Gutter Calculator
Match roof area and rainfall intensity to a gutter size from an editable capacity table, plus downspouts.
Processed locally in your browser — nothing is uploaded.
Gutter Calculator
Footprint area draining to this gutter run
Local design storm; 1–4 in/hr covers most of the US
Enter the roof area and rainfall to size the gutter.
About Gutter Calculator
Drainage demand is roof area × rainfall intensity (in/hr). The tool compares that against a capacity table in the style of SMACNA sizing guides — 5" K-style handles roughly 5,520 sq ft·in/hr — and recommends the smallest adequate gutter. Every capacity is editable. Downspouts are counted at about one per 35 ft of gutter run.
Common uses
- Decide between 5" and 6" K-style gutters for a large roof
- Count downspouts for a long gutter run
Example
Input
1,400 sq ft draining area, 2 in/hr storm, 120 ft of gutter
Output
2,800 demand → 6" half-round or 5" K-style · 4 downspouts
Limitations
- Capacities are rough table values assuming slight slope; steep roofs and valleys concentrate flow
- Local rainfall design intensities vary — check your region's figure
Frequently asked questions
What size gutter do I need for my roof?
Multiply the roof area draining into that run by your local rainfall intensity, then pick the smallest gutter whose capacity covers it. A 1,400 sq ft area in a 2 in/hr storm is a demand of 2,800, which is past the 5 in half-round's 2,500 but well inside the 6 in half-round at 3,840 or the 5 in K-style at 5,520.
How many downspouts do I need?
About one per 35 ft of gutter run, which is the rate this calculator uses — a 120 ft run gets 4. Downspouts are usually the real bottleneck rather than the gutter profile, so adding one is often more effective than upsizing the trough.
Does roof pitch affect gutter sizing?
Yes. A steep roof throws water into the gutter faster and lets wind drive rain further along the slope, so sizing tables scale the drainage area up for pitch. Enter an increased area for steep roofs, and treat any run catching a valley as carrying more than its share.
What rainfall intensity should I use?
Your area's design storm intensity in inches per hour, not the annual rainfall — most of the US falls between 1 and 4 in/hr, with the Gulf Coast and Southeast at the high end. Regions with frequent but gentle rain, like the Pacific Northwest, have surprisingly low peak intensities.
Is my roof information uploaded when I size gutters?
No. The demand, gutter, and downspout calculations happen in your browser — roof areas and capacity edits are never sent to a server or stored.