Most central-Illinois driveways take 12–18″ HDPE plastic pipe, and 15 inch is the size we set most often. HDPE won't rust and lasts 50+ years; metal works but corrodes in wet ground. Size chart by diameter below.
Want it done rather than sized? We set and replace driveway culverts across central Illinois — see culvert repair or culvert replacement. Going in a road right-of-way? Start with the Illinois driveway culvert permit.
A driveway culvert is the pipe that carries the roadside ditch underneath your driveway entrance. Get it right and you forget it's there for decades. Get the material or the size wrong and you're hauling gravel back into a washed-out approach every spring. Two questions decide the job: what size, and plastic or metal. Here's how I answer both.
Driveway Culvert Size Chart
Culvert pipe is made in standard diameters — 12, 15, 18, 24, 30 and 36 inch. There's nothing between 12 and 15 or between 18 and 24, so sizing is always a step up or a step down, never a split. Here's what each size is actually for on a driveway around here, and what I'd set it in.
| Pipe Size | What It's For | Plastic Or Metal |
|---|---|---|
| 12 inch | A short entrance off a shallow grass swale with almost nothing draining to it. This is the floor, not a default — plenty of road districts won't take anything under 15 inch in their right-of-way. | HDPE. Metal this small rusts out along the bottom long before the driveway is done with it. |
| 15 inch | The size we set most often on residential driveways. Handles a normal roadside ditch carrying a few acres of yard, road surface and field edge. | HDPE dual-wall. This is the workhorse pipe in this county. |
| 18 inch | A deeper ditch, more ground draining to the crossing, or a drive that has already overtopped with a 12 or 15 inch pipe in it. Step up here instead of replacing in kind. | HDPE. Still light enough to set clean and bed properly. |
| 24 inch | A real ditch, not a swale — it's moving water off crop ground or off several properties. Common at field entrances and long shared lanes. | HDPE is standard; metal if a county or township spec calls for it. |
| 30–36 inch | Channel crossings and field entrances where the ditch runs close to bank-full in a hard rain and loaded grain trucks cross it. | Either. Metal and concrete both show up at these sizes; HDPE works but wants careful bedding and more cover. |
Past 36 inch you're not talking about a driveway culvert anymore — that's a road crossing, sized to a design storm, and it's the road district's call, not yours or mine.
Plastic (HDPE) vs Metal (CMP) — The Honest Tradeoff
There are really two common driveway culvert materials around here: HDPE — the black, double-wall corrugated plastic pipe — and CMP, corrugated galvanized metal pipe. (Concrete pipe exists too, but it's for heavy or permanent road crossings, not a typical driveway.) Here's how they stack up.
HDPE Plastic
- Doesn't rust — the big one. Wet, acidic ground doesn't touch it.
- Lasts 50+ years in most installs.
- Light — easy to handle and set, which keeps install simple.
- Smooth interior flows well and resists clogging.
- Best all-around pick for most residential driveways.
Corrugated Metal (CMP)
- Rusts — galvanized metal corrodes from the inside out, faster in wet or acidic soil.
- Lasts 20–50 years depending on the ground it sits in.
- Can be cheaper in larger diameters and stands up to heavy loads.
- Still a fine choice for a dry site or where a county spec calls for it.
- The rusted-through pipe we replace most often is old metal.
My default recommendation for a central-Illinois driveway is HDPE. We dig out a lot of rusted metal culverts that gave up after a couple decades, and the plastic simply outlasts them in our wet clay ground. Metal still has its place — I'll tell you straight when it's the better value for your site.
How To Size A Driveway Culvert
Sizing is the part people guess at and get wrong. A culvert that's too small backs water up, overtops the driveway, and washes the whole approach out. Too big is just money in the ground. The size depends on how much water the ditch has to move in a hard rain — which comes down to the drainage area feeding it and the slope.
The rule of thumb
- Most residential driveways: 12″ to 18″ diameter.
- Minimum: many counties and townships require a 12″ or 15″ minimum in the road right-of-way — and they may set the size for you.
- Bigger drainage areas, larger ditches, or a lot of upstream water: 24″ and up.
- Match the ditch: the culvert should carry what the ditch carries — if water already runs deep and wide in your ditch, you need a bigger pipe.
When to call
A rule of thumb gets you in the ballpark, but the real number comes from looking at the ditch, the slope, and how much ground drains to that spot. If your driveway already washes out, if the ditch runs hard after a storm, or if a road commissioner is involved, that's when we come measure it. I'd rather size it right once than set it twice.
Cover, Length And Ends — The Three Things The Chart Can't Tell You
Diameter is one number out of four. I've dug out plenty of pipe that was the right size and still failed, because one of these was wrong.
Cover — how much is on top of it
A culvert carries traffic through the fill above it, not through the pipe wall itself. On a 12 to 18 inch driveway pipe I want at least a foot of compacted material over the crown before anything drives across, and more than that where loaded trucks turn in. Skimp on cover and the pipe deflects — it goes oval, the top cracks, and the drive settles in a line right over the pipe. That's the failure people blame on bad gravel.
Length — always longer than the driveway is wide
People measure the width of the approach and order that many feet of pipe. It's never enough. The fill has to slope away on both sides of the drive, and the pipe has to daylight past the toe of that slope on each end — bury the ends in the side slope and you've built a dam with a hole in it. A 16-foot approach with normal side slopes usually wants something in the 24 to 30 foot range. Measure the top width, then add the slope run off both ends.
Grade — the pipe follows the ditch, not the driveway
The culvert has to sit on the ditch flowline with a little fall from inlet to outlet, and that has nothing to do with how your driveway slopes. Lay it dead flat and it silts in from the inlet end. Set it high and it holds a pool of standing water and freezes solid. Set the outlet lower than the ditch it dumps into and it backs up every rain. We shoot the flowline with a laser level so the pipe goes in on the grade the ditch already runs.
Ends — where most of the trouble starts
The pipe barrel usually outlives the ends. Bare cut ends let the fill slough into the ditch, catch a mower deck, and let inlet water scour a hole around the pipe until it undermines it. Rock aprons at the inlet and outlet, or flared end sections where the site wants them, are the cheapest insurance on the whole job. Keep the ends mowed and open — a pipe plugged with sod and silt is the same as no pipe at all.
How To Measure The Culvert You Already Have
If you're calling about an existing pipe, here's what to check before you pick up the phone. Five minutes with a tape gets us a long way.
- Measure the inside diameter at the good end. If one end is mushroomed or crushed, use the other. Round to the nearest standard size — 12, 15, 18, 24, 30 or 36 inch.
- Measure it both ways. Vertical and horizontal. If the vertical is noticeably shorter than the horizontal, the pipe has deflected under traffic and it's on its way out.
- Look for the stamp. HDPE pipe has the size and spec printed right on it. Metal won't, but you can count the corrugations to compare ends.
- Measure the ditch. Depth from the shoulder down to the flowline, and top width. If the ditch cross-section dwarfs the pipe, the pipe is undersized — that's the single most common thing I find.
- Look inside for a silt line. Silt filling the bottom third means it's laid too flat or the outlet is buried. That's a grade problem, and dropping in the same pipe won't fix it.
- Check for daylight end to end. If you can't see through it, something in the middle is collapsed, separated, or packed full.
- Take a couple photos — both ends and the ditch after a rain. I can tell a lot from those.
What A Driveway Culvert Costs
There's no honest sticker price on a culvert, because the number rides on a handful of things: pipe diameter and length, material (HDPE vs metal), how deep it has to be buried, whether we're digging out an old pipe first, and how much rock the approach needs rebuilt. A short, shallow new install with a small-diameter pipe is straightforward; a deep replacement that rebuilt a washed-out crossing is a bigger day. The way to get a real figure is a quick look at the site — that part's free.
Install & Repair — How It Goes
Whether it's a fresh install or swapping out a dead pipe, the fundamentals are the same, and they're what make a culvert last:
- Size & locate — match the pipe to the drainage area and check for utilities before digging.
- Bed it — set the pipe on a clean gravel base at the right depth and grade so water actually flows through it.
- Backfill in lifts — compact both sides as we build up so the crossing doesn't settle later.
- Rock the ends — stone aprons at the inlet and outlet are cheap insurance against washout.
- Finish grade — rebuild the driveway surface over it and grade it to drain.
If your pipe is rusted through, crushed, or just too small, that's a replacement — we dig it out and set a new one at the right size and grade. If the pipe's still sound but a washed-out end or a settled crossing failed around it, that's a repair. We do all three: repair, replacement, and brand-new installs. If you already know yours needs to come out, we walk through the whole job — the tell-tale signs, the dig, and what the day costs — in driveway culvert replacement in central Illinois. More on the service itself on the culvert replacement page, and the bigger picture on our drainage page.
Frequently Asked Questions
Is a plastic or metal culvert better for a driveway?
For most residential driveways, HDPE plastic is the better pick — no rust, light to set, lasts decades. Metal still works and can be cheaper in larger diameters, but in wet or acidic soil it rusts from the inside out. I recommend HDPE for most driveway culverts around here.
What size culvert do I need for my driveway?
Most residential driveway culverts run 12 to 18 inches, sized to the ditch and the water it carries in a heavy rain. Many counties set a 12- or 15-inch minimum in the right-of-way. The honest answer is we size it on site — undersized backs water up, oversized is wasted money.
What sizes do driveway culverts come in?
Culvert pipe is made in standard diameters — 12, 15, 18, 24, 30 and 36 inch are the ones you see on driveways in central Illinois. There is nothing between 12 and 15 or between 18 and 24, so sizing is always a step up or a step down, never a split. Anything past 36 inch stops being a driveway culvert and becomes an engineered road crossing.
Is a 15 inch culvert big enough for a driveway?
For most central-Illinois residential entrances, yes — 15 inch is the size we set most often, and it handles a normal roadside ditch draining a few acres of yard, road surface and field edge. It is not enough where a real channel crosses, where crop ground drains to the spot, or where the road district specifies bigger. If a 15 inch pipe is already in the ground and the drive still overtops, the answer is to step up to 18 inch, not to replace it in kind.
How long does a driveway culvert last?
HDPE plastic commonly lasts 50 years or more because it doesn't corrode. Galvanized metal typically lasts 20 to 50 years depending on soil and water — wet, acidic ground rusts it faster. Either fails early if it's the wrong size or set at the wrong grade.
Do I need a permit to put in a driveway culvert?
If it ties into a county or township road right-of-way, you usually need a permit from the local road commissioner, and they may dictate the minimum pipe size. We help you sort out the local rules before we dig. Which office issues it depends on whose road your entrance meets — see the Illinois driveway culvert permit guide.
Why does my driveway culvert keep washing out?
Usually it's undersized for the water it carries, set at the wrong grade, or missing rock aprons at the ends. We size it right, set it true to grade, and armor the inlet and outlet with stone so it stops washing out.
Tell Me What You're Dealing With — I'll Size It Right
I install, repair, and replace culverts across Coles County and the surrounding area — Mattoon, Charleston, Effingham, Champaign, and out to the edges of the 60-mile radius. You don't need the spec memorized. Call or text (217) 809-0779 with what your ditch and driveway are doing, and I'll figure the pipe and the price.