Gutter Calculator
Measure each eave where a gutter will hang, enter the roof area that drains to it, and this calculator works out the total length, how many downspouts you need and whether a standard 5-inch gutter can handle your local rainfall. It uses the same roof-slope adjustment and capacity approach that sheet-metal contractors use, and shows every step so you can check the math.
Gutter Calculator
Estimate how many feet of gutter and how many downspouts a house needs, check whether a 5-inch or 6-inch gutter is likely big enough for your rainfall, and get a budget cost range.
Your results appear here and update as you change the numbers.
How this calculator works
- Linear feet = sum of all gutter runs.
- Adjusted drainage area = plan area × roof-slope factor (1.00 up to 3/12, 1.05 to 5/12, 1.10 to 8/12, 1.20 to 11/12, 1.30 for 12/12) — steeper roofs catch more wind-driven rain.
- Design load per run = adjusted area × rainfall intensity ÷ number of runs. If it exceeds the profile’s capacity at 1 in/hr, consider a larger gutter or splitting the run with more downspouts.
- Downspouts = the larger of: adjusted area ÷ area per downspout, or one per run plus one for every additional spacing interval along long runs.
- Cost = linear feet × (material + labor per ft) + downspouts × installed price, shown as a ± range.
Worked example
Two 40 ft runs draining 2,000 sq ft of roof at 6/12, design rainfall 5 in/hr, 5-inch K-style, 2×3 downspouts: adjusted area 2,000 × 1.10 = 2,200 sq ft; load per run 2,200 × 5 ÷ 2 = 5,500 sq ft-equivalent, under the 5,520 capacity, so 5-inch gutters work. Downspouts: 2,200 ÷ 600 = 3.7 → 4. Cost with $7 + $4 per ft and $120 per downspout: 80 × $11 + 4 × $120 = $1,360 (example prices; your settings may differ).
When to get a professional check
Gutter sizing here is preliminary. Final gutter and downspout sizes depend on your local rainfall data, the gutter manufacturer’s capacity tables and run layout — confirm with your installer.
Assumptions used — example values, not yet reviewed against current local prices
| Assumption | Value |
|---|---|
| 5" K-style capacity (roof area per run at 1 in/hr) | 5,520 sq ft |
| 6" K-style capacity (roof area per run at 1 in/hr) | 7,960 sq ft |
| 5" half-round capacity (roof area per run at 1 in/hr) | 2,500 sq ft |
| 6" half-round capacity (roof area per run at 1 in/hr) | 3,840 sq ft |
| Roof area per 2×3 downspout (≈1 sq in per 100 sq ft) | 600 sq ft |
| Roof area per 3×4 downspout | 1,200 sq ft |
| Maximum downspout spacing along a run | 40 ft |
| Vinyl gutter material | $4 /ft |
| Seamless aluminum gutter material | $7 /ft |
| Galvanized steel gutter material | $10 /ft |
| Copper gutter material | $25 /ft |
| Installation labor | $4 /ft |
| Downspout installed (pipe, elbows, straps) | $120 /each |
| Range width (± share of the estimate) | 15% |
Sources and references
Ready for real prices? Learn how to find and vet a local roofer.
How to choose a contractorResults are planning estimates, not quotes. Prices vary by location, contractor and the condition of your roof. Verify sizes and code requirements locally before buying or building.
Frequently asked questions
How many downspouts do I need?
At least one per gutter run, more on long runs (roughly every 35–40 ft), and enough outlet area for the roof that drains to them — about 1 sq in of downspout per 100 sq ft of roof as a rule of thumb.
Should I get 5-inch or 6-inch gutters?
5-inch K-style handles most homes. Steep roofs, large roof sections draining to one run, or heavy-rain regions often need 6-inch. The calculator flags when your rainfall exceeds the 5-inch capacity.
Where do I find my rainfall intensity?
NOAA Atlas 14 (hdsc.nws.noaa.gov/pfds) — use the 5-minute, 10-year value in inches per hour.
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