Gutter Calculator

Work out gutter sections, hangers and how many downspouts a roof area needs.

Draft — figures checked, awaiting an editorial read. Not indexed.

Formula: Sections = eave length ÷ stock length. Downspouts = roof area ÷ 600 ft². Hangers = eave length ÷ 2 ft.

What this calculates

This sizes a gutter run: how much gutter to buy, how many hangers to hold it up, and — the part that actually matters — how many downspouts the roof needs. Gutter length is simple arithmetic. Downspout count is the design decision, because a gutter is only as good as its ability to get water out, and an under-drained gutter overflows in exactly the heavy rain it was installed to handle. Two hundred feet of gutter running to a single downspout will spill over the front edge in any real storm.

How it works

The gutter length is just the total eave length you enter, divided by the stock length to give a section count. Hangers are spaced every two feet, which is the standard for a 5 inch K-style gutter and tightens to 18 inches in regions with snow load. Downspouts use the common residential rule of roughly one per 600 square feet of roof area drained — enter the roof surface area from a roofing calculation rather than the building footprint, since a pitched roof sheds noticeably more water than its footprint suggests.

Worked example

A house with 80 feet of eave draining 1,400 square feet of roof, using 10 foot sections. Gutter: 80 linear feet, which is 8 sections. Hangers: 80 ÷ 2 = 40. Downspouts: 1,400 ÷ 600 = 2.33, rounded up to 3. Plus two end caps. Three downspouts on 80 feet of gutter puts one at each end and one in the middle, which is a sensible layout as well as an adequate one.

Common mistakes

Using the building footprint as the roof area, which under-counts downspouts on any pitched roof. Second, spacing hangers too far apart — gutters sag under the weight of water long before they sag under their own, and a sagging gutter pools rather than drains. Third, treating downspout placement as purely cosmetic: they need to discharge well away from the foundation, and a downspout that dumps against the wall creates a basement problem to solve a roof one.

Common questions

How many downspouts do I need?

About one per 600 square feet of roof drained is the common residential rule of thumb, with a practical maximum of roughly 40 feet of gutter run per downspout. Heavy rainfall regions should be more generous.

How far apart should gutter hangers be?

Every 24 inches in most climates, tightening to 18 inches where snow and ice load the gutter. Closer spacing is cheap insurance against sagging.