Catch Basin Sizing Calculator

Overview

Uses drainage area and rainfall intensity to estimate flow, then selects a planning basin and outlet-pipe size tier.

Assumptions

• The rational-method-style estimate uses a fixed 0.9 runoff coefficient.
• The tool assumes a 20-ft outlet run.

Worked example

For 1,000 sq ft at 2 in/hr, the implemented conversion produces about 12.4 GPM, placing it in the 9 × 9-in basin / 3-in pipe tier.

Common mistakes

• Using average monthly rain rather than a design storm intensity.
• Assuming basin size alone proves pipe or outlet capacity.

Limitations

It is not a hydraulic design and does not model downstream restrictions or debris load.

Safety and local requirements

Keep grates suitable for the location and check local storm-drain connection rules before excavation.

Sources

• FHWA HEC-22 — Urban Drainage Design Manual
• EPA — Soak Up the Rain