A 40 × 28 ft house with a gable roof and 12 in overhangs needs two 42 ft gutters, 84 feet in all. At a design rainfall of 3 in/h its 1,260 ft² roof sheds 39 gallons a minute — well within a 5 in gutter and four 2 × 4 in downspouts, one at each end of each gutter, because no run should be longer than about 40 ft. The calculator above sizes gutters and downspouts for any rectangular gable or hip roof and counts every part you need to order.
How the calculator works
Enter the building length and width, the overhang and the height of the eaves, choose a gable or a hip roof, and set the rainfall, gutter and downspout sizes. The result is the gutter length, the number of downspouts and what sets it, and the parts list: 10 ft sections, hangers, connectors, end caps or corners, outlets, downspout sections, elbows and straps.
Runoff from the roof
Rain falls vertically, so what counts is the roof's plan area, overhangs included, not its sloped area. One inch of rain an hour on one square foot is 0.0104 gallons a minute: Q = area × rainfall ÷ 96.2. The rainfall is the 100-year, 1-hour rate for your location, which the International Plumbing Code asks for (IPC §1106.1); it is about 3 in/h across much of the Midwest and Northeast, 4–5 in/h in the Southeast and over 6 in/h on parts of the Gulf Coast. Your building department has the figure.
Gutter and downspout capacity
| Gutter (IPC Table 1106.6, ⅛ in/ft) | gpm | Downspout (Table 1106.3) | gpm |
|---|---|---|---|
| 5 in half-round | 74 | 2 × 2 in | 30 |
| 6 in half-round | 110 | 2 × 4 in, 3 in round | 92 |
| 8 in half-round | 247 | 3 × 4¼ in, 4 in round | 192 |
K-style gutters are not in the code table. They carry more than a half-round of the same width, so rating a 5 in K-style as a 5 in half-round is on the safe side.
How many downspouts
Each gutter gets enough downspouts to satisfy three limits at once: no run longer than the maximum (40 ft is the usual rule), no downspout carrying more than its table capacity, and no gutter carrying more than its own. The 84 ft example is governed by length: each 42 ft gutter is just over 40 ft, so it gets two downspouts, one at each end, with the gutter sloped from the middle both ways. At 6 in/h the flow doubles to 79 gpm, still under 20 gpm per downspout. A hip roof has one continuous gutter all round, 144 ft on the same house, with four corners instead of four end caps.
Parts list
- Sections. Gutter length ÷ section length, offcuts reused: 84 ft is nine 10 ft sections.
- Hangers. One every 24 in plus one at the end of each gutter: 44. Use 16 in where snow and ice sit in the gutter.
- Connectors. One at each joint within a straight run: 8 for the two gutters. Seamless gutters need none.
- Downspouts. Each is the eave height plus the overhang, 11 ft here; four of them take five 10 ft sections, three elbows each and two straps each.
Common mistakes
- Measuring the walls instead of the eaves. Gutters run along the roof edge, overhangs included.
- Using the sloped roof area. A steep roof catches no more rain than a flat one of the same plan.
- One downspout for a long gutter. Past about 40 ft the gutter must be pitched so far that it drops visibly — add a downspout and slope both ways.
To work out the roof itself, use the Rafter Length Calculator or the Hip Roof Calculator.