French Drain Gravel Weight
The stone in a French drain is clean, open-graded rock, not a base that packs. This calculator uses 1,600 kg/m³, which is 1.35 US tons per cubic yard and 1.60 tonnes per cubic meter. That matches typical #57 limestone. Granite or a wet load can reach about 1.5 tons per yard. The volume is the trench minus the pipe, so the weight is only the stone that actually fits around the pipe.
| Stone | Tonnes / m³ | US tons / yd³ | In a drain? |
|---|---|---|---|
| #57 limestone, the default | 1.60 | 1.35 | Yes. Open, almost no dust. |
| Granite #57 | 1.70–1.75 | 1.43–1.47 | Yes. Heavier clean stone. |
| 3/4 clean | 1.60–1.70 | 1.35–1.43 | Yes, if it is washed. |
| Crusher run, 304, class 5 | 1.8–2.4 | 1.5–2.0 | No. Fines clog the pipe. |
Trench Minus Pipe
Gravel volume = (trench width × trench depth − pipe area) × length. The pipe area is a circle using the outside diameter, because that is the space the pipe occupies. A 4 inch PVC pipe is not 4 inches outside. This tool uses schedule-40 outside diameters: 3 inch pipe is 3.5 inches outside, 4 inch pipe is 4.5 inches outside, 6 inch pipe is 6.625 inches outside. Pick the nominal size and the diameter fills in, or type an outside diameter if the pipe is different.
| Nominal pipe | Outside diameter used | Typical job |
|---|---|---|
| 3 in | 3.500 in | Short footing or downspout drain |
| 4 in | 4.500 in | Most house French drains |
| 6 in | 6.625 in | Long yard drain or a wet lot |
A residential trench is often 12 inches wide and 18 to 24 inches deep, sloped about 1 inch in 8 feet toward daylight or a dry well. Wider and deeper moves more water and uses more stone. The pipe subtraction matters more as the pipe gets large relative to the trench.
Coverage for a Straight Trench
All rows are 12 inches wide, 18 inches deep, 4 inch pipe (4.5 inch outside), at 1,600 kg/m³. Yards are after the pipe is removed.
| Length | yd³ | m³ | US tons | 0.5 ft³ bags |
|---|---|---|---|---|
| 20 ft | 1.03 | 0.79 | 1.39 | 56 |
| 50 ft | 2.57 | 1.97 | 3.47 | 139 |
| 100 ft | 5.15 | 3.93 | 6.94 | 278 |
A 50 foot run is a partial landscape-truck load (5 to 7 tons). A 100 foot run is about one small truck. Bags only make sense for a short test section.
Worked Example: 50 ft, 4 in Pipe
- Trench box: 50 ft × 1 ft × 1.5 ft = 75 ft³ = 2.78 yd³ before the pipe.
- 4 inch pipe, 4.5 inch outside, along 50 ft = 0.20 yd³.
- Gravel = 2.78 − 0.20 = 2.57 yd³, which is 1.97 m³.
- At 1,600 kg/m³: 3.47 US tons and 3.15 tonnes.
- 0.5 ft³ bags: 139. Order the tons.
Enter length 50 ft, width 12 in, depth 18 in, and pipe size 4. The order lines should read 2.57 cubic yards, 1.97 cubic meters, 3.47 tons, and 3.15 tonnes. The formula does not add extra for a sloppy trench. Add about 10 percent if the walls cave as you dig.
Fabric, Stone, and What Not to Put in the Trench
Line the trench with nonwoven drainage fabric, set the perforated pipe on a few inches of stone with the holes aimed as your detail requires, fill with the same clean stone to cover the pipe, and fold the fabric over the top before the soil goes back. The fabric keeps silt out of the voids. A coated weed barrier or a plastic sheet does the opposite: it holds water. Do not wrap only the pipe in a tight sock and then dump dirty fill around it if the design calls for a stone envelope. The stone is the drain. The pipe is the collector.
Use #57 or washed 3/4. Leave crusher run, 304, class 5, and quarry dust for bases. Pea gravel drains, but the round stones are harder to keep in a narrow trench and are a poor choice next to a foundation. Bedding for a solid utility pipe is a different section: use the pipe bedding calculator. Soil that goes back over the fabric is the backfill calculator. The cut, the swell, and the spoil that will not fit back are the trench calculator. Stone size, the fabric wrap, and how the bed and the cover share the trench depth are the French drain gravel guide.
French Drain Questions
How much gravel does a French drain need?
A 50 foot trench, 12 inches wide and 18 inches deep, with a 4 inch pipe, needs 2.57 cubic yards after the pipe is removed. That is 3.47 tons at 1,600 kg/m³. Shorter runs scale almost in proportion: 20 feet is 1.03 cubic yards.
Why subtract the pipe volume?
The trench holds both the pipe and the stone. Counting the whole trench as gravel orders stone that cannot fit. On the 50 foot example the 4 inch pipe takes 0.20 cubic yards, about 8 percent of the trench.
What gravel size goes in a French drain?
Clean #57 or washed 3/4 inch stone. Both are about 1/2 to 1 inch with almost no dust, so water can move. Crusher run and class 5 are the wrong product because the fines seal the pipe.
What pipe size should I use?
Most house drains use 4 inch perforated PVC, which is 4.5 inches outside. A short downspout run can be 3 inch (3.5 inches outside). A long, wet yard often steps up to 6 inch (6.625 inches outside).
Do I need filter fabric?
Yes. Use nonwoven drainage fabric around the stone, folded over the top before you backfill. Skip plastic and coated weed barrier. Those stop the water the drain is there to collect.
How deep and wide should the trench be?
A common residential trench is 12 inches wide and 18 to 24 inches deep, falling about 1 inch every 8 feet toward the outlet. Go deeper if you are intercepting water at a footing.
How much does French drain gravel weigh?
At the default 1,600 kg/m³, one cubic yard is 1.35 US tons and 1.60 tonnes. The 2.57 yard example weighs 3.47 tons. Granite can be closer to 1.5 tons per yard. Change the density if the ticket says so.
Can I use pea gravel in a French drain?
It drains, but the round pebbles are a weak choice in a foundation trench. Use angular #57 or washed 3/4 so the envelope stays put. Keep pea gravel for paths and beds.