French Drain Gravel Calculator
Installing a French drain to manage yard runoff or water near your foundation? Enter your trench dimensions and pipe size to calculate the exact cubic yards and tons of drain rock or washed gravel required.
Results
How much rock a trench actually swallows
A French drain is a gravel-filled trench with a perforated pipe in it, carrying groundwater away from a foundation or a low spot in the yard. The volume is straightforward, but the number surprises people, because a trench is deeper than it looks from above.
The calculator works in this order:
cu ft = length × (width ÷ 12) × (depth ÷ 12), minus the pipe, then cubic yards = (cu ft ÷ 27) × waste factor, and tons = cubic yards × 1.35.
| Trench | Cubic yards | Tons | 50 lb bags |
|---|---|---|---|
| 25 ft × 12 in × 18 in | 1.44 | 1.94 | 78 |
| 50 ft × 12 in × 18 in | 2.88 | 3.89 | 156 |
| 50 ft × 18 in × 24 in | 5.93 | 8.01 | 321 |
| 100 ft × 12 in × 24 in | 7.79 | 10.52 | 421 |
The bag column is there to make a point rather than to be used. A routine 50-foot drain needs 156 bags of rock, which is roughly four pallets and several trips. Past about a cubic yard, bulk delivery is cheaper and very much faster.
The pipe displaces less than people expect
It is tempting to skip the pipe subtraction, and for once the intuition is nearly right. A 4-inch pipe in a 12 × 18 inch trench takes up 5.8% of the volume: on a 50-foot run that is 4.4 cubic feet, or 0.18 cubic yards out of 3.06.
| Pipe | Displaces | Gravel needed |
|---|---|---|
| No pipe | — | 3.06 cu yd |
| 4 in perforated | 4.4 cu ft | 2.88 cu yd |
| 6 in perforated | 9.8 cu ft | 2.66 cu yd |
At 6 inches it starts to matter, and the calculator subtracts it either way. The larger effect is the waste factor: gravel settles into the trench walls and into the soil beneath, so ordering the exact computed volume reliably leaves you short.
Washed stone only, and why the fines matter
The stone specification is the part of a French drain that decides whether it still works in five years. Water has to move through the gaps between stones, so the stone has to have gaps.
Use 3/4 inch washed drain rock or clean #57 stone. Do not use crusher run, quarry process, or anything sold as a compacting base material. Those contain fines — sand and rock dust deliberately graded in so the material locks up solid under a driveway. That is the opposite of what a drain needs. The fines migrate down, silt the filter fabric, and the trench becomes a buried strip of damp concrete-like fill that carries no water at all.
The same logic sets the pipe orientation: perforations face down. Water rises into the pipe from the trench bottom and is carried away; holes facing up only collect water that has already traveled the full depth of the gravel.
What this number does not cover
The calculator prices gravel. A working drain needs several things it does not count:
- Filter fabric to wrap the trench, which is what keeps surrounding soil out of the stone. Budget the trench perimeter plus generous overlap.
- The perforated pipe itself, plus couplings, and a solid (unperforated) run wherever the line crosses under a path or driveway.
- An outlet. A French drain moves water; it does not absorb it. It needs somewhere lower to discharge — daylight on a slope, a dry well, or a storm connection where that is legal.
- Spoil removal. Every cubic yard of rock going in means roughly a cubic yard of soil coming out, and it will not fit back.
Slope is not a material cost but it decides whether any of this works: aim for a fall of about 1% along the run, an inch every 8 to 10 feet, consistently downhill toward the outlet.
Frequently asked questions
What size gravel is best for French drains?
3/4 inch washed river rock or clean #57 drain rock is ideal. Do not use gravel with fines (like crusher run) as small particles will clog the filter fabric and pipe.
Should the pipe sit at the bottom of the trench?
Place a 2-inch bedding layer of gravel at the bottom first, then lay the pipe on top with perforations facing down, and cover with remaining gravel.
How much gravel do I need for a 50 foot French drain?
A 50-foot trench 12 inches wide and 18 inches deep needs about 2.9 cubic yards, or roughly 3.9 tons, of 3/4 inch drain rock — including a 10% settling allowance and subtracting the volume of a 4-inch pipe.
Do I need filter fabric around a French drain?
Yes, in almost every soil. Fabric lets water pass while keeping silt and clay out of the stone. Without it, fine soil works into the gaps between stones over a few seasons and the trench slowly stops draining. Wrap the trench, place the stone, then fold the fabric over the top before backfilling.