MachineCalcs

Catch Basin Sizing Calculator

Size a yard-drain or catch-basin screen from drainage area, rainfall intensity, runoff coefficient, grate open area and gravity outlet pipe capacity.

Hydraulics 12 inputs 9 results
Used in Piping Trades 28

Calculator

Inputs

View results
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
Edit inputs
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.

Catch basin sizing starts with the Rational Method screen Q = C*i*A. This calculator uses local rainfall intensity, drainage area, runoff coefficient and a safety/clogging allowance to estimate design runoff, then checks grate open area with A_open = Q/v and outlet pipe capacity with Manning full-pipe flow. It is a site-drainage screen only; final work still needs local stormwater criteria, manufacturer inlet data, debris, tailwater, overflow and civil review.

Continue workflow

All Hydraulics
Workflow placement Piping Trades Stage 2: Lay out the install
Next 1 Lay out pipe slope Lay out outlet pipe fall, pitch and invert elevations after the catch-basin flow screen. Next 2 Size pipe ID Back-solve pipe ID from design flow when velocity is the controlling first-pass criterion. Next 3 Check pipe pressure drop Use the selected actual pipe ID for water-like pressure or head-loss checks on pressurized portions. Chart Use reference data Steel pipe schedule chart

How to use this calculator

  1. Enter the drainage area. Use the roof, pavement, patio or yard area that actually drains to the basin.
  2. Set rainfall and runoff. Use local rainfall intensity and a runoff coefficient for the surface.
  3. Enter grate geometry. Add grate count, width, length and open area from manufacturer data when available.
  4. Check the outlet. Enter actual pipe ID, slope and Manning n for the gravity outlet.
  5. Read the bottleneck. Compare grate utilization, pipe utilization and the limiting drainage area.

How it works

Catch basin sizing starts with a design runoff flow: Q = C i A where C is runoff coefficient, i is rainfall intensity and A is drainage area. MachineCalcs keeps rainfall intensity as a user input so the page does not hard-code a local storm event.

The grate check converts that flow to required open area: Aopen = Q / v The outlet check uses Manning full-pipe capacity for a round gravity drain: Q = (1/n) A Rh^(2/3) S^(1/2)

For adjacent piping work, use the pipe slope calculator, pipe size by flow velocity calculator and pipe flow pressure drop calculator.

Worked example

Verified against the live calculator

With the default 2,000 ft^2 drainage area, 3 in/h rainfall intensity, C = 0.90 and a 20% allowance, the design runoff is about 67 gpm. Two 12 in x 12 in grates at 55% open area are comfortably below the open-area target, while the default 4 in outlet pipe at 1% slope is the tighter check at about 79% utilization.

Frequently asked questions

How do you estimate catch basin flow?

This calculator uses the Rational Method screen Q = C i A, where C is runoff coefficient, i is rainfall intensity and A is the drainage area.

Does this choose a code-required storm event?

No. Enter the rainfall intensity required by the project, local stormwater manual, owner criteria or civil engineer. Design storms vary by location, duration and return period.

Is grate open area the same as inlet capture?

No. Open area is a first screen only. Real capture depends on grate geometry, approach flow, ponding head, slope, debris, bypass and manufacturer data.

How is the outlet pipe capacity calculated?

The outlet check uses Manning full-pipe gravity capacity from pipe ID, slope and Manning n. Tailwater, inlet control and partial-flow hydraulics still need separate review.

Method & assumptions

  • Runoff uses the Rational Method as a sizing screen, not a jurisdiction-specific stormwater design manual.
  • Rainfall intensity must be entered for the applicable location, duration and return period.
  • Grate capacity is based on open area and target velocity; actual inlet capture needs manufacturer data and site hydraulics.
  • Manning pipe capacity assumes a circular gravity pipe flowing full at the entered slope and roughness.
  • Final drainage design still needs debris/clogging factors, ponding limits, overflow routing, outlet tailwater, pipe cover, bedding, frost and local approval.
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