hole. If the top 6 inches of a trench will be topsoil and sod, leave that out.
Worked examples
Yard drainage trench: 20 × 3 ft, 12 in of rock, $45/ton
- Cubic feet: 20 × 3 × 1 = 60 ft³
- Cubic yards: 60 ÷ 27 = 2.22 yd³
- Tons: 2.22 × 1.4 = 3.11 tons, $140.00
- With 10%: 3.42 tons, $154.00
Downspout dry well: 6 × 6 ft pit, 36 in of rock, $45/ton
- Cubic feet: 6 × 6 × 3 = 108 ft³
- Cubic yards: 108 ÷ 27 = 4.00 yd³
- Tons: 4 × 1.4 = 5.60 tons, $252.00
- With 10%: 6.16 tons, $277.20
A 4 in perforated pipe takes up only about 0.1 cubic feet per foot of trench, so there is no need to subtract it. The waste allowance covers it.
Drain rock per 10 feet of trench
| Trench width | Rock depth | Cubic yards | Tons |
|---|---|---|---|
| 12 in | 12 in | 0.37 | 0.52 |
| 12 in | 18 in | 0.56 | 0.78 |
| 18 in | 18 in | 0.83 | 1.17 |
| 18 in | 24 in | 1.11 | 1.56 |
| 24 in | 24 in | 1.48 | 2.07 |
| 24 in | 36 in | 2.22 | 3.11 |
| 36 in | 12 in | 1.11 | 1.56 |
Multiply by your trench length in tens of feet. A 60 ft trench, 18 in wide with 24 in of rock, needs about 6 × 1.56 = 9.36 tons.
Common drain rock types
| Stone | Typical size | Good for |
|---|---|---|
| #57 washed stone | about ½–1 in | French drains, footing drains, under slabs |
| ¾ in clean crushed | about ¾ in | General drainage, behind walls |
| 1½ in washed | about 1–1½ in | Dry wells, large voids |
| Washed river rock | 1–3 in | Visible swales and dry creek beds |
Names and gradations vary between quarries. Ask for “washed” or “clean” stone with no fines.
How to measure
- Measure the trench as dug. Walls are rarely straight, so take the width in a few places and average it.
- For a dry well, measure the pit’s length, width, and the depth of stone you will place.
- Long trenches are often easiest with the trench volume calculator, then convert to tons here.
- Check the fall of the trench with the drainage slope calculator before you order.
Common mistakes
- Buying crusher run or road base. The dust fills the voids and the drain stops working.
- Skipping the fabric. Without a filter fabric wrap, silt washes in over a few seasons. Size it with the landscape fabric calculator.
- Compacting it. Drain rock only needs light tamping. Heavy compaction crushes stone and closes voids.
- Counting the topsoil cap as rock. Measure stone depth only.
For pipe-wrapped trenches, the French drain gravel calculator works around the pipe size, and the French drain cost calculator prices the whole job.
Frequently asked questions
What is drain rock, and how is it different from crusher run?
Drain rock is clean, washed stone with the dust screened out, so the gaps between stones stay open for water. Crusher run keeps its dust so it packs tight, which is great for a driveway base and bad for drainage. Never swap one for the other.
What size drain rock should I use?
Washed ¾ in to 1½ in angular stone is a common all-purpose choice for French drains, dry wells, and footing drains. #57 stone (roughly ½ to 1 in) is widely used. Bigger stone leaves larger voids; very small stone such as pea gravel drains but can migrate into a perforated pipe's holes.
Why does drain rock need to be washed?
Unwashed stone carries fine dust that settles into the gaps and slowly clogs them. Washing removes most of it before delivery. Even then, keep the rock away from mud during installation.
Does drain rock need to be wrapped in fabric?
Usually. Lining the trench or pit with non-woven filter fabric keeps surrounding soil from washing into the stone. Fold the fabric over the top of the rock before backfilling. Use the landscape fabric calculator for roll counts.
How many tons of drain rock for a dry well?
A 6 × 6 ft pit filled 36 in deep is 4.00 cubic yards. At 1.4 tons per cubic yard that is 5.60 tons, or 6.16 tons with the 10% waste box ticked. Pit size depends on roof area and how quickly your soil drains, so a percolation test helps.
Should I add waste when ordering drain rock?
Yes. Tick the 10% box for uneven trench walls and stone lost at the edges. Drain rock is not compacted much, so you do not need the larger allowance used for road base.
Estimates only. Verify quantities with your supplier before purchasing.