What Size Drain Tile Do I Need?

Published July 31, 2026 · Updated October 10, 2026 · Brohez Trucking LLC · Mattoon, IL

"What size drain tile do I need?" comes up on nearly every drainage call, usually right before somebody buys a roll of pipe at the farm store. The short answer is that 4-inch handles most of what people are actually trying to fix. The longer answer is worth two minutes, because the size question is often standing in for a different problem.

Here's what each size is for, and the three things that matter more than diameter.

The Sizes And What They're Actually For

• 3-inch — short residential runs, downspout and sump discharge lines, and a lot of older systems. Fine for a single, short, well-pitched run. Plugs easier than 4-inch and there's rarely enough money saved to justify it.
• 4-inch — the default. Standard field lateral, standard yard line, standard French drain pipe. If somebody says "drain tile" with no other qualifier around here, they mean 4-inch. It's the size to use unless there's a reason not to.
• 6-inch — a small main picking up a few laterals, or a lateral that's running unusually long or serving an unusually wet area. Also common where a yard system collects several inlets before heading to one outlet.
• 8-inch — a real main. Several laterals feeding it, or a long run with meaningful acreage behind it.
• 10-inch and 12-inch — submains and mains on larger systems, and outlets where several mains combine. This is where undersizing gets expensive, because everything upstream backs up behind it.

Above 12-inch you're into designed systems and, frankly, into a conversation with an engineer or a tiling outfit rather than a blog post. If you're not sure which of those you're supposed to be calling, start with farm drainage tile contractors — who does what.

Why 4-Inch Is The Default

A 4-inch line carries far more water than most people assume, and the limiting factor on a lateral almost never turns out to be pipe capacity. It's how fast water can get into the pipe through the soil around it. In central Illinois clay, that's slow — the ground meters the water, not the pipe.

So a 4-inch lateral running full is rare. What you see instead is a 4-inch lateral running at a trickle while the ground above it is still wet, and the instinct is "we need bigger pipe." It isn't the pipe.

Three Things That Matter More Than Diameter

1. Grade. Tile runs on gravity and very little of it. A line with too little fall holds water and silts in regardless of size. A 4-inch line at proper grade will outperform an 8-inch line laid flat every time. This is the single most common reason a new line disappoints.

2. The outlet. Every foot of pipe upstream is limited by where the water is allowed to leave. If the outlet is crushed, silted, submerged, or mowed off, the system is only as good as that. We check outlets first on "the tile quit" calls, and it's the answer more often than anybody expects.

3. Whether the line is broken. If a spot that used to drain has started ponding, the size was fine — something failed. Replacing a good 4-inch line with a bigger one because it "isn't keeping up" fixes nothing when the actual problem is a collapsed run forty feet away. That's drain tile repair, not a resize. How to find a broken or blown field tile covers the surface clues.

How Deep, And At What Grade?

This is the question the size question usually turns into, and the honest answer is that neither number starts with a table. Both start at the outlet.

Depth is decided from the bottom up. Wherever the water is allowed to leave — a ditch bank, a road ditch, an existing main — that elevation is the deepest the far end of your line can ever be. You work back upstream from it. If the outlet is shallow, the whole system is shallow, and no amount of digging at the upper end changes that. On a repair the question mostly answers itself: we match the depth of the line that is already there, because a repair that comes out of the ground at a different elevation than the pipe either side of it builds in the exact low spot we are trying to remove.

Then two limits squeeze it from the other direction. Too shallow and the line lives where the equipment does — deep tillage and a ripper shank are the most common thing we dig up a crushed line to find, and that is a depth problem, not a pipe-quality problem. Too deep and in heavy clay you stop buying much: water moves through our ground slowly enough that past a point you are paying for trench, not drainage. Where the real answer lands is a function of your soil, your tillage and your outlet, and it is read on the ground rather than off a chart.

Grade is simpler than people expect and stricter than they expect. Tile runs on gravity, so what the line needs is continuous fall in one direction, the whole way to the outlet. The number that matters is not a percentage — it is whether there is any uninterrupted fall from the upstream end to where the water leaves. What kills a line is not a gentle grade, it is a belly: one low spot mid-run where water stands, velocity drops to nothing and silt settles out until the pipe is a sediment trap. That is why a 4-inch line at steady grade beats an 8-inch line laid flat, and why we shoot the run with laser grade control before anything goes in the ground. If there is no fall to a legal outlet, no diameter fixes it — that is a different conversation about where the water can go, and it is worth having before you buy pipe.

Spacing is a different trade, and we will say so. How far apart to run parallel laterals across a whole field is a full-field pattern-tile design question, and pattern tile is plowed in — a tile-plow outfit’s work, not ours. We do field tile repair and replacement and targeted new lines trenched with the excavator, and we are glad to point you to a tiling contractor for a full-field system. If you want the design numbers for your own ground, the University of Illinois publishes the Illinois Drainage Guide, whose General Recommendations program takes your soil’s permeability and natural-drainage group and returns depth and spacing for it. That is the right source for that question, and it is a better answer than a number we made up.

Sizing A Main Instead Of A Lateral

Laterals are close to standardized. Mains aren't, and a main is where size actually decides whether a system works.

A main has to carry everything feeding it, and the number that drives it is the acreage draining to that point plus the drainage coefficient the system was designed to — the depth of water per day it's meant to remove. In practice: count the laterals, know the acres behind them, and size so the main isn't the bottleneck. Undersize it and every lateral upstream backs up behind the restriction, which looks exactly like a field-wide failure and gets misdiagnosed as one.

If you're adding to an existing system, the honest move is to look at what the main already carries before you tie one more line in. We'll do that as part of the walk — the same look we start every drainage job with.

Perforated Or Solid, And Sock Or No Sock

Two questions that ride along with size:

Perforated vs. solid. Perforated pipe takes water in through the wall — that's what you want anywhere the line is supposed to collect. Solid pipe just conveys: outlet runs, crossings under a driveway, the stretch between a collection point and where the water leaves. Using perforated where you meant solid means picking water up in the wrong place; using solid where you meant perforated means the line drains nothing.

Sock or no sock. A filter sock keeps fine material out of the pipe and it's cheap insurance in sandy or silty ground. In heavy clay it's more of a judgment call — clay doesn't migrate the same way, and a sock can blind over in the wrong conditions. We call it by what's in the hole.

Culvert Pipe Is A Different Question Entirely

People sometimes bring the tile-size question to a driveway culvert, and the logic doesn't transfer. A culvert isn't collecting water out of soil — it's carrying an open ditch under a crossing, and it's sized on how much watershed drains to that ditch and how hard it rains. That's why culverts start at 12–15 inches where tile laterals are 4.

If that's the job, see driveway culvert: plastic vs. metal and what size you need, or the culvert repair and installation page.

Drain Tile Size — Common Questions

What size drain tile is standard?

Four-inch. It's the standard field lateral, the standard yard line, and the standard French drain pipe in central Illinois. Unless there's a specific reason to go bigger — a main carrying several laterals, or an unusually long run — 4-inch is the size to use.

Is 3-inch drain tile big enough?

For a short, well-pitched residential run or a sump and downspout discharge line, usually yes. It plugs more easily than 4-inch and the material savings are small, so most of the time 4-inch is the better buy for the same trench.

When do I need 6-inch or larger drain tile?

When the line is a main picking up several laterals, when a single lateral is running unusually long or serving an unusually wet area, or at an outlet where multiple runs combine. Mains in the 8 to 12-inch range are sized on the acreage draining to them, not on preference — undersizing a main backs up every lateral behind it.

Will bigger drain tile drain my yard faster?

Usually not. The limiting factor on a lateral is how fast water moves through the soil into the pipe, not the pipe's capacity — and in heavy central Illinois clay that's slow. If a line isn't keeping up, the cause is much more often too little fall, a bad outlet, or a break in the line than the diameter.

What's the difference between perforated and solid drain tile?

Perforated pipe takes water in through the wall and is used anywhere the line is meant to collect. Solid pipe only conveys — outlet runs, crossings under a driveway, and the stretch between a collection point and where the water leaves. Getting the two swapped means either collecting water where you didn't want it or draining nothing at all.

Is drain tile the same size as a driveway culvert?

No, and they're sized on different logic. Tile collects water out of the soil and 4-inch handles most laterals. A culvert carries an open ditch under a crossing and is sized on the watershed draining to it, which is why culverts typically start at 12 to 15 inches.

How deep should drain tile be buried?

There is no single number, because depth is set by your outlet rather than by a rule. Wherever the water is allowed to leave is the deepest the far end of the line can ever be, and you work back upstream from that elevation — a shallow outlet means a shallow system no matter how deep you dig at the top. Two limits then squeeze it from the other side. Too shallow and the line sits where the equipment works, and a ripper shank or deep tillage finding the pipe is the single most common crushed line we dig up. Too deep and in heavy central-Illinois clay you stop buying much drainage per foot of trench. On a repair the question largely answers itself: we match the existing line, because coming out of the ground at a different elevation than the pipe either side builds in the low spot you are paying to remove. The real depth is read on the ground off your soil, your tillage and your outlet — we shoot it before we dig.

How much slope does drain tile need?

Enough to keep water moving one direction the entire way to the outlet — and the useful test is continuity, not a percentage. A line can run on very little fall and work fine for decades as long as that fall never reverses. What fails a line is a belly: a single low spot mid-run where water stands, velocity drops off and silt settles out until the pipe becomes a sediment trap. That is why a 4-inch line at steady grade outperforms an 8-inch line laid flat, and why grade matters more than diameter on almost every job we look at. We shoot the run with laser grade control before the pipe goes in, so the fall is measured rather than eyeballed. And if there is no fall available to a legal outlet, no pipe size solves it — that becomes a conversation about where the water is allowed to go, which is worth having before anybody buys material.

What spacing should drain tile be?

Honestly: that one is not ours to answer, and we would rather say so than guess. Spacing — how far apart parallel laterals run across a whole field — is a full-field pattern-tile design question, and pattern tile is plowed in with a tile plow. We do not own one, so full-field systems are a tiling contractor’s trade and we are happy to point you to one. What we do is field tile repair and replacement and targeted new lines trenched with the excavator. If you want real spacing numbers for your own ground rather than a rule of thumb, the University of Illinois publishes the Illinois Drainage Guide, and its General Recommendations program returns depth and spacing from your soil’s permeability and natural-drainage group. That is the authoritative source for your field, and a better answer than a number we invented.