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Gutter Calculator

A gutter calculator works out how much guttering to buy, how many downspouts you need and how many hangers to hold it up — from your eave length and the roof area each run drains. It sizes the drainage so the system actually keeps up in a downpour.

Your measurements

Result

The number that matters beyond length is downspouts: too few and the gutters overflow no matter how big they are. This applies the one-per-35-feet and one-per-800-square-feet rules and takes the larger.

How it is calculated

Gutter length = eave length. Downspouts = the greater of eave length ÷ 35 ft and roof area ÷ 800 sq ft (rounded up, minimum 1). Hangers = eave length ÷ hanger spacing. Corners, end caps and elbows depend on your roof's shape and are added separately.

Steps

Measure the total eave length the gutters will run along.
Enter the roof area that drains into them.
Pick a hanger spacing (24 in is typical).
Read the gutter footage, downspouts and hangers.
Add corners, end caps and elbows for your layout.

Worked example

120 ft of eaves draining 1,600 sq ft of roof needs about 120 ft of gutter, 4 downspouts and ~60 hangers at 24 in spacing.

Common questions

How many feet of gutter do I need?

It equals the total length of the eaves the gutters run along. The calculator also counts 10 ft sections.

How many downspouts do I need?

About one per 35 ft of gutter and one per 800 sq ft of roof drained — use whichever gives more so the gutters don't overflow.

How far apart should gutter hangers be?

Every 24 inches is typical; go to 16 inches where heavy snow or ice loads the gutter.

What else do I need?

End caps, inside/outside corners, downspout elbows and outlets — these depend on your roofline, so add them to the run length.