Gutter & Downspout Calculator
Sections, downspouts, elbows, and hangers — the parts list for the eave, counted two ways so the downspouts do not come up short.
Counts are planning counts. Gutter and downspout capacity sizing follows the manufacturer and local code tables for your roof and rainfall — this page does not certify a size, it counts parts once the layout is chosen.
How the math works
Sections = eave length ÷ section length. Downspouts are counted by BOTH common planning rules — one per your max spacing along the eave, and one per your planning roof area — and the larger count wins, because the rule that asks for more is the one the storm will use. Runoff is roof area × rainfall rate × 0.623 gallons per inch per sq ft ÷ 60, in gallons per minute. Capacity sizing itself follows manufacturer and local code tables; this page counts parts for a chosen layout.
A worked example
An 80 ft eave in 10 ft sections takes 8 sections. Downspouts: 2 by the 40 ft spacing rule, but 3 by the 1,500 sq ft roof-area rule at 600 sq ft each — so 3, with 9 elbows at three per downspout, 3 downspout sections for a 10 ft drop, and 41 hangers at 2 ft spacing. At a 5 in/hr planning rain rate, that roof sheds 77.88 gallons a minute.
Questions from the field
How many downspouts do I need? Run both rules — spacing along the eave and roof area served — and take the larger. Long eaves with small roofs are ruled by spacing; big roofs on short eaves are ruled by area.
How often do gutter hangers go in? About every 2 ft is the common planning spacing, closer in snow country (not a Long Beach problem) and at corners.
What rainfall rate should I plan for in Long Beach? Design storms, not annual averages — atmospheric-river hours are the test. The rate is an editable planning input here; your local code table is the authority.
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