Gutter Calculator
Size your gutters and downspouts from eave length and roof area.
What you'll need to order
- K-style gutter
- downspouts
- hidden hangers
- end caps
- outlet drops
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
downspouts = roof area ÷ 600 · hangers = eave length ÷ 2
One standard 2 by 3 inch downspout drains roughly 600 sq ft of roof in typical rainfall. In high-intensity rainfall regions, size down to 400 sq ft per downspout or step up to 3 by 4 inch.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- SMACNA Architectural Sheet Metal Manual 2012, 8th edition — gutter and downspout sizing against roof area and rainfall intensity
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With total eave length 100 ft, roof area draining to it 1800 sq ft, this calculator returns 3 .
Downspouts needed for roofs of common size
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | Downspouts needed |
|---|---|---|
| Small ranch | 70 ft × 1200 sq ft | 2.00 |
| Typical two-storey | 100 ft × 1800 sq ft | 3.00 |
| Large house | 140 ft × 3000 sq ft | 5.00 |
| Detached garage | 44 ft × 600 sq ft | 1.00 |
| Long low outbuilding | 120 ft × 1500 sq ft | 2.50 |
In high-intensity rainfall regions, drop to 400 sq ft per downspout or step up to 3×4 in.
Frequently asked questions
How many downspouts do I need?
One 2 by 3 inch downspout per 600 sq ft of roof area is the usual rule. A 1,800 sq ft roof needs three. In heavy-rainfall regions, work to 400 sq ft per downspout instead.
What size gutter should I use?
Five inch K-style handles most homes. Go to 6 inch where the roof is large, steep, or the rainfall intensity is high — a 6 inch gutter carries nearly twice the water of a 5 inch.
How far apart should gutter hangers be?
Twenty-four inches in most climates, and 18 inches where snow and ice load the gutter. Hidden hangers screwed into the fascia hold far better than the old spike-and-ferrule method.
What slope should a gutter have?
About 1/4 inch of fall per 10 ft toward the downspout. Too little and water stands; too much and the gutter looks visibly crooked against the fascia line.