Culvert Capacity Calculator

Inputs

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Contributing area draining to this road, driveway or embankment crossing.
ft²
Use the local design-storm intensity for the selected duration and return period.
in/h
Dimensionless runoff coefficient for the contributing surfaces. Impervious areas are higher; meadow or wooded areas are lower.
Extra flow allowance for uncertainty, partial blockage, debris and local judgment.
%
Choose the installed barrel geometry to check. The alternate required-size output still reports a rough comparison.
Actual inside diameter for a circular culvert barrel.
in
Longitudinal barrel slope as fall divided by run times 100.
%
Roughness coefficient. Smooth concrete/PVC can be lower; corrugated metal or rough barrels can be higher.
Velocity screen for outlet erosion and downstream sensitivity. Local criteria control final values.
ft/s

Results

Default result
Edit inputs
Design runoff flow (Qd)
733.2 gpm
Pass

Raw runoff before allowance: 2313 L/min.

Also computed

Barrel capacity (Qbarrel) Pass 2,554 gpm

Manning full-flow capacity for the selected circular or box barrel.

Capacity utilization (Uq) Pass 28.71 %

Full-flow velocity (v) Pass 3.22 ft/s

Velocity at full-flow barrel capacity, not only at design runoff.

Velocity utilization (Uv) Pass 53.66 %

Required circular ID (D req) Pass 11.27 in

Circular full-pipe ID needed by Manning at the entered slope and roughness.

Required box width (W req) Pass 6.292 in

Box width needed at the entered box height, slope and roughness.

Culvert preview: circular barrel capacity 9667 L per minute, design runoff 2775 L per minutedesign runoff2775 L/minbarrel capacity9667 L/minCapacity29%VelocityScreen only. Inlet control, headwater, tailwater, overtopping, erosion, debris, cover, structural loading and permits remain separate design checks.
Method notes 3 notes
  • Rational Method screen: runoff flow Q = C * i * A. Rainfall intensity and runoff coefficient must come from the local design storm and site surface assumptions.
  • Barrel capacity uses Manning full-flow capacity for a circular or rectangular conduit. Actual culverts can be inlet-controlled or outlet-controlled before this barrel capacity is reached.
  • Final culvert design still needs inlet geometry, headwater, tailwater, roadway overtopping, outlet erosion, debris, sediment, cover, structural loading, traffic loading, local stormwater criteria and permitting review.

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