Downspout Calculator

How many downspouts do I need?

sq ft
Downspout size

Order this much 2 × 3 in rectangular downspout

4downspouts

Typical rating of about 600 sq ft per downspout. Cut that by a third in heavy-rain regions, and place downspouts near the middle of long runs.

2,400 sq ft of roof4 downspouts
Area per downspout600 sq ft
Downspout opening6 sq in
Rule of thumb need24 sq in total

Find how many downspouts your roof needs in the size you plan to use.

How it works

Roof area ÷ the typical drainage area per downspout, rounded up.

Before you order

  • 3 × 4 inch downspouts move far more water than 2 × 3.
  • Keep elbows to a minimum, since each one slows flow.
  • Place an outlet near valleys and long runs.

Common questions

What is the rule for downspout size?

A common rule is about 1 square inch of opening per 100 square feet of roof.

How much roof can one 2 × 3 downspout drain?

About 600 square feet in normal rainfall, and less in heavy rain regions.