Drainage stone has one job and one enemy. The job is to hold open a volume of connected air space that water can move through faster than the surrounding soil. The enemy is fines — silt, clay, sand and stone dust — which fill that air space and turn a functioning drain into a buried gravel-shaped rock.
Nearly every drainage failure traces back to that single mechanism. Not undersized pipe, not the wrong slope, not bad luck. Fines got into the stone, either because the wrong stone was ordered in the first place or because nothing stopped surrounding soil from washing in. Get the rock and the fabric right and a drain will still be moving water in thirty years. Get them wrong and you will be digging it up in five.
Buying tip: Order by the phrase “washed, clean, open-graded” and specify a number — #57 or #67 for most residential work. Never accept “drainage gravel,” “pit run” or “bank run” as a description; those are pit terms with no gradation behind them and they routinely arrive full of sand and silt.
Washed vs Unwashed: The Only Distinction That Matters
Washed stone has been rinsed after crushing to remove everything below roughly a No. 8 sieve. What is left is a narrow band of similarly sized angular pieces that cannot nest tightly, leaving 36 to 42 percent of the placed volume as connected air. That air is the entire product. You are not buying rock; you are buying the space between the rock.
Unwashed material — pit run, bank run, crusher run, road base, anything dense-graded — still contains its full complement of sand and dust. Those fines are excellent when you want a compacted, load-bearing layer, which is exactly what crusher run is for. They are catastrophic in a drain, because they occupy the void space and drop permeability by two or three orders of magnitude. A trench backfilled with crusher run does not drain; it holds water like a sponge and then heaves in winter.
Yards will sometimes hand you a marginal load because a washed pile ran low. The field test is simple: grab a handful and rub it. If your hands come away dusty, or the pile leaves a chalky film, it is not clean enough for a drain. Send it back.
Choosing Size by Application
Bigger stone means bigger individual voids and faster flow, but also harder placement, more settlement as it seats, and a higher risk that fines wash straight through it. Smaller stone filters better and places easier but clogs sooner. The right size is a function of what the drain is doing and what soil surrounds it.
| Application | Recommended stone | Typical depth / volume | Fabric? |
|---|---|---|---|
| French drain (perforated pipe) | #57 washed | 12–18 in trench, pipe 2 in off bottom | Yes — wrap the whole trench |
| Dry well | #57 or #3 | Sized to storage volume at 40% voids | Yes — full envelope |
| Foundation / footing drain | #57 washed | Min 12 in wide, up to within 12 in of grade | Yes, plus filter sock on pipe |
| Downspout splash / dispersal | #57 or river rock | 3–4 in over a 3–4 ft pad | Optional |
| Swale lining (low flow) | #57 to 1.5 in stone | 4–6 in | Yes |
| Swale or channel (high flow) | #2, #1 or small riprap | 6–12 in | Yes, heavy woven |
| Retaining wall backfill | #57 washed | 12 in behind wall, full height | Yes — separate from soil |
| Under slab / vapor break | #57 washed | 4 in | Vapor barrier above |
| Septic leach field | #57 (check local code) | Per permit | Per permit |
When to Go Larger Than #57
Move up to #3 (1–2 inch) or #2 (1.5–2.5 inch) when you need maximum storage volume in a small footprint, such as a deep dry well or an infiltration trench, or when the drain will carry high, fast flow that would displace smaller stone. Larger stone is also standard for temporary construction entrances and access over saturated ground, where the goal is to bridge mud rather than filter water. For channels where scour is the real threat, you are into riprap territory — size that with the riprap calculator.
When to Go Smaller Than #57
Drop to #67 (up to 3/4 inch) or #8 (3/8 inch) in shallow trenches, tight planter drains, and permeable paver bedding where placement space is limited and flow volumes are modest. Smaller washed stone still has the same void structure — the voids are just smaller and more numerous, which means slightly slower flow but noticeably better resistance to coarse sediment working down through the layer. Size it on the #67 stone calculator.
What you should never do is use pea gravel in a structural drain. It is rounded, so it does not interlock; it shifts under load and around pipe, and it does not hold a trench wall. Pea gravel is a decorative and walking-surface material — see river rock vs pea gravel for where it does belong.
The Fines-Clog Failure Mode
Here is how a good drain dies. The stone goes in clean and works perfectly. Over the following seasons, water moving through the surrounding soil carries silt and fine sand with it. Those particles are small enough to enter the void space between the stones and too small to be caught by anything. They settle in the bottom of the voids. Year by year, the effective porosity drops.
Nothing visible happens until the drain crosses a threshold, at which point it simply stops keeping up during heavy rain, and the homeowner concludes the pipe is broken. It usually is not. The pipe is fine; the stone around it is now a low-permeability soil.
Three things prevent it. First, start with genuinely washed stone so you are not importing your own fines. Second, separate the stone from the soil with a non-woven geotextile filter fabric on all sides — and overlap the seams by at least 12 inches, folding the top flap fully closed over the stone rather than leaving it open at the surface. Third, cap the trench correctly: either bring clean stone all the way to grade, or if you must cover with topsoil, keep the fabric envelope sealed beneath it.
Non-woven fabric is what you want for filtration — it passes water through its fiber matrix while catching sediment. Woven fabric is a separation and strength product used under driveways and roads; it has a lower flow rate and is the wrong choice inside a filter drain. Our guide to landscape fabric under gravel covers the difference in detail, and the French drain installation guide walks the trench build step by step.
Calculating How Much You Need
Drainage stone is bought by the ton or the cubic yard, and the two are related at about 1.5 tons per cubic yard for crushed stone. For a trench, multiply length by width by depth in feet, divide by 27 for cubic yards, then subtract the volume of any pipe. A 60-foot trench 12 inches wide and 18 inches deep is 90 cubic feet, or 3.3 cubic yards — call it 5 tons.
| Trench size | Per 10 linear feet | Cubic yards | Approx. tons |
|---|---|---|---|
| 12 in wide × 12 in deep | 10 cu ft | 0.37 | 0.55 |
| 12 in wide × 18 in deep | 15 cu ft | 0.56 | 0.83 |
| 18 in wide × 24 in deep | 30 cu ft | 1.11 | 1.67 |
| 24 in wide × 36 in deep | 60 cu ft | 2.22 | 3.33 |
For dry wells, work backwards from storage. If you need to hold 200 cubic feet of stormwater and the stone gives you 40% void space, the excavation needs to be about 500 cubic feet — roughly 18.5 cubic yards of stone. Always order 5–10% extra; stone settles as it seats and trenches are never as neat as the drawing.
Run the numbers with the drainage gravel calculator, the #57 stone calculator, or convert between units with the cubic yards to tons calculator.
One thing no amount of stone will fix: a drain with nowhere to go. The trench needs a minimum 1% fall — about an inch of drop every eight feet — and a daylighted outlet, a dry well or a storm connection at the low end. A perfectly built, perfectly stoned trench that dead-ends into soil is just an expensive underground bathtub, and it will back up in the first serious rain.
Rock Type: Does It Matter?
Less than size and washing, but it is not nothing. Granite and trap rock are hard, low-porosity and effectively inert — they will not degrade in a permanently wet trench, and they do not shed fines as they age. Limestone is softer and slowly dissolves in acidic groundwater, so over decades a limestone drain generates its own fines and raises the pH of water leaving it.
For most residential French drains, local limestone is perfectly serviceable and the price advantage is real. For anything permanently submerged, anything in acidic soil, or anything you genuinely never want to dig up again, spend the extra on granite. The full breakdown is in limestone vs granite gravel.
Bottom Line
Buy washed, open-graded stone with a real gradation number on it. Use #57 for the overwhelming majority of residential drainage work, go up to #2 or #3 for high-volume storage and fast flow, drop to #67 or #8 in tight spaces. Wrap everything in non-woven filter fabric with generous overlaps, and never let dense-graded material anywhere near a drain.
The stone is cheap — $25–45 a ton — and the excavation is not. Spending an extra $60 on the right rock and proper fabric is the cheapest insurance in the entire project. For more on why #57 became the default, see what #57 stone is.
