★ REFERENCE TABLE · CONSTRUCTION · GUTTERS
A 1,200 sq ft roof section on an ordinary pitch already needs a 6-inch gutter. With a pitch factor of 1.1 its adjusted drainage area is 1,320 sq ft — above the 1,050 sq ft a 5-inch gutter serves per downspout at 4 inches per hour — so it takes a 6-inch K-style gutter with 3x4 downspouts. Enter your own section below.
Gutters are sized per roof section, not per house: the area draining into one run, steepened by its pitch, decides the size. A 5-inch K-style gutter handles sections up to 1,050 sq ft; a 6-inch one takes anything bigger, at an assumed design rain rate of 4 inches per hour. Enter your section below.
Adjusted drainage area = roof area × pitch factor. Pitch factors: flat to 3:12 counts once (1.0), 4:12 to 6:12 counts 1.1 times, 7:12 to 9:12 counts 1.2 times, and 10:12 and steeper counts 1.3 times — steeper roofs shed water faster, so the same roof counts as a bigger drainage load. The capacity basis is stated once and used throughout: HUD Durability by Design, Table 4-5, at a design rain rate of 4 inches per hour — a 5-inch K-style gutter serves up to 1,050 sq ft per downspout with 2x3 downspouts, and a 6-inch K-style gutter serves up to 1,709 sq ft per downspout with 3x4 downspouts. If the adjusted area is 1,050 sq ft or less, the answer is 5-inch; above that, 6-inch. Downspout count = adjusted area ÷ the chosen size's per-downspout capacity, rounded up. Capacities scale inversely with rain rate: at 8 inches per hour, halve these per-downspout areas. This is a planning method, not a building code — local code and the manufacturer's capacity table govern the final install.
Setup: roof area 1,200 sq ft, pitch 4:12 to 6:12 (factor 1.1), design rain rate 4 inches per hour.
Line 1 — adjusted area. 1,200 × 1.1 = 1,320 sq ft.
Line 2 — size. 1,320 is above 1,050, so 6-inch K-style with 3x4 downspouts.
Line 3 — downspouts. 1,320 ÷ 1,709 = 0.77, rounded up to 1 downspout.
Small flat section: 500 sq ft, flat to 3:12. Adjusted area 500 sq ft, so a 5-inch K-style gutter with 2x3 downspouts and 1 downspout (500 ÷ 1,050, rounded up).
Steep section: 1,500 sq ft, 10:12 and steeper. Adjusted area 1,500 × 1.3 = 1,950 sq ft, so a 6-inch K-style gutter with 3x4 downspouts and 2 downspouts (1,950 ÷ 1,709 = 1.14, rounded up).
Right at the boundary: 1,050 sq ft, flat. Adjusted area 1,050 sq ft, exactly at the 5-inch capacity, so a 5-inch K-style gutter with 2x3 downspouts and 1 downspout — but with no margin, a 6-inch upgrade is the safer call if your design rain rate exceeds 4 inches per hour.
Large section: 3,000 sq ft, 7:12 to 9:12 pitch. Adjusted area 3,000 × 1.2 = 3,600 sq ft, so a 6-inch K-style gutter with 3x4 downspouts and 3 downspouts (3,600 ÷ 1,709 = 2.11, rounded up).
| Gutter | Downspout | Area per downspout at 4 in/hr | Best for |
|---|---|---|---|
| 5-inch K-style | 2x3 rectangular | Up to 1,050 sq ft | Sections up to 1,050 sq ft of adjusted area |
| 6-inch K-style | 3x4 rectangular | Up to 1,709 sq ft | Larger, steep, or rain-heavy sections |
Basis: HUD Durability by Design, Table 4-5, at 4 inches per hour. Do not mix this table with vendor charts that quote far larger areas — see Accuracy & limitations for why they differ.
Aluminum is the default: light, rust-proof, and cheap enough that 6-inch costs only about 20 to 25 percent more than 5-inch. Galvanized or Galvalume steel is stronger and heavier, good under snow and ladders, but needs coating care in wet or coastal air. Copper lasts decades and needs compatible fasteners — never mix copper gutters with aluminum or steel fasteners — but costs several times aluminum. Vinyl is the cheapest and lightest, but it expands in heat, gets brittle in cold, and usually needs closer hanger spacing. Cross-section tells the capacity story: a 5-inch K-style gutter holds about 7.2 square inches of water against about 10.5 for a 6-inch, which is why the 6-inch serves roughly 60 percent more roof per downspout. Hang gutters with a slight slope toward the downspout, about 1/16 to 1/8 inch per foot, and extend downspouts 5 to 10 feet from the foundation.
| Output | Formula | What it means |
|---|---|---|
| Adjusted drainage area | Roof area × pitch factor | 1,320 sq ft at the defaults — the number the size is actually chosen from. |
| Recommended gutter | 5-inch if adjusted area ≤ 1,050, else 6-inch | 6-inch K-style at the defaults — the 1,200 sq ft section already exceeds the 5-inch limit. |
| Downspout size | 2x3 with 5-inch; 3x4 with 6-inch | 3x4 at the defaults — the outlet must match the gutter, or the gutter is not the bottleneck that matters. |
| Downspout count | Adjusted area ÷ per-downspout capacity, rounded up | 1 at the defaults — never round down, because one missing downspout is an overflow. |
Published sources disagree sharply, and this page does not hide that. Vendor charts sometimes quote a 5-inch gutter at up to 5,500 sq ft — but those figures typically describe a whole system, or assume a much lower rain rate, not one downspout at 4 inches per hour, so they cannot be compared with the HUD table. HUD Durability by Design, Table 4-5, at 4 inches per hour gives 5-inch = 1,050 sq ft and 6-inch = 1,709 sq ft per downspout, and that single basis is what this calculator uses. SMACNA's rule of thumb — 1 square inch of downspout per 100 sq ft of roof — sizes the downspout, not the gutter, which is another reason published numbers differ: they answer different questions. Valleys, long gutter runs, elbows, and gutter guards all cut real capacity, and a run over about 40 feet usually needs a downspout at each end regardless of the count here. This is a planning estimate, not a code calculation: check the manufacturer's capacity table and local code before buying. Error policy: roof area must be a number above 0; anything else is reported as an error rather than calculated.
Bright Side Kit. "Gutter Size Chart: 5-Inch vs 6-Inch K-Style." https://brightoolbox.com/gutter-size-chart/ Accessed October 24, 2026.For big, steep, or rain-heavy sections, yes. A 6-inch K-style gutter holds about 10.5 square inches of water against about 7.2 for a 5-inch, and serves up to 1,709 sq ft per downspout against 1,050 at 4 inches per hour. For an ordinary small section, 5-inch is enough and looks less bulky.
Divide the adjusted area by the per-downspout capacity and round up. A 1,320 sq ft adjusted section on 6-inch gutters needs 1,320 ÷ 1,709 = 0.77, rounded up to 1. Never round down, and put a downspout at each end of any run over about 40 feet.
2x3 inches rectangular, always on this page. The pairing is fixed: 5-inch K-style pairs with 2x3 downspouts and 6-inch with 3x4. A larger outlet does not change the gutter's own capacity on the HUD basis used here.
3x4 inches rectangular, always on this page. The pairing is fixed: 6-inch K-style pairs with 3x4 downspouts. Larger outlets belong to larger commercial profiles, not to a bigger residential number on this basis.
One roof section at a time. Gutters are sized per run: the area draining into that gutter. Entering the whole house area will oversize every run and overstate the downspout count.
Yes — steeper roofs shed water faster. The sizing rules multiply the area by 1.0 for flat to 3:12 pitch, 1.1 for 4:12 to 6:12, 1.2 for 7:12 to 9:12, and 1.3 for 10:12 and steeper, which is why a steep 1,500 sq ft section can need 6-inch gutters where a flat one would not.
Usually, when the math says so. Six-inch aluminum runs about 20 to 25 percent more than 5-inch, which is cheap insurance against overflow, fascia rot, and foundation water — the expensive problems gutters exist to prevent.