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.
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