Catch Basin Sizing Calculator

Inputs

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Paved, roof, patio or yard area draining to this basin.
ft²
Use the local design-storm intensity for the selected duration and return period.
in/h
Dimensionless runoff coefficient. Impervious paving is often high; lawn or soil areas are lower.
Extra factor applied to runoff flow for debris, partial blockage and uncertainty.
%
Number of identical grates or inlet openings serving the basin.
Gross width of one grate or opening.
in
Gross length of one grate or opening.
in
Open fraction of the grate. Use manufacturer data when available.
%
Velocity used to convert design flow into required open area.
ft/s
Actual inside diameter of the gravity outlet pipe, not only nominal pipe size.
in
Pipe slope as fall divided by run times 100.
%
Roughness coefficient for gravity-pipe capacity. Smooth PVC is lower; corrugated or rough pipe is higher.

Results

Default result
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Design runoff flow(Qd)
67.32gpm
Pass

Raw runoff before allowance: 212.4 L/min.

Also computed

Required open area(Ao req)Pass14.4in²

Open area needed at the target inlet velocity.

Installed open area(Ao)158.4in²

Capacity at target velocity: 2803 L/min.

Grate utilization(Ug)Pass9.091%

Installed inlet velocity(v)Pass0.1364ft/s

Outlet pipe capacity(Qp)Pass85.41gpm

Manning full-pipe gravity capacity at the entered ID and slope.

Pipe utilization(Up)Pass78.82%

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.
  • Required grate area uses open area = Q / target inlet velocity. Actual capture can be lower because of grate geometry, debris, bypass, ponding head and curb or trench hydraulics.
  • Outlet pipe capacity uses Manning full circular pipe flow. Final site drainage still needs local stormwater criteria, manufacturer inlet data, tailwater/outfall checks, pipe cover, bedding, overflow path and civil review.

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