A trench is a box with a width you did not choose
Volume for a trench is the same multiplication as a slab — length times width times depth — and the same conversion to tons. What makes trenches different is that two of the three dimensions are decisions made with a shovel:
cubic feet = length × width × (depth ÷ 12)
tons = cubic feet ÷ 27 × tons per yard
Crushed stone runs 1.45 tons per cubic yard; the calculator rounds yards to a quarter and tons to a tenth, and both up.
A 30-foot run at 12 inches wide and 12 inches deep is 30 cubic feet. That is 1.11 cubic yards, and at 1.45 tons per yard it is 1.61 tons — rounded up to 1.7 tons, or 68 bags of 50 lb. Fill the same trench 18 inches wide and it is 45 cubic feet, 2.5 tons, 100 bags. The length never changed.
Chart: gravel for a 12 × 12 trench, by length
Crushed stone, 12 inches wide, 12 inches deep — the cross-section most residential french drains and downspout runs end up at.
| Trench length | Cubic feet | Cubic yards | Tons | 50 lb bags |
|---|---|---|---|---|
| 15 ft | 15 | 0.75 | 0.9 | 36 |
| 20 ft | 20 | 0.75 | 1.1 | 44 |
| 30 ft | 30 | 1.25 | 1.7 | 68 |
| 50 ft | 50 | 2 | 2.7 | 108 |
| 75 ft | 75 | 3 | 4.1 | 164 |
These are the figures the gravel calculator returns for the same trench, rounded up. Note the cubic yards column rounding to a quarter: a 20-foot trench is 0.74 yards of stone, rounded to 0.75, which is why the tonnage on short runs steps in visible jumps.
Bags stop making sense somewhere around 30 feet
A 15-foot drain is 36 bags, which is a reasonable morning. Thirty feet is 68 bags — more than a ton — and 75 feet is 164 bags, which is four trips and a stack of empties. The arithmetic is the same tone as the driveway decision: a ton is 40 bags, so anything past a ton or so is easier delivered in bulk and moved once with a wheelbarrow.
Bulk stone is sold by the cubic yard, and a yard of crushed stone is about 1.45 tons. The calculator shows both, so the conversion is already done when you call: 1.25 yards for a 30-foot run, 3 yards for 75 feet.
Chart: width and depth on a 30-foot trench
The two dimensions you control, and the reason a trench measured after digging is a different order from a trench measured on paper.
| Width × depth (in) | Cubic feet | Tons | 50 lb bags |
|---|---|---|---|
| 12 × 12 | 30 | 1.7 | 68 |
| 18 × 12 | 45 | 2.5 | 100 |
| 24 × 12 | 60 | 3.3 | 132 |
| 12 × 18 | 45 | 2.5 | 100 |
| 12 × 24 | 60 | 3.3 | 132 |
Width and depth are interchangeable in the arithmetic: 18 inches wide and 12 deep is the same 45 cubic feet as 12 wide and 18 deep, and both are 2.5 tons. They are not interchangeable in the ground. Width is what the shovel produced, and depth is what the pipe and its bedding need — so the honest way to size the order is to measure a couple of finished sections rather than trust the number on the plan.
Over-dig is the hidden tonnage
Two extra inches of depth across a 30-foot run is 30 × 1 × 0.167, which is 5 cubic feet — 0.185 yards, about 0.27 tons. Round it up and you are ordering a third of a ton of stone for a mistake nobody would call a mistake. A trench that wanders to 16 inches wide for half its length adds more.
Bags absorb this badly, because bag counts are only sold whole and the step between 68 and 72 is a trip. Bulk absorbs it well, which is the second argument for bulk on a long run: ordering a half yard over is a rounding decision rather than a return trip.
What actually goes in the trench
- Washed, open-graded stone. #57 and similar. The point of the stone is the void space between pieces, and stone with fines in it fills those voids rather than draining through them.
- Filter fabric. It lines the trench and laps over the stone, keeping soil out. Fabric is cheap against re-excavating a filled trench.
- Pipe bedding, not pipe burial. Six inches of stone under the pipe and six over it is the common section; the stone is a path for water, not a replacement for the pipe.
- A fall to a real outlet. Water has to go somewhere that drains. A trench with nowhere to daylight is a storage tank that fills with silt.
None of that changes the tonnage — it changes the shape of the box the stone occupies. If the design calls for fabric-wrapped stone and soil above it, the depth in the calculator is the stone depth, not the trench depth.
The mistakes that cost a ton
01
Using the planned width instead of the dug width
A trench dug for a 12-inch bucket is rarely 12 inches wide at the top. Width scales the volume linearly, so a trench that finishes 18 inches wide across its full length takes half again the stone — 2.5 tons instead of 1.7 on a 30-foot run.
02
Filling the whole trench with stone
A french drain is stone around the pipe, then filter fabric, then soil. Stone all the way to the surface is a trench that collects silt from above, and the tonnage for that version is the whole box rather than the box minus the backfill.
03
Using crusher run in a drain
Dense-grade stone is designed to bind, and the fines in it wash into the pipe's openings and clog the drain. Washed stone — #57 or similar — is what keeps the voids open. Base gravel belongs under a slab, not around a perforated pipe.
04
Skipping the filter fabric
Fabric is what keeps soil out of the stone. Without it the drain silts up from the trench walls, and the fix is re-excavating a trench you already filled — the stone is the cheap part of that job.
05
Buying bags for a full run
A 30-foot trench at 12 × 12 inches is 68 bags of 50 lb — well over a ton. Bags make sense for a short downspout run; for a drain around a foundation, bulk is fewer trips and less plastic.
Run your own trench count
The link below opens the gravel calculator on the 30-foot trench from this page — 1 foot wide, 12 inches deep, crushed stone — showing 30 cubic feet, 1.25 cubic yards, 1.7 tons, and 68 bags. Change the length, the width, or the depth and all four columns follow.
Working out a surface instead of a trench? How many square feet a ton covers runs the same tonnage from the coverage side, and gravel for a driveway covers the multi-layer case where this arithmetic is repeated three times.