The Short Answer: Washed 3/4 Inch Angular Stone
If you are standing at an open trench with the supply yard on the phone, here is the order. Washed, clean, angular crushed stone in the 3/4 inch range. At most yards that is sold as #57 stone. Some yards will offer #67, which is a slightly tighter version of the same idea, and it works fine.
The usable range runs roughly 3/4 inch up to about 1 1/2 inch. Below about 1/4 inch the pieces pack too tightly and water has nowhere to move. Much above 1 1/2 inch the stone gets awkward to place around a pipe without knocking the pipe out of line, and you end up with large hollow pockets that are hard to backfill evenly. Three quarter inch is the default because it threads that needle.
Two words on the order matter more than the number itself. Washed means the dust and fines have been rinsed off at the quarry. Clean means the stone is a single narrow size range rather than a blend that includes sand and powder. A pile can be labeled 3/4 inch and still be the wrong product if it arrives full of dust.
Here is what to actually say at the yard. Ask for 3/4 inch washed clean stone, no fines, for a drainage trench. If they ask for a number, say #57.
If you are choosing stone for the job, our drainage rock page covers sizes, depth, and how much to order.
Key takeaway: the pipe is not what drains your yard. The stone around it does the collecting. The pipe is just the exit route once the water gets there.
Why Void Space Is the Whole Point
Pour a bucket of clean stone into a trench and look at what you actually created. The pieces touch each other at a few points and the rest is open air. Those gaps between the stones, the void space, are where water travels. Wet soil leaks into the gaps, the water runs downhill through them, and the perforated pipe at the bottom is simply the drain line that carries it out to daylight, a dry well, or whatever outlet you have chosen.
Once you understand that, every rule about stone selection falls out of the same logic. Anything that fills the gaps reduces both how much water the trench can hold and how fast that water can move. Stone dust does it. Sand does it. Silt washing down from the surface does it. Loose soil crumbling in from the walls does it. They all do the same damage, which is to turn open channel into packed solid.
This is why the grade of the stone matters as much as the diameter. Two piles of 3/4 inch rock sitting side by side can perform completely differently if one of them has dust in it. The dust finds the gaps, settles between the big pieces, and compacts under the weight of the backfill above. You bought the right size and still ended up with a reduced drain.
Angular stone does one more thing worth paying for. The broken faces catch on each other and lock the mass together. A bed of crushed stone holds its shape under the weight of soil and whatever drives or walks across the lawn above it. That matters because the trench is buried. You cannot inspect it, and you will not find out it has slumped or shifted until water shows up somewhere it should not, usually at the lowest point of a basement wall.
3/4 Inch Clean Stone vs. Pea Gravel
Pea gravel is the most common substitution homeowners make, usually because there is already a pile of it somewhere on the property or because it looks like drain rock in the bin at the yard. It is rounded, and it typically runs smaller than 3/4 inch clean stone. Both of those traits work against it below grade.
Shape and Interlock
Rounded stones do not lock together. They roll past each other under load the same way marbles do. Over several seasons of soil weight, freeze cycles and saturation, a pea gravel bed shifts, settles unevenly and can migrate around the pipe rather than staying put as a stable bed. Angular crushed stone wedges tight on the first compaction and then stays where you put it.
That shifting is not a cosmetic problem. When the bed slumps, the stone that was holding the pipe in alignment moves, the pipe can lose its fall, and low spots develop where water sits instead of running.
Size and Flow
Smaller rounded pieces also nest together more tightly than larger broken ones. Less air per shovel load means less water moving per minute through the same trench cross section. You dug the same ditch, bought the same volume of material, and got a drain that carries less.
In a yard with real runoff volume, that difference is the gap between a trench that keeps up during a heavy rain and one that backs up and sheets across the lawn anyway.
Where Pea Gravel Does Belong
None of this makes pea gravel a bad product. It is an excellent surface material. It is comfortable underfoot, it looks good, and it works well for garden paths, dry creek beds, play areas, pot drainage and decorative top dressing around a patio. The smooth rounded shape that fails underground is exactly what makes it pleasant to walk on.
So if there is already a pile of it in the yard, spread it somewhere visible and buy the right stone for the trench. The price difference between pea gravel and washed crushed stone is small compared to the cost of digging the same ditch a second time.
One honest caveat. A shallow, low volume run in naturally sandy ground backfilled with pea gravel will move some water. It is not inert. It just moves less, and it is far more likely to shift and lose capacity over the years than the angular alternative. If you are putting in the labor of a trench, put in the stone that holds up.
Why Crusher Run Is the Wrong Choice Here
Crusher run, sold in different regions as road base, dense grade or simply crush and run, is crushed stone with all the fines deliberately left in. It runs from roughly 3/4 inch at the top end all the way down to stone dust, blended on purpose across that whole range.
It is one of the highest volume products a quarry moves and it is usually one of the cheaper ones on the board, so it is a common default suggestion when a homeowner calls and asks for gravel without specifying. That is how crusher run ends up buried in drain trenches where it has no business being.
The fines are a genuine feature for the jobs crusher run was designed for. The small particles and dust fill the spaces between the bigger pieces, bind when compacted and wetted, and produce a solid load bearing surface. That is what you want under a driveway, a paver patio or a shed pad. It is dense on purpose and it holds a wheel load without rutting.
It is also the precise opposite of what a drain needs. Backfill the trench with crusher run and you get a buried strip of compacted base material. Water hits it and runs across the top rather than soaking in and dropping through to the pipe. You dug a ditch and installed a barrier.
The same logic applies to any unwashed or pit run stone that shows up with visible dust coating the pieces, regardless of what the ticket says the size is. If handling the pile leaves a chalky gray film on your gloves, that material belongs under something, not in a drain.
Practical check when the truck tips the load. Clean washed stone looks bare and gray or tan depending on the quarry, individual pieces are distinct, and the pile does not throw a cloud when you push a shovel into it. If it puffs dust, call the yard before you backfill anything.
Gravel Size vs. Pipe Perforation Size
This is the worry that sends people searching in the first place, and it is a reasonable one. If the stone is small enough to work its way into the pipe slots, the pipe gradually fills with rock and sediment and stops carrying water. The drain then fails from the inside out with no visible warning.
The plain rule is that the stone should be clearly larger than the openings in the pipe. With standard perforated drain pipe, 3/4 inch washed stone will not pass through the slots or holes. That size match is a large part of why 3/4 inch became the default recommendation in the first place. It is big enough to bridge the perforations and small enough to shovel neatly around a pipe.
It is also another argument against blended material. Even if most of a mixed pile is the correct size, the fine fraction is exactly what migrates down through the bed and into the pipe over time. Single size stone has nothing small enough to migrate.
Two placement notes worth getting right while the trench is open. First, in most installs the perforations face down, so water enters the pipe from the bottom of the stone bed rather than only once the trench has flooded to the top of the pipe. Second, set stone underneath the pipe as well as over it, so it is not resting on the raw trench floor where silt collects.
Filter fabric handles the other half of the problem. Stone size protects the pipe from the stone. Fabric protects the stone from the soil. Line the trench so the fabric wraps up the sides and folds over the top of the finished stone, and surrounding soil cannot wash into the void space and choke it. Skipping the fabric is the single most common reason a correctly stoned trench still silts up within a few years.
How Much Stone to Order
The math is simple enough to do in your head at the trench. Multiply the width by the depth of stone you plan to place by the length of the run. All three in feet. That gives you cubic feet. Divide by 27 to get cubic yards, because a cubic yard is three feet on every side.
Here is a common run worked through. Fifty feet long, 12 inches wide, with 12 inches of stone depth. One foot by one foot by fifty feet is 50 cubic feet. Divided by 27, that comes to a little under two cubic yards. Round up and order two.
For your own dimensions, run the numbers through the gravel calculator rather than scaling from the example, since a trench that is 18 inches wide instead of 12 changes the answer by half again.
One thing that confuses people at order time is that supply yards quote stone both by the cubic yard and by the ton, and the two are not interchangeable. Stone weight varies by rock type and by how wet the pile is. If a yard gives you a per ton price, ask them what a cubic yard of their 3/4 inch washed stone weighs so you can convert and compare honestly against another quote.
Order a little extra either way. Trench walls are never perfectly straight, the floor is never perfectly flat, and hand dug ditches always swallow more stone than the tape measure predicted. Running short halfway through an install means an open trench sitting in the yard waiting on a second delivery, possibly through a rain.
For anything beyond a very short run, bulk washed crushed stone delivered loose is noticeably cheaper than bagged and dramatically less work. Two cubic yards is a lot of bags to buy, load, haul, carry and split open one at a time. A dumped pile near the work is one wheelbarrow route and done.
The Order Line, One More Time
Ask for 3/4 inch washed clean stone with no fines, for a drainage trench. If the yard works by number, that is #57, and #67 is an acceptable substitute. Skip pea gravel, which is rounded and shifts under load. Skip crusher run and anything else that arrives dusty, because the fines pack the void space you just paid to dig for. Get the stone right and the rest of the install is shovel work.
How We Started
We started Mulch Mound because we got tired of the hassle that came with buying landscaping materials. The options were either loading bags into your car at a garden center or calling around to local suppliers, trying to figure out pricing, minimums, and delivery schedules. Neither option felt convenient or transparent.
Three of us – Alec, Mo, and Tyler – decided there had to be a better way. Alec and Tyler got their start back in 2013 running a landscaping business during college, moving mulch and mowing lawns to pay tuition. That experience taught them how frustrating it was to source materials, and years later, that frustration turned into Mulch Mound.
We focus on making it simple to get mulch, stone, and soil delivered directly to your home. Order online, pick your delivery date, and we handle the rest. No loading bags. No calling multiple suppliers. No wondering if you bought enough or paid a fair price.
We work with quality local suppliers in the areas we serve and aim to be straightforward about what we offer and what it costs. Landscaping is hard work. Buying the materials for it shouldn't be.
Frequently asked questions
Does French drain gravel need to be washed, or is unwashed stone acceptable?
Washing is a processing step at the quarry, not a different rock. The same nominal size is commonly sold both washed and unwashed, and the unwashed version is cheaper, so the distinction has to be made on the order rather than assumed. Say washed explicitly when you book the delivery, and confirm it on the ticket if the yard provides one.
Then check the pile when it lands, before you start backfilling. For a small patio sized project you can rinse questionable stone on site with a hose over a screen or a wheelbarrow with the plug pulled, but that is tedious work and completely impractical for a full run of trench. Catching it at delivery is far easier than fixing it later.
Can I mix sizes of stone to save money?
No, and the reason is the same engineering that makes road base work. When you blend a smaller stone into a larger one, the small pieces settle into the gaps between the big pieces. That is literally how dense graded base is designed, by choosing a spread of sizes that packs with minimal space left over. You would be building the wrong product on purpose.
A deliberate single size pile is part of what you are paying for when you buy #57 or #67 stone. The uniformity is the product.
What about river rock?
River rock is hard, durable and attractive, and it drains better than soil, so it gets suggested for trenches fairly often. The problem is shape and size. It is rounded like pea gravel, so it has the same lack of interlock and the same tendency to shift under load. Most river rock sold at landscape yards also runs larger than ideal, which makes it clumsy to seat around a perforated pipe without nudging the pipe out of alignment.
Keep it for dry creek beds and bed borders where it is seen, and use crushed stone where it is buried.
Can I skip the stone and use a pipe in a sock product instead?
Those wrapped pipe products do reduce digging, material and haul time, and for a short run handling a small nuisance wet spot they can be a reasonable choice. The honest tradeoff is capacity. Without a surrounding bed of clean stone there is very little void space to collect and hold water, so the system carries far less volume than a stone filled trench of the same length.
If the yard has real runoff, water against a foundation, or a persistently soggy low area, the stone trench is the version that keeps up.
How deep should the stone layer be around the pipe?
Think of it as three layers rather than one number. A bed on the floor so the pipe is not resting on bare soil, the pipe itself, then enough stone over the top that the pipe is fully surrounded. Deeper stone above the pipe means more storage capacity during a downpour.
Leave room at the top for fabric and whatever surface you want, whether that is a few inches of topsoil and grass or a decorative stone finish left open to the sky. Just keep soil out of the stone itself.