Gutter Calculator
Enter your roof edge length to estimate gutters, downspouts, and hangers — the full system, not just the trough.
Runs entirely in your browser — your numbers never leave this page.
How it works
Gutters are a system: trough + downspouts + hangers + slope. Size it for your worst storm, not the average one:
- 5-inch vs 6-inch: 6-inch K-style carries ~40% more water for little extra cost — the right call for steep or large roofs.
- Downspouts every 30–40 ft: too few and gutters overflow in heavy rain, dumping water at the foundation — defeating the purpose.
- Slope 1/4 in per 10 ft toward downspouts. Dead-level gutters hold water, breed mosquitoes, and rust from the inside.
- Hangers every 24 in (not spikes-and-ferrules — hidden hangers hold through ice and ladders).
- Extensions: downspouts must discharge 4–6 ft from the foundation — into daylight, a dry well, or buried drain pipe.
Frequently asked questions
How much gutter do I need?
Measure all eaves (horizontal roof edges), add 5% for cuts. A 40×30 ft house with gutters on both 40-ft eaves needs ~84 ft.
How many downspouts do I need?
One per 30–40 ft of gutter, minimum 2 per run. Long runs need a downspout at each end or the middle sags with water weight.
5-inch vs 6-inch gutters?
6-inch carries ~40% more water. Choose 6-inch for steep roofs, large roof areas, or heavy-rain regions — the cost difference is small.
What slope do gutters need?
1/4 inch per 10 feet toward the downspout. Too flat holds water; too steep looks wrong and wastes capacity.
How far apart are gutter hangers?
Every 24 inches. Hidden hangers screwed to the fascia beat old spike-and-ferrule, which loosens as wood expands.
Where should downspouts drain?
4–6 feet from the foundation minimum — extensions, splash blocks, or buried pipe to daylight. Dumping at the foundation causes basement water.
Do I need gutter guards?
If you have trees, yes — they cut cleaning from twice yearly to occasional brushing. Micro-mesh beats screens and foam inserts.
Seamless vs sectional gutters?
Seamless (formed on-site) leaks only at corners and ends; sectional DIY leaks at every 10-ft joint eventually. Seamless costs more, leaks less.
Why do gutters overflow in heavy rain?
Undersized (5-in on a big roof), too few downspouts, or clogs. Water shooting over the front edge in storms is a capacity problem, not a slope problem.
How long do gutters last?
Aluminum: 20–30 years. Copper: 50+. Vinyl: 10–20 (brittle in sun). Galvanized steel rusts in 15–20 without maintenance.