French drains are one of those deceptively simple projects: a trench, a pipe, some gravel, and suddenly water stops pooling where it shouldn’t. But anyone who’s ever dealt with a soggy yard, a damp basement corner, or a driveway that turns into a mini river knows the details matter. And the biggest “detail” that makes or breaks a French drain is the gravel—especially the size of it.

Pick the right gravel size and you get fast drainage, fewer clogs, and a system that lasts for years with minimal upkeep. Pick the wrong size and you can end up with a drain that silts up, compacts, or barely moves water at all. This guide walks through the practical basics: what gravel size works best, why it matters, and how to choose materials that match your soil, your water problem, and your budget.

Even if you’re not building the drain yourself, understanding gravel sizing helps you ask better questions, avoid upsells that don’t help, and recognize when a contractor is cutting corners. Let’s get into it.

What a French drain actually does (and what it doesn’t)

A French drain is a subsurface drainage system designed to intercept and redirect water. It usually consists of a perforated pipe placed in a gravel-filled trench, wrapped in landscape fabric, and sloped so water flows toward an outlet or a safe discharge area. The gravel creates open voids that let water move quickly down to the pipe and along it.

It’s important to know what a French drain is good at: managing groundwater and surface water that infiltrates the soil. It’s not a magic fix for every water issue. If you have a roof runoff problem, for example, you may need gutters, downspout extensions, or a solid pipe line in addition to (or instead of) a French drain.

Gravel size matters because it controls the void space around the pipe. Void space is where water travels. Too little void space and water can’t move quickly. Too much void space with the wrong filter strategy and soil can wash in and clog the system.

Why gravel size is the “engine” of the system

People tend to focus on pipe diameter or whether the pipe should be rigid or flexible. Those things matter, but the gravel is what creates the drain’s capacity. Think of the gravel bed as a highway for water. The pipe is the exit ramp. If the highway is narrow, full of debris, or poorly built, the exit ramp won’t save you.

The size of the stone affects three big performance factors: how fast water enters the trench, how well the system resists clogging, and how stable the trench stays over time. Gravel that’s too fine can compact and behave more like soil than a drainage layer. Gravel that’s too large can allow soil migration if you don’t use the right fabric or if the surrounding soil is very silty.

In other words, gravel size isn’t a cosmetic choice. It’s a functional design decision that should match your soil type and the kind of water you’re trying to move.

The most recommended gravel size for French drains

For most residential French drains, the sweet spot is a clean, washed, angular gravel in the 3/4 inch range. You’ll often see this described as “3/4-inch washed gravel,” “#57 stone,” or “3/4-inch crushed rock,” depending on local naming conventions. The key is that it’s washed (low fines) and angular enough to lock in place while still leaving plenty of voids.

This size drains quickly, supports the pipe without shifting too much, and provides a good balance between void space and filtration. It’s also widely available, which matters because consistency is a big part of building a drain that performs the same way throughout its length.

If you’re building a typical yard drain to relieve pooling water, a 3/4-inch clean gravel is usually the default choice unless your soil or site conditions demand a different approach.

Washed vs. unwashed: the hidden deal-breaker

Two gravels can look similar from a distance and perform completely differently once buried. The difference is fines—tiny particles of sand, silt, and stone dust. Unwashed gravel (or “crusher run” and many mixed base materials) contains fines that can pack tightly. That packing reduces void space and slows water movement.

Fines also migrate. As water flows, those tiny particles can move into the perforations of the pipe or collect in low spots, creating a sludge layer that gradually reduces capacity. It’s one of the most common reasons French drains fail early.

Washed gravel costs a bit more, but it’s usually cheaper than re-digging a trench later. If you only remember one gravel rule, make it this: prioritize clean, washed aggregate for the drainage zone.

Angular rock vs. rounded river rock

Rounded river rock is popular in landscaping because it looks nice and is easy to handle. But for French drains, angular rock often performs better. Angular pieces interlock, which helps the trench stay stable and reduces settling over time. That stability matters because settling can change slope and create low points where water and sediment collect.

Rounded rock, on the other hand, can shift more easily. It still drains well if it’s clean and properly sized, but it may require more careful compaction and bedding to keep the pipe aligned and the slope consistent.

If you’re installing a drain near a structure, under a walkway, or anywhere you really don’t want settlement, angular washed gravel is usually the safer pick.

How soil type changes the best gravel choice

Clay soils: slow infiltration, high payoff for proper gravel

Clay holds water and drains slowly, which is why properties with clay often have persistent soggy zones. In clay, a French drain can still work well, but it must be built to maximize the path for water to enter the trench. Clean gravel with strong void space becomes even more important because the surrounding soil won’t “give up” water quickly.

In many clay-heavy yards, you’ll benefit from a wider trench and a generous gravel envelope around the pipe. The gravel acts as a collection zone, giving water a place to move once it finally seeps out of the clay.

Clay also makes fabric selection more important. A good nonwoven geotextile can reduce the risk of fine clay particles migrating into the gravel bed. The gravel size itself is often still 3/4 inch, but the overall system design needs to account for slow soil drainage.

Sandy soils: fast infiltration, higher risk of migration

Sandy soils drain quickly, which sounds great—until sand starts moving. If the surrounding soil is very sandy, water can carry fine particles into the gravel bed. Over time, that can fill voids and reduce performance.

In sandy conditions, the gravel size can still be 3/4 inch, but the filtration strategy becomes critical. A properly selected fabric wrap (and careful installation so it doesn’t tear) helps keep sand where it belongs.

Some installers also consider slightly smaller clean gravel in certain sandy sites to reduce the size difference between soil particles and stone voids. The goal is to maintain drainage while discouraging soil migration. It’s a balancing act, and it’s one reason “whatever gravel is cheapest” can come back to bite you.

Silty soils: the clogging challenge

Silt is notorious for clogging drainage systems because the particles are small enough to move easily but large enough to settle and accumulate. If your site has silty soil, your French drain needs both clean gravel and a robust filter layer.

In silty conditions, washed gravel is non-negotiable. Unwashed material can combine with silt to form a concrete-like layer that blocks water movement. You also want to avoid creating dead-flat sections where water slows down and drops sediment.

Many silty sites benefit from thoughtful outlet placement and maintenance access, because even a well-built drain can gradually collect fines over years. Gravel size is only part of the solution, but it’s a big part.

Common gravel sizes and when they make sense

3/8-inch pea gravel: tempting, but often problematic

Pea gravel is small, rounded, and easy to shovel. It’s often used in walkways and decorative applications. For French drains, though, pea gravel can be a mixed bag. Because it’s small and rounded, it can pack more tightly than angular stone, reducing void space and slowing drainage.

Pea gravel can also shift around the pipe. In some cases it can work for short runs, low-flow areas, or where the drain is more about relieving minor seepage than handling heavy water loads. But it’s not the go-to for most functional French drains.

If you’re considering pea gravel, be extra cautious about fabric and slope. And make sure it’s washed—pea gravel with fines is a fast track to clogging.

3/4-inch washed gravel (#57): the versatile workhorse

This is the most common recommendation for a reason. It provides strong void space, good structural stability, and reliable performance across many soil types. It’s also easy to source and fairly predictable in grading.

It works well around 4-inch perforated pipe, which is the standard for many residential installations. It also scales up reasonably for larger pipes if you’re building a bigger system.

If you’re unsure what to choose, this is usually the safest starting point for a typical French drain.

1 to 1.5-inch clean stone: higher flow, more attention needed

Larger stone increases void space and can move water very quickly. That can be helpful for high-flow situations, like drains that intercept hillside runoff or collect water from multiple sources.

The tradeoff is filtration: larger voids make it easier for soil particles to migrate into the gravel bed if the fabric isn’t right or if the installation is sloppy. Larger stone can also be harder to grade smoothly, which matters when you’re trying to keep consistent slope along a long trench.

It’s not “wrong,” but it’s usually a better choice when the design includes strong filtration and the installer is comfortable working with it.

Crusher run / road base: good for compaction, bad for drainage zones

Crusher run (often a mix of stone sizes plus fines) compacts extremely well, which is why it’s used under driveways and pavers. But that same compaction makes it a poor choice for the main drainage layer in a French drain.

Because it contains fines, it can behave like a semi-impermeable layer once compacted. Water movement slows dramatically, and the trench can become a muddy channel rather than a free-draining system.

There are situations where a compactable base is useful above the drain (for example, to support a walkway), but it shouldn’t replace the clean gravel envelope around the pipe.

How gravel size affects pipe performance

Perforated pipe relies on water entering through holes or slots. If the surrounding gravel has good void space, water reaches the pipe quickly and enters from all sides. If the gravel compacts or clogs, water may never reach the pipe efficiently, and you end up with a trench that holds water instead of moving it.

Gravel size also affects how well the pipe stays in place. A stable gravel bed helps maintain slope, and slope is what keeps water moving. Even a small sag can become a sediment trap, especially in silty conditions.

Finally, gravel acts as protection. The right stone size creates a cushion around the pipe and reduces the chance of crushing or deformation when the trench is backfilled and the surface is used.

Trench design: gravel size is only half the story

Trench width and gravel envelope

A French drain isn’t just a pipe in a skinny trench. The gravel around the pipe is part of the drainage capacity. A wider trench gives you more gravel volume, which gives you more void space to store and move water during heavy rain.

For many residential projects, trench widths around 12 inches are common, but the best width depends on water volume, soil type, and available space. In clay soils or high-flow areas, going wider can noticeably improve performance.

Whatever width you choose, keep the gravel envelope consistent. Random narrow spots can become bottlenecks, and inconsistent bedding can cause slope issues.

Slope and the “it’ll find its way” myth

Water does not magically flow through a French drain if the slope is inconsistent. You want a steady fall toward the outlet—often around 1% (about 1/8 inch per foot) as a practical target, though site realities vary.

Gravel size interacts with slope because larger stone can create a slightly rougher internal flow path. That usually isn’t a problem, but it means you can’t rely on “big rock” to compensate for poor grading.

If you’re doing the work yourself, a string line and a simple line level can save you from the most common DIY failure: a drain that looks great on day one and slowly turns into a buried puddle.

Fabric wrap: keeping soil out without choking water

Landscape fabric (geotextile) is there to keep surrounding soil from migrating into the gravel. But not all fabric is equal. Some cheap woven fabrics can clog over time, especially in silty soils, because the pores are too small or the fabric isn’t designed for filtration.

A quality nonwoven geotextile is often preferred for French drains because it allows water through while trapping soil particles. The fabric should wrap the gravel like a burrito, overlapping at the top so soil doesn’t fall in during backfill.

Gravel size and fabric work together. If you use larger stone, fabric becomes even more important. If you use smaller stone, you still need fabric, but the risk of migration changes. The goal is always the same: keep the void space open for water.

How deep should the gravel be?

A common setup is a few inches of gravel under the pipe (as bedding), then gravel covering the pipe by at least several inches. Many builds aim for 6 inches of gravel below the pipe and 6 inches above it, but real-world depths depend on trench depth and surface needs.

More gravel generally means more capacity, but there’s a point of diminishing returns if the surrounding soil can’t feed water into the trench quickly. In clay, extra gravel volume can still help by providing storage during storms. In sandy soil, extra volume may be less critical, but filtration and fabric integrity become more important.

If you plan to top the trench with soil and turf, leave enough room for a healthy soil layer so grass can root. If you’re topping with decorative stone, consider how it will interact with the fabric and whether it could introduce fines over time.

Real-world scenarios and the gravel choice that fits

Yard pooling after rain

If your yard holds water for hours or days after rain, you’re usually dealing with poor infiltration, poor grading, or both. A French drain can help by giving water a faster path away from the problem area.

In this scenario, 3/4-inch washed gravel is typically the best match. It provides quick intake and reliable flow without being so large that it increases the chance of soil migration.

Pair it with a nonwoven fabric and make sure the trench actually intercepts the low spot where water collects. A drain placed “nearby” often doesn’t solve pooling because water takes the easiest path—usually straight down in the lowest point.

Downspout tie-ins and roof runoff

When you’re moving roof water, you’re often dealing with higher volumes delivered quickly. Many people use solid pipe for downspouts (to keep debris out) and then transition to a French drain section for dispersal or for picking up groundwater along the way.

Gravel size around a French drain section can still be 3/4-inch washed gravel, but the bigger design question is whether you need a cleanout and how you’ll prevent leaves and roof grit from entering the perforated section.

If you’re primarily transporting roof water, a solid pipe to daylight can be simpler. Use the French drain where you truly need infiltration or groundwater interception.

Basement seepage along a foundation

Foundation drains are a more sensitive application because mistakes can be expensive. You want strong drainage capacity and long-term clog resistance. Clean, angular gravel is usually the right call here, often in the 3/4-inch range.

Depth and outlet reliability matter as much as gravel size. If the outlet is too high or can freeze, water can back up. If the slope is inconsistent, sediment can accumulate in low points.

For foundation work, it’s also worth thinking about waterproofing, surface grading, and gutter extensions. A French drain can be part of the solution, but it shouldn’t be the only layer of defense if you have persistent moisture.

When rip rap enters the conversation

Rip rap is larger rock—often used for erosion control, slope stabilization, and protecting outlets where fast-moving water could wash soil away. While rip rap is not typically used as the main gravel envelope in a French drain, it can be extremely useful at the discharge point, along a swale, or anywhere concentrated flow needs armoring.

If your French drain outlet dumps water onto a slope, into a ditch, or near a vulnerable area, adding a rip rap apron can prevent erosion and keep the outlet from undermining itself. The drain can be perfectly built underground, but if the outlet creates a mini waterfall that eats away soil, you’ll end up with a mess (and possibly a broken pipe) later.

If you’re planning that kind of protection, you can order rip rap today and size it appropriately for your flow and slope. The goal is to slow water down and spread it out, not to create a rock pile that redirects water back toward your home.

Material sourcing: consistency beats “whatever is on sale”

French drains fail when materials vary too much along the trench. If half your trench is clean 3/4-inch stone and the other half is a mixed, dusty base material, water will slow down where it hits the tighter section, and sediment will settle out right there. That’s how you get localized clogs.

Try to source the full amount of gravel from the same supplier and the same product designation. If you’re unsure what you’re being sold, ask whether it’s washed, what the nominal size is, and whether it contains fines. “Drain rock” can mean different things in different places.

Delivery matters too. Gravel is heavy, and multiple small loads often cost more than one properly planned delivery. It also reduces the temptation to “stretch” the gravel by mixing in soil or cheaper base material mid-project.

Delivery and logistics: planning for the least fun part

It’s easy to underestimate how much gravel a French drain consumes. Even a modest trench can require a surprising volume once you account for width, depth, and the gravel envelope around the pipe. Calculating cubic yards ahead of time keeps the project moving and prevents last-minute substitutions.

If you’re working in Southern California and coordinating materials for a drainage project, having reliable delivery can make or break your timeline. For example, if you need Long Beach sand and gravel delivery, scheduling it for the day you’re ready to place rock (not the day you start digging) helps you avoid piles sitting around getting contaminated with soil and debris.

Also consider access. Can a truck get close to the trench without damaging landscaping or cracking a driveway? Sometimes a smaller truck or a different drop location is worth it to avoid cleanup and repairs.

How to calculate gravel quantity without getting lost in math

You don’t need to be an engineer to estimate gravel. Measure the trench length, width, and the depth of gravel you plan to place. Multiply them to get cubic feet, then divide by 27 to convert to cubic yards (since 1 cubic yard = 27 cubic feet).

For example, a trench 50 feet long, 1 foot wide, with 1 foot of gravel depth is 50 cubic feet. Divide by 27 and you get about 1.85 cubic yards. Add a little extra for waste, uneven trenching, and settling—often 10% is a reasonable buffer.

If your trench depth varies, break it into sections and estimate each one. Over-ordering a small amount is usually cheaper than stopping mid-job and paying for a second delivery.

Gravel placement tips that protect performance

Keep the gravel clean during install

One of the easiest ways to sabotage a French drain is to contaminate the gravel with excavated soil. If you dump soil into the trench and then pour gravel on top, you’ve effectively added fines back into the system.

Place fabric in the trench first, then add a bedding layer of gravel, set the pipe, and continue filling with gravel. Keep excavated soil on a tarp or in a separate pile away from the trench edge so it doesn’t slough back in.

If it rains during your project, cover the trench. Muddy water flowing into an open trench can deposit silt directly into your clean gravel before you even finish.

Don’t crush the void space with over-compaction

Gravel does settle, and a little settling is normal. But aggressively compacting the gravel envelope around the pipe can reduce void space and make the system less effective.

You want the gravel to be stable, but you also want it to remain open. Typically, placing it in lifts and lightly tamping for stability (especially near the surface if you’re supporting something) is enough.

If you need a compacted surface for pavers or a walkway, keep the drainage zone separate and use the correct base layer above the fabric, not mixed into the drain gravel.

Protect slope while backfilling

Pipe slope is easiest to maintain when the bedding layer is even and stable. Once the pipe is set, avoid stepping directly on it or dragging heavy tools over it.

Backfill gravel evenly along the trench rather than dumping a huge pile in one spot and raking it out. Large dumps can knock the pipe out of alignment.

If you’re using flexible corrugated pipe, be extra careful: it can sag between support points if the bedding isn’t consistent. Rigid PVC with holes can hold slope more reliably, but either can work if installed carefully.

Local aggregate options and why “close enough” isn’t always close enough

When people hear “gravel,” they assume it’s all basically the same. In reality, aggregate products vary by quarry source, crushing method, and grading. Two “3/4-inch” gravels can have different amounts of fines or different particle shapes, and that changes how they drain.

If you’re comparing suppliers, ask for the product spec or at least confirm that the material is washed and intended for drainage. If you’re sourcing materials for a project near Orange County, for example, it can help to know what’s available as aggregate in Santa Ana so you can match the right product to the drain design instead of settling for whatever happens to be in stock.

Consistency matters even more if you’re building multiple drains or extending a system later. Using the same type and size of gravel reduces the chance of performance differences along the line.

Mistakes that cause French drains to fail early

Using the wrong gravel (or the right gravel, but dirty)

The most common failure is using gravel with fines. It drains okay at first, then slowly silts up. The system doesn’t “break” overnight—it just becomes less effective until you’re back to puddles and soggy soil.

Another issue is mixing materials: a little washed gravel, a little road base, a little leftover sand. It’s understandable when you’re trying to save money, but it creates inconsistent permeability and invites clogging.

Stick to clean, washed drainage rock for the envelope around the pipe. Save the compactable materials for surface layers where compaction is actually helpful.

Skipping fabric or using the wrong one

Fabric is not optional in most soils. Without it, soil migrates into the gravel and reduces void space. With the wrong fabric, water flow can be restricted or the fabric can clog like a coffee filter.

Nonwoven geotextile is often a good all-around choice for French drains. The key is proper installation: overlap seams, avoid tears, and keep soil out of the gravel during backfill.

If you’re in a very sandy soil, fabric helps prevent migration. If you’re in silty soil, fabric helps prevent clogging. Different problem, same tool.

No real outlet (or an outlet that can’t keep up)

A French drain needs somewhere to send the water. “It’ll soak into the ground” can be true in some sandy sites, but in many yards—especially with clay—an outlet is essential.

Outlets can be daylight discharge, a storm drain connection (where legal), a dry well, or a swale. Whatever you choose, make sure it can handle peak flow and won’t cause erosion.

This is also where rip rap or an energy-dissipating feature can protect the outlet area from washing out.

Maintenance habits that keep drains working for years

A well-built French drain is low maintenance, not zero maintenance. If your system has a cleanout, flushing it occasionally can remove early sediment before it becomes a stubborn blockage. This is especially helpful if you know your soil is silty or if you’ve had construction nearby.

Keep surface debris under control. If your drain is near trees, try to prevent leaves from accumulating where water enters the ground. Organic debris breaks down into fines, and fines are the enemy of void space.

After big storms, check the outlet. If you see muddy discharge, that can be normal at first, but persistent sediment could mean soil is migrating into the system or the fabric was compromised during installation.

A practical cheat sheet for choosing gravel size

If you want a simple way to decide, here’s a practical approach. For most residential French drains: choose clean, washed, angular 3/4-inch gravel (often #57). It’s the best all-around mix of drainage speed, stability, and availability.

If you’re dealing with high-flow discharge areas, consider using larger rock like rip rap at the outlet to prevent erosion, while still using 3/4-inch washed gravel around the pipe. If your soil is very sandy or silty, focus on filtration (fabric choice and installation quality) as much as the stone size.

And if someone suggests using road base or mixed gravel “because it compacts,” remember: compaction is great under a driveway, but it’s the opposite of what you want in the water-moving zone of a French drain.