Important: This tool provides reference guidance only. The county engineer or road authority always specifies the final required culvert diameter, length, and material for your specific location — based on the actual drainage area, existing culverts in your neighborhood, and local standards. Never order pipe before getting official specifications.
Culvert Sizing Reference Lookup
Typical Specifications for Your Situation
Accepted Pipe Materials for Your State/Road
What the County Inspector Will Verify
Culvert Material Requirements by State and Road Type
| State / Authority | CMP (Metal) | HDPE (Plastic) | RCP (Concrete) | Min. Diameter | Notes |
|---|---|---|---|---|---|
| TxDOT (TX state highways) | Required | Not accepted | Not accepted | 18" | CMP is the ONLY accepted material in TxDOT ROW for culverts |
| Travis County, TX (county road) | Required | Not accepted | Not accepted | 18" | Same CMP-only rule as TxDOT; safety end treatments required |
| Harris County, TX (county road) | Accepted | Accepted | Accepted | 15" | Multiple materials accepted; county engineer specifies |
| City of Pattison, TX | Check | Accepted | Accepted | 15" | HDPE (ADS N-12) or RCP; $250 permit fee |
| Lake County, FL | Accepted | Accepted | Accepted | 15" | Match neighborhood; 15" min, 30-ft min length; mitered ends |
| FDOT (FL state highways) | Accepted | Accepted | Accepted | 18" | Material per FDOT standard specs; safety ends required |
| Ohio county roads (typical) | Accepted | Accepted | Accepted | 15" | County engineer specifies; Clinton County: free permit |
| PennDOT (PA state highways) | Accepted | Accepted | Accepted | 15" | Per PennDOT RC standards; part of HOP review |
| Caltrans (CA state highways) | Accepted | Check | Accepted | 18" | Engineering calcs often required; district-specific rules |
| Generic county road (national avg.) | Accepted | Accepted | Accepted | 15" | Most counties accept all three; CMP most commonly specified |
Culvert Length Reference
| Driveway Width | Minimum Culvert Length | Notes |
|---|---|---|
| 10 ft (single lane) | 16–20 ft | Pipe extends 2–3 ft beyond each driveway edge |
| 12 ft | 18–22 ft | Standard minimum in most county specs |
| 14 ft | 20–24 ft | Most common residential driveway width |
| 18 ft (two-car) | 24–28 ft | Note: many counties set absolute minimum of 30 ft regardless |
| 24 ft (wide apron) | 30–36 ft | Grate inlets may be required for lengths over 40 ft |
| Any width — absolute minimum | 30 ft | Lake County FL, many OH counties set 30-ft floor regardless of width |
The Inspection You Cannot Skip
Before backfill: A culvert installation requires a mid-construction inspection before the trench is backfilled. If you cover the pipe before calling for inspection, most jurisdictions will require you to excavate and expose it again — at your cost. Call the permit office the same day the pipe is set in the trench.
Pre-installation site visit (before any digging)
County engineer visits and specifies exact diameter, slope, length, and end treatment. This visit is free in most counties. Request it when you apply for the permit.
Purchase pipe AFTER getting specifications
Don't buy anything until you have written specifications. The most common and costly mistake is buying 12-inch corrugated black plastic pipe — which is almost never accepted for permitted culvert installations in a public ROW.
Excavate trench and set pipe at correct slope
Pipe must slope with the ditch flow — typically 0.5–2% slope. Flat pipe backs up with debris. Bedding material (compacted gravel) supports the pipe and prevents settlement.
Install end treatments before calling for inspection
Mitered, flared, or concrete end sections must be in place before the inspector arrives. End treatments prevent erosion, protect vehicle tires, and are required by virtually every jurisdiction.
Call for mid-construction inspection — pipe exposed
Inspector verifies diameter, slope, length, material, and end treatments. Do NOT backfill yet.
Backfill, grade driveway, and call for final inspection
Final inspection verifies driveway grade, apron surface, ditch restoration, and sodding. Permit closes when inspector signs off.
Related guides: Full Culvert Permit Guide · Permit Cost Estimator · Texas Culvert Rules · Ohio Culvert Rules
Disclaimer: This tool provides reference guidance only. Always obtain official culvert specifications from your county engineer or road authority before purchasing materials. Requirements vary significantly by jurisdiction.