Downspout Calculator

Enter your roof drainage area and get the number of downspouts to plan — sized by a drainage-area rule of thumb, with the total downspout length and a metric equivalent.

Inputs
sq ft
ft
Results update as you type — no submit
ResultsLIVE
Downspouts
2 outlets
≈ 20 ft of pipe (6.1 m)
Downspout pipe20 ft (6.1 m)
Roof area drained2,000 sq ft (185.8 m²)
Formula
downspouts = ceil(2,000 ÷ 1,000) = 2
pipe = 2 × 10 ft wall = 20 ft
Diagram — drainage planeNTS
54.8 ft36.5 ft2 DS · 2,000 SQ FTDRAINAGE AREA · OUTLETS
MeasureShedmeasureshed.com/calculator/downspout-calculator
Downspouts — material listUNITS: FT · SCALE: NTS
Inputs
Roof area2,000 sq ft
Outlet size1,000 sq ft each
Wall height10 ft
Results
Downspouts2
Downspout pipe20 ft
Drainage planeNTS
54.8 ft36.5 ft2 DS · 2,000 SQ FTDRAINAGE AREA · OUTLETS
Shopping list
☐ 2 downspout outlets + elbows
☐ 20 ft downspout pipe
Formula
downspouts = ceil(2,000 ÷ 1,000) = 2
pipe = 2 × 10 ft = 20 ft
Estimates for planning only — verify before purchaseSHEET 1 OF 1
How it works

A downspout can only carry the runoff from a limited patch of roof before it overwhelms in a downpour, so the number you need scales with drainage area. Divide the roof area by the area one downspout handles and round up.

How much one downspout handles depends on its outlet size and your local rainfall intensity. As a planning rule, a `2` by `3` inch outlet drains about `600` square feet, a `3` by `4` inch outlet about `1,200`, and a round `3`-inch pipe falls in between. This calculator uses a midpoint of about `1,000` square feet per downspout by default and lets you switch to the smaller or larger allowance to match your outlets.

The tool also estimates the pipe you will run down the wall. Each downspout drops from the gutter to grade, so the total downspout length is the number of downspouts times the wall height, plus a little for the elbows and the kick-out at the bottom. That gives you a length to price alongside the count.

This is a rule-of-thumb estimate for planning. In a region with intense storms, or on a roof with valleys that concentrate flow, err toward more and larger downspouts, and consider a proper sizing that accounts for your rainfall rate.

Worked example
Roof area `2,000 sq ft` · outlet serves `1,000 sq ft` · wall height `10 ft`
downspouts = `2,000 ÷ 1,000` = `2.0` → `2` downspouts
downspout pipe = `2 × 10 ft` wall = `20 ft`
Smaller `2×3` outlets at `600 sq ft` each → `2,000 ÷ 600` = `3.3` → `4` downspouts
Questions about downspouts
Plan on one downspout for every 600 to 1,200 square feet of roof, depending on the outlet size. A common midpoint is one per 1,000 square feet. A 2,000-square-foot roof needs about two large outlets or four small ones.
SOURCES — SMACNA Architectural Sheet Metal Manual · 2×3 ≈ 600 sq ft, 3×4 ≈ 1,200 sq ft per outlet
Estimates for planning only — verify before purchase
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