Culvert Capacity Calculator

Inputs

View results
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)
733.2gpm
Pass

Raw runoff before allowance: 2313 L/min.

Also computed

Barrel capacity(Qbarrel)Pass2,554gpm

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

Capacity utilization(Uq)Pass28.71%

Full-flow velocity(v)Pass3.22ft/s

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

Velocity utilization(Uv)Pass53.66%

Required circular ID(D req)Pass11.27in

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

Required box width(W req)Pass6.292in

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.

Powered by MachineCalcs ↗