Size gutters and downspouts the way sheet-metal contractors do. Enter each gutter run and the roof area that drains into it, your roof pitch, and your local rainfall intensity, and the calculator applies the SMACNA method to recommend 5-inch or 6-inch K-style (or half-round) gutters, the downspout size and count, and a full parts list: total length, hangers, end caps, corners, outlets, elbows, and downspout pipe.
Need the pitch first? The roof pitch calculator converts rise and run into a pitch, and the roofing calculator estimates shingles for the same roof. Sending downspout water away from the foundation? The French drain calculator sizes the trench.
Frequently asked questions
Do I need 5-inch or 6-inch gutters?
It depends on how much roof drains into each gutter, how steep the roof is, and how hard it rains where you live. The SMACNA method this calculator uses multiplies the roof's footprint by a pitch factor, then compares it with what each gutter can carry at your local 5-minute rainfall intensity. A level 5-inch K-style gutter is rated for 5,520 sq ft at 1 inch per hour, so at 7 in/hr it handles about 790 sq ft per outlet; a 6-inch K-style handles about 1,140. Many houses with long roof slopes or steep pitches need 6-inch gutters.
How many downspouts do I need?
Each square inch of downspout drains about 1,200 ÷ rainfall intensity square feet of design roof area, per the SMACNA tables. A 2×3-inch downspout (6 sq in) therefore drains about 1,000 sq ft at 7 in/hr, and a 3×4 (12 sq in) about twice that. The popular rule that a 2×3 drains 600 sq ft and a 3×4 drains 1,200 is the same formula at a conservative 12 in/hr. Independently of capacity, plan on at least one downspout for every 50 feet of gutter, and more if a single gutter cannot carry its whole watershed to one outlet.
What rainfall intensity should I use?
Gutters are sized for short, intense bursts, not daily totals, so use the 5-minute rainfall intensity that is expected about once every 10 years. The city list comes from the SMACNA table based on U.S. Weather Bureau records — for example Chicago 6.8 in/hr, Houston 8.2, and Seattle 2.1. For your exact address, look up NOAA Atlas 14 at hdsc.nws.noaa.gov/pfds, choose the 5-minute duration and 10-year return period, and switch the output to intensity. Some local codes require a 100-year storm instead, which gives larger numbers.
Why does roof pitch change the gutter size?
Rain rarely falls straight down. Wind-driven rain hits a steep roof face-on, so a steep roof catches more water than its footprint alone suggests. SMACNA handles this with a pitch factor applied to the plan area: 1.00 for roofs up to 3/12, 1.05 for 4/12–5/12, 1.10 for 6/12–8/12, 1.20 for 9/12–11/12, and 1.30 for 12/12 and steeper. Enter the footprint area that drains into each gutter run — not the sloped surface area from a roofing takeoff — and let the factor handle the slope.
How far apart should gutter hangers be?
Space hidden hangers every 24 to 36 inches. Use 24 inches in areas with snow and ice, under long roof valleys, or anywhere ladders will lean against the gutter; 36 inches is the usual maximum elsewhere. The calculator counts hangers per run at the spacing you choose, plus one so both ends are supported. It also counts end caps (two per straight run, minus two for every corner that joins runs), drop outlets, three elbows per downspout, and 10-foot sections in case you are not installing seamless gutters.