Gutter Length & Downspout Calculator

Estimate total gutter footage, downspout count, and downspout footage from your roof perimeter.

Eave Perimeter (ft)

Total linear feet of roof edge that needs gutters.

Downspout Spacing (ft)

Rule of thumb: one downspout per 30-40 ft of gutter.

Downspout Drop Height (ft)

Vertical distance from the gutter to the ground or splash block.

Gutter length is simply the total linear footage of roof eave that needs coverage, but downspout count is not something you eyeball — too few downspouts and the gutters overflow in a heavy storm, backing water up under the roofline. The standard trade rule of thumb is one downspout for every 30-40 linear feet of gutter, or roughly one per 600-800 sq ft of roof area draining into that run, whichever is more restrictive for your roof's pitch and rainfall intensity.

This calculator divides your total eave perimeter by the spacing rule to get a downspout count, then multiplies that count by the vertical drop height to get total downspout footage — the number you actually take to the supply counter alongside the gutter footage itself.

Frequently Asked Questions

How many downspouts does a house need?

A common rule is one downspout per 30-40 linear feet of gutter, though homes in heavy-rainfall regions or with steep roof pitches often benefit from spacing them closer, around every 20-30 feet.

How far apart should gutter downspouts be?

Downspouts are typically placed no more than 35-40 feet apart along a gutter run, and always at the low point of any sloped section so water has somewhere to drain.

What size downspout do I need?

Standard residential downspouts are 2x3 inches, which pair with a 5-inch K-style gutter. Larger roofs or high-rainfall regions often use 3x4 inch downspouts with 6-inch gutters to handle higher water volume.