This gutter calculator sizes a gutter system the way codes and manufacturers do: from roof drainage area and design rainfall it computes peak runoff, recommends a gutter size, and counts the downspouts the run needs.
Sizing follows the standard drainage-area method (adjusted area = area × rainfall × pitch factor, compared against published capacities for a 1-in/hr baseline). Local codes set the design rainfall for your area — Gulf Coast cloudbursts can demand 4–8 in/hr design values while arid regions use less. Valleys concentrating flow onto one segment deserve one size up.
Why Roof Area, Not House Size
Gutters drain the roof plane's footprint projected with pitch adjustment — only the sections sloping toward each gutter run, with valleys doubling the local load where two planes meet. The physics: one inch of rain on one square foot is 0.623 gallons; a modest 1 in/hr design storm on 1,500 sq ft already delivers ~15 gallons a minute to the eaves. Design rainfall, not average rainfall, is the input that matters — systems are sized for the storm, not the climate mean. Roof quantities themselves (squares, pitch, bundles) are our Roofing Calculator's job, and pitch conversion lives at the Roof Pitch Calculator. Part of our Construction Calculators collection.
Worked Example (Verified)
1,500 sq ft draining to 80 ft of gutter, 1 in/hr design rain, 6/12 pitch (factor 1.05):
- Peak runoff: 1,500 × 1 × 0.01038 × 1.05 = ≈ 16.3 gallons/minute at the peak of the design storm
- Downspouts: length rule ⌈80 ÷ 40⌉ = 2; capacity rule ⌈1,575 ÷ 900⌉ = 2 → 2 downspouts, ~790 adjusted sq ft each
- Size: 790 falls in the high-capacity band → 5-inch high-capacity or 6-inch K-style with 3×4 downspouts if the 6-inch is chosen
- Sanity contrast: the same house in a 4 in/hr Gulf design storm quadruples the flow (~65 gpm) and pushes segments to commercial sizing — rainfall input is destiny
Installation Rules of Thumb Behind the Numbers
- One downspout per ~40 ft of gutter run, and never let a single downspout serve more than ~900–1,200 adjusted sq ft without upsizing.
- Slope gutters ¼ inch per 10 ft toward the outlet — level-looking gutters that hold water breed mosquitoes and ice.
- Downspout sizing pairs with gutter sizing: 5-inch K typically runs 2×3" outlets; 6-inch wants 3×4" — an undersized outlet bottlenecks a big gutter.
- Discharge matters as much as capture: extend downspouts 4–6 ft from the foundation or to a drain — a perfect gutter dumping at the footing is a basement-watering device.
Related Tools
- Roofing Calculator — squares and bundles for the roof above
- Rainwater Harvesting Calculator — catching this runoff in a barrel or tank instead
- Construction Calculators — the full collection
Frequently Asked Questions
How do I know what size gutters I need?
From adjusted drainage area per downspout: under ~700 sq ft → 5-inch K-style suffices; 700–1,200 → high-capacity 5-inch or 6-inch; beyond that, 6-inch (or commercial profiles). This calculator applies those bands to your area, rainfall and pitch inputs.
How many downspouts do I need?
The greater of: one per 40 ft of gutter run, and total adjusted drainage area ÷ ~900 sq ft. An 80 ft, 1,575-adjusted-sq-ft system needs 2 by both rules. More downspouts also buy resilience when one clogs mid-storm.
What rainfall intensity should I enter?
Your area's code design value — commonly 1 in/hr baseline comparisons in manufacturer tables get scaled by your local figure (many regions design at 3–6 in/hr short-duration intensities from NOAA data). Your building department or a local roofer knows the number; erring high costs one gutter size, erring low costs a flooded eave.
Do 6-inch gutters look too big on a house?
Modern 6-inch K-style reads nearly identical from the street (the extra inch is depth), holds ~40% more water, and clogs less. On large or steep roofs it's the default professional recommendation; on small shallow roofs 5-inch remains proportionate.
Why does roof pitch change the calculation?
Steeper roofs shed water faster and catch more wind-driven rain per horizontal foot — the adjustment factor (≈1.0 flat to 1.3 at 12/12+) scales the effective drainage area, a standard correction in SMACNA-based sizing tables.
Is this gutter calculator free?
Yes — free, instant, no sign-up.