Free calculator
Culvert Manning's Flow Capacity
Calculate the flow capacity of a circular culvert and check whether it's large enough for your site's drainage.
Results
Enter your dimensions and hit Calculate.
Manning's full-flow capacity is ONE input to culvert design — not a complete sizing method.
Real culvert sizing also requires inlet-control analysis, headwater depth, tailwater conditions, debris/blockage margin, and local drainage standards.
The 'Recommended min size' (advanced) assumes outlet control and full flow. A licensed engineer or county drainage engineer must confirm sizing before purchase or installation.
Culvert undersizing can cause road washout, property flooding, and safety hazards.
Assumptions
- Manning's equation, full-flow (pipe flowing full), outlet-control condition.
- Hydraulic radius R = D/4 for a full circular pipe.
- US customary coefficient: 1.486.
- Velocity range: < 2.5 ft/s may allow siltation; > 10 ft/s risks scour at outlet.
- Standard culvert size series: 12, 15, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72 in.