Catch Basin & Downspout Calculator

Build a branch-specific takeoff for roof runoff, basin inlets, solid-pipe runs, and outlet pieces without forcing those fittings into the French-drain math.

Downspout extension Catch basin runs Pipe, fittings, and outlet pieces
By: CalcHub Editorial Operated by: Cloudtopia
Maintenance: Updated when formulas, supplier packaging, or guidance change.
Method: Research + supplier packaging + formula verification.
Units:
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Which Basin / Downspout Branch Fits The Job?

Downspout extension

Move roof water away from the house with solid pipe and a simple outlet.

Catch basin run

Collect surface water into basins and route it to a controlled outlet.

Downspout to basin

Combine roof leaders with basin capture before the shared outlet run.

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How The Branch Counts Work

This calculator treats the branch as a pipe and fitting system. You add the segment lengths first, apply a modest waste allowance, and then translate the total into sticks or coils depending on how you plan to buy the pipe.

Pipe length with waste = Σ(segment lengths) × (1 + waste %)

Adapters, elbows, branch connectors, cleanouts, and outlet pieces are then counted around that pipe run. The goal is to give you a real residential branch takeoff, not just a single line item for “pipe.”

The page stays intentionally narrow. It does not decide whether the branch is the right family in the first place, and it does not size dry-well storage. Use the runoff calculator for the family decision and a later drainage phase for storage or final discharge design.

Frequently Asked Questions

When should I use this instead of the French drain calculator? +
Use this page when roof runoff, basin inlets, or solid-pipe surface capture owns the job. If the main question is trench rock, perforated pipe, and fabric around a subsurface drain, stay with the French drain calculator.
Why do the fittings matter so much on basin and downspout jobs? +
These branches often fail from the accessory count, not the pipe footage. Adapters, elbows, tees, wyes, cleanouts, and outlet pieces determine whether the pipe can actually connect all the inlets you measured.
Does this calculator size dry-well storage? +
No. The dry-well option is a handoff only. This page counts the branch materials up to the connection point, but it does not replace runoff-volume storage sizing for the dry well itself.
Why does 3-inch pipe trigger warnings on some runs? +
Three-inch pipe can work for a short single downspout extension, but it becomes tight once several inlets or longer shared runs feed the same line. Four-inch pipe is the safer residential starting point when the branch gets more complex.

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Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements depend on site conditions, compaction, grading, and local building codes. Always verify measurements on-site and consult with your material supplier before purchasing.