The number to be careful with
Some cost and sizing guides list a 5-inch K-style gutter at 5,520 square feet and a 6-inch at 7,960, as if those were the most roof each can serve. Fine Homebuilding's method shows what that figure really means: 5,520 is the area a 5-inch K-style drains at a rainfall rate of 1 inch per hour. You divide it by your local rain rate to get the area one run can serve between downspouts.
Huntsville's design rate isn't 1 inch per hour. It's 8.20 or more. Read as a whole-house limit, the chart number overstates capacity about eightfold at that rate. This Old House's 7,960 for 6-inch is treated here as being on the same 1-inch basis, which is this site's reading of its table.
Huntsville's rain rates
Gutters have to handle short bursts, so sizing uses the 5-minute rate, stated in inches per hour. Sheet-metal trade group SMACNA's calculator follows its manual and uses 5-minute rainfall at the 10-year or 100-year level. These are NOAA Atlas 14 values for downtown Huntsville:
| How rare | 5-minute rate, in/hr |
|---|---|
| 1-year | 4.79 |
| 2-year | 5.60 |
| 5-year | 6.98 |
| 10-year | 8.20 |
| 25-year | 9.92 |
| 50-year | 11.3 |
| 100-year | 12.8 |
These values move slightly by address. Over a full hour the rates are far lower: 2.30 inches per hour at 10-year and 3.53 at 100-year. That gap is why a monthly rain total tells you little about gutter size.
The method, in four steps
- Find the plan area. Length of the eave × the horizontal distance from eave to ridge. Measure the footprint, not the slope.
- Apply a pitch factor. The copper handbook and This Old House use the table below.
- Divide the gutter's 1-inch figure by the rain rate. That's the area one run can serve. Pitching the gutter 1/8 inch per foot multiplies it by 1.4, and doubling the downspouts doubles it, per Fine Homebuilding.
- Size the downspout. At 1 inch per hour, each square inch of downspout drains 1,200 square feet, and doubling the rain rate halves that, per the copper handbook.
| Roof pitch | Factor |
|---|---|
| Flat to 3-in-12 | 1.00 |
| 4 or 5-in-12 | 1.05 |
| 6 to 8-in-12 | 1.10 |
| 9 to 11-in-12 | 1.20 |
| 12-in-12 and steeper | 1.30 |
Worked Huntsville example
Assumptions
One rear eave 40 feet long, 15 feet horizontally from eave to ridge, 6-in-12 pitch, one downspout at one end, K-style gutter. This is for teaching, not a design for your house.
- Plan area: 40 × 15 = 600 square feet.
- Design area: 600 × 1.10 = 660 square feet.
- Rain: 8.20 inches per hour (10-year) and 12.8 inches per hour (100-year).
| Gutter | 10-year, level | 10-year, pitched | 100-year, level | 100-year, pitched |
|---|---|---|---|---|
| 5-inch K | 673 | 942 | 431 | 604 |
| 6-inch K | 971 | 1,359 | 622 | 871 |
Against 660 square feet: a level 5-inch barely clears the 10-year rate, and it misses the 100-year rate even when pitched. A 6-inch clears the 10-year rate level and the 100-year rate pitched. A second downspout doubles every figure in the table.
One practical note: pitched here means 1/8 inch per foot, which is 5 inches of fall over a 40-foot run. An editor at Green Building Advisor notes that sloped gutters can open a gap under the drip edge, so ask how much slope your eave can really take.
The plain takeaway: at Huntsville's design rain rates, the answer turns on the length of each run and the number of downspouts, not just on the size of the house. Ask how each run was sized and how many outlets it has.
The downspout in that example
At 8.20 inches per hour: 1,200 ÷ 8.20 = 146 square feet per square inch, so 660 square feet needs 4.5 square inches. At 12.8: 94 square feet per square inch, so it needs 7.0. The common rule of thumb in the Department of Energy's Building America guide and This Old House, 1 square inch per 100 square feet, gives 6.6, which is the same rule at 12 inches per hour.
A 2x3 downspout nominally has 6 square inches and a 3x4 has 12, and the actual area of a corrugated or rounded downspout may be less. The copper handbook's minimum is 7 square inches, except for small areas. So a 2x3 passes at the 10-year rate and falls short at the 100-year rate and of the handbook's minimum.
Run your own eave
Where the sources disagree
- How much more a 6-inch holds. Angi says 50% more. An Ohio exterior company's article says 67%, based on gallons held per foot. This Old House's table implies about 44%. This page uses the table, so it follows the 44% end.
- Steep roofs. A 2015 Fine Homebuilding article says to multiply by 0.85 above 5-in-12, which doesn't match the pitch table. This page uses the table.
- Slope. Sources range from 1/16 inch per foot to 1/2 inch per 10 feet, and forum discussion goes as far as hanging them flat. No single number is the rule.
- SMACNA. The manual's tables weren't accessed and its calculator wasn't run for this page, so these results aren't SMACNA's.
What this math misses
Valleys that dump on one spot, upper roofs that drain to lower ones, and elbows aren't covered. A 2015 Fine Homebuilding article names roofs that concentrate flows among common gutter mistakes, along with defaulting to 5-inch and using too few downspouts. If your house has a valley ending right over a run, the plain area figure understates what that spot receives.
When you don't need this calculator
- Your gutters overflow only when they're full of leaves. Start with cleaning.
- You've got a leak, not an overflow. See repair.
- You want a design you can rely on. This page teaches the idea. Ask the contractor how they sized each run, and ask to see the numbers.
Permits
Gutter size isn't a permit question on any page read for this site, and those pages don't say whether gutter work needs a permit. Call the office for your address: Huntsville Inspection at 256-427-5331, Madison Building at 256-772-5644, or Madison County Inspection at 256-746-2950.
Questions homeowners ask
Do I need the charts, or should I just buy 5-inch?
Defaulting to 5-inch is the mistake Fine Homebuilding's 2015 article warns about. It may well work on a short run. Run the numbers for your longest run first.
Can a 5-inch gutter handle 5,520 square feet of roof?
Not at Huntsville's rain rates. That figure applies at 1 inch per hour. At 8.20 inches per hour, the same gutter, level with one outlet, serves 673 square feet.
Should I size for the 10-year or the 100-year rate?
SMACNA's calculator uses either one. The 100-year rate leaves more margin. Ask your contractor which they used, and why.
Is one downspout enough on a long run?
Doubling the downspouts doubles the area a run can serve, and spacing guidance varies by source. See downspouts and drainage.
Why is pitch in the math at all?
The published tables raise the design area as pitch rises, so a steep roof counts for more than its footprint alone. Check your pitch before picking a row.
How this site helps: your request goes to one independent local contractor, who sizes and prices the actual job. This site does no gutter work.
Related guides
Sources
- NOAA Atlas 14, Vol. 9 v2, at latitude 34.7304, longitude -86.5861. The raw file doesn't label its columns, so this table follows NOAA's standard layout.
- Fine Homebuilding, sizing method, undated; common gutter blunders, 2015.
- Copper trade handbook, gutters and downspouts.
- This Old House, how to size gutters, and gutter sizes, both updated April 20, 2026.
- Angi, seamless gutters, "2025 Data"; standard sizes, undated.
- Building America Solution Center, updated September 26, 2021.
- SMACNA calculator page.
- Green Building Advisor, pitch for gutters, March 2018.
- An Ohio exterior company's article on 5-inch and 6-inch gutters, modified April 1, 2026.