Hydraulics and pipes
Culvert Capacity Calculator (Screening)
Culverts can be limited by the inlet (inlet control) or by the barrel (outlet control). This screening tool estimates both with simple equations and reports the lower one. Final design should use FHWA HDS-5 nomographs or HY-8 software.
Formula
Inlet (submerged orifice): Q = Cd × A × √(2gh), g = 32.2 ft/s², h = head above the centerline of the opening. Barrel (Manning, flowing full): Q = (1.49/n) × A × (D/4)^(2/3) × S^(1/2), with D in ft. The smaller value is the screening capacity.
Worked example
A 24 in corrugated metal pipe (n = 0.024) on a 1% slope with 3 ft of head over its centerline: A = 3.14 ft². Inlet: 0.6 × 3.14 × √(2 × 32.2 × 3) = 26.2 cfs. Barrel: (1.49 ÷ 0.024) × 3.14 × 0.5^(2/3) × 0.1 = 12.3 cfs. Estimated capacity: 12.3 cfs (barrel-limited).
Assumptions and limits
- Screening only: real culverts flowing full under outlet control can carry more than Manning's normal-depth flow because headwater adds driving head; the HDS-5 energy equation includes entrance, friction and exit losses and tailwater.
- Orifice equation is valid only when the inlet is submerged (headwater roughly 1.2 times the diameter or more).
- Road crossings in public right of way must follow the agency's drainage manual.
- For estimation only. A licensed Professional Engineer (PE) must design and stamp any structural, foundation or life-safety element.
Frequently asked questions
What is inlet control versus outlet control?
Under inlet control, the entrance geometry limits flow and the barrel can carry more. Under outlet control, barrel friction, length, slope and tailwater limit flow. HDS-5 has you compute both and use the higher headwater.
What size culvert do I need for my driveway?
Many counties require at least a 12 to 18 in pipe for driveway culverts, but the right size depends on the drainage area and design storm. Estimate peak flow with the rational method, then check the culvert, or ask the local road department.
What software do engineers use for culverts?
FHWA's free HY-8 program implements the HDS-5 methods for inlet and outlet control, roadway overtopping and tailwater.