A gutter job is really a parts list: gutter sections for every eave, enough downspouts for the roof area, hangers every couple of feet, end caps, outlets, elbows and pipe down to the ground. This gutter calculator builds that list from your eave length, number of runs, roof area and wall height. It sizes downspouts with a common rule of thumb, one square inch of downspout opening per 100 ft² of roof, and a maximum gutter length per downspout. A house with 120 ft of eaves in four runs and a 1,800 ft² roof needs about 13 ten-foot sections, 4 downspouts, 64 hangers and 12 elbows.
Find the roof area with the roofing shingles calculator, and see how much water those downspouts can feed into a barrel with the rainwater harvesting calculator.
How to use the gutter calculator
- Measure the length of every eave that gets a gutter and add them up.
- Count the separate runs. A run ends where the gutter stops, for example at a gable or an inside corner you don't wrap.
- Enter the roof area that drains into the gutters, measured along the slope (plan area × the pitch factor). Steep roofs catch more wind-driven rain.
- Choose the downspout size and the maximum length of gutter you want per downspout.
- Enter the eave height for downspout pipe, plus your section length and hanger spacing.
Formula
The 5% on gutter length covers overlaps and miters. Three elbows per downspout is typical: two to bring the pipe back to the wall and one at the bottom.
Worked example
120 ft of eaves in 4 runs, a 1,800 ft² roof, 2 × 3 inch downspouts, a downspout at least every 35 ft, 10 ft eaves, 10 ft sections and hangers every 24 inches.
- Downspouts by roof area: 1,800 ÷ 600 = 3. By length: 120 ÷ 35 = 3.4, so 4. By runs: 4. Use 4 downspouts.
- Sections: 120 × 1.05 = 126 ft ÷ 10 = 12.6, so 13 sections.
- Hangers: 1,440 in ÷ 24 = 60, + 4 = 64 hangers. End caps: 8. Outlets: 4.
- Pipe: (10 + 2) ÷ 10 = 1.2, so 2 pieces per downspout = 8 pieces. Elbows: 4 × 3 = 12.
Downspout sizing rule of thumb
| Downspout | Outlet area | Roof area per downspout (rule of thumb) | Downspouts for a 2,000 ft² roof |
|---|---|---|---|
| 2 × 3 in rectangular | 6.0 in² | 600 ft² | 4 |
| 3 × 4 in rectangular | 12.0 in² | 1,200 ft² | 2 |
| 3 in round | 7.1 in² | 707 ft² | 3 |
| 4 in round | 12.6 in² | 1,257 ft² | 2 |
This rule assumes moderate rainfall intensity. Engineers size gutters and downspouts from the local design rainfall intensity (NOAA publishes precipitation frequency data for every U.S. location) using sheet-metal industry tables such as SMACNA's. In regions with intense downpours, upsize to 3 × 4 inch downspouts or add more of them.
Gutter size and slope
Residential gutters are most often 5 inch K-style, with 6 inch gutters for large or steep roofs. Slope gutters toward the outlets, commonly about 1/4 inch per 10 ft, and hang them so the roof edge drains into the gutter. The International Residential Code (Section R801.3) requires a roof drainage system that discharges water away from the foundation where expansive or collapsible soils are present, and many builders follow that practice everywhere. Run downspouts at least a few feet from the house with extensions or splash blocks. Replacing siding at the same time? Do it before the gutters with the siding calculator; for roof slope, the roof pitch calculator helps.
Caveats and assumptions
- Seamless gutters are made to length on site; order by the foot instead of sections.
- Inside and outside corners replace some end caps; adjust the fittings list for your layout.
- The downspout rule of thumb is not an engineered design. Heavy-rain areas may need larger gutters and downspouts.
- Hanger spacing should follow the manufacturer's instructions and local snow loads.
Frequently asked questions
How many downspouts do I need?
At least one per gutter run, one for every 30 to 40 ft of gutter, and enough opening for the roof area: a rule of thumb is one 2 × 3 inch downspout per 600 ft² of roof or one 3 × 4 inch per 1,200 ft².
How far apart should gutter hangers be?
Commonly every 24 inches, and every 16 to 18 inches in heavy-snow areas, or as the gutter manufacturer specifies.
How many gutter sections do I need?
Divide the total eave length by the section length and add about 5% for overlaps. 120 ft of eaves needs 13 ten-foot sections.
What size downspout do I need?
2 × 3 inch downspouts suit many small roofs; 3 × 4 inch downspouts carry about twice the water and are better for large roofs, steep roofs and heavy-rain areas.
How much slope should a gutter have?
A common guideline is about 1/4 inch of fall per 10 ft of gutter toward the downspout.
Sources
- NOAA — Precipitation Frequency Data Server (Atlas 14) — local design rainfall intensity
- ICC — 2021 International Residential Code, Chapter 8: Roof-Ceiling Construction — Section R801.3 roof drainage
- SMACNA — Sheet Metal and Air Conditioning Contractors' National Association — Architectural Sheet Metal Manual gutter and downspout sizing
- U.S. EPA — Soak Up the Rain: rain barrels — downspouts and rain barrels
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.