Culvert Sizing: FHWA HDS-5 & HY-8 Guide
The Culvert Size Calculator determines minimum diameter or rise for circular, box, and arch culverts using Manning’s equation and FHWA HDS-5 nomographs. It checks inlet/outlet control, headwater, velocity, and outlet protection per HY-8 standards.
What is a Culvert?
Conduit under roadway for stream or drainage. Types:
- Circular: CMP, RCP, PVC
- Box: Precast concrete
- Arch: Corrugated metal
Why Proper Sizing Matters?
Prevents:
- Flooding: Road overtopping
- Erosion: High velocity
- Clogging: Debris
- Cost Overrun: Oversized pipe
Use with our Concrete Volume Calculator for headwalls.
Flow Control Types
| Control | When | Equation |
|---|---|---|
| Inlet | Steep slope, small culvert | HW = f(D, Q, inlet) |
| Outlet | Mild slope, long culvert | Q = (A R^{2/3} S^{1/2}) / n |
How to Size Culvert
Step-by-step:
- Find Q: Rational method (Q = CiA)
- Set HW/D: ≤ 1.5 (roadway), ≤ 2.0 (field)
- Iterate D: Until Q_capacity ≥ Q_design
- Check Velocity: 3–10 fps
- Add Outlet Protection: Riprap apron
Our calculator automates inlet/outlet control.
Practical Applications
Used for:
- Highway Crossings: Stream culverts
- Driveways: Ditch relief
- Storm Drains: Urban runoff
- Railroads: Under-track drainage
Pair with our Aggregate Calculator for riprap.
Tips for Code Compliance
Best practices:
- Min D = 18": Fish passage
- Embed 20%: Natural bed
- Bevel Edges: Improve inlet
- Trash Rack: Debris control
Common Mistakes to Avoid
Avoid:
- Wrong n: Use 0.024 for CMP
- No HW Check: Flooding
- High Velocity: Scour
- No Outlet Protection: Erosion
Advanced Considerations
For complex cases:
- Multiple barrels
- Irregular shape
- Storage routing
- Fish passage (baffles)
Conclusion
Culvert sizing ensures safe, efficient drainage. Our Culvert Size Calculator delivers FHWA-compliant results instantly. Combine with our Steel Weight Calculator for CMP. Explore our full suite of Construction Calculators for complete site drainage.