Quick answer
Size a gutter from the roof area that drains into it and your local 100-year, 1-hour rainfall rate. At 3 inches per hour and 1/16 inch of slope per foot, the 2015 International Plumbing Code lets a 5 inch half-round gutter drain up to 834 sq ft of roof and a 6 inch up to 1,280 sq ft. A typical 40 × 15 ft roof section is 600 sq ft, so 5 inch is enough there, while the same roof in Houston, Miami or New Orleans needs 6 inch.
What decides the size
The International Plumbing Code sizes gutters from three numbers:
- Roof area draining into the gutter, measured flat as seen from above. A steep roof catches no more rain than a flat one with the same footprint.
- Rainfall rate: the heaviest hour of rain you can expect once a century on average, called the 100-year, 1-hour rate. The 2015 code sizes storm drainage on this rate (Section 1106.1).
- Gutter slope, because water moves faster in a steeper gutter.
Table 1106.6 in the 2015 code then gives the largest roof area each gutter size may drain. A heavier rainfall rate means a smaller allowed area, so the same roof can need a bigger gutter in one city than in another.
5 inch vs 6 inch: roof area limits
Largest roof area per half-round gutter at 1/16 inch of slope per foot, the flattest slope in the table:
| Rainfall rate | 5 inch gutter | 6 inch gutter |
|---|---|---|
| 1 in/hr | 2,500 sq ft | 3,840 sq ft |
| 2 in/hr | 1,250 sq ft | 1,920 sq ft |
| 3 in/hr | 834 sq ft | 1,280 sq ft |
| 4 in/hr | 625 sq ft | 960 sq ft |
| 5 in/hr | 500 sq ft | 768 sq ft |
| 6 in/hr | 416 sq ft | 640 sq ft |
A 6 inch gutter drains about half again as much roof as a 5 inch at every rate. Hanging it steeper helps too: at ⅛ inch per foot and 3 inches per hour, a 5 inch gutter takes 1,172 sq ft instead of 834.
Which size for your city
This table gives NOAA's 100-year, 1-hour rainfall for 16 cities and the most roof one gutter may drain in each. The code table lists whole-inch rates. For a rate like 3.08, we use the 4 inch column, which gives the smaller, safer area. The last column is the smallest size the code table allows for a 600 sq ft roof section: a 40 ft eave with 15 ft from the eave edge to the ridge, measured flat.
| City | 100-year, 1-hour rain | 5 inch gutter drains up to | 6 inch gutter drains up to | Smallest size for 600 sq ft |
|---|---|---|---|---|
| Los Angeles, CA | 1.63 in/hr | 1,250 sq ft | 1,920 sq ft | 4 inch |
| Phoenix, AZ | 2 in/hr | 1,250 sq ft | 1,920 sq ft | 4 inch |
| Denver, CO | 2.26 in/hr | 834 sq ft | 1,280 sq ft | 5 inch |
| Boston, MA | 2.62 in/hr | 834 sq ft | 1,280 sq ft | 5 inch |
| New York, NY | 2.89 in/hr | 834 sq ft | 1,280 sq ft | 5 inch |
| Philadelphia, PA | 2.92 in/hr | 834 sq ft | 1,280 sq ft | 5 inch |
| Chicago, IL | 3.08 in/hr | 625 sq ft | 960 sq ft | 5 inch |
| Nashville, TN | 3.13 in/hr | 625 sq ft | 960 sq ft | 5 inch |
| St. Louis, MO | 3.13 in/hr | 625 sq ft | 960 sq ft | 5 inch |
| Charlotte, NC | 3.19 in/hr | 625 sq ft | 960 sq ft | 5 inch |
| Atlanta, GA | 3.25 in/hr | 625 sq ft | 960 sq ft | 5 inch |
| Minneapolis, MN | 3.58 in/hr | 625 sq ft | 960 sq ft | 5 inch |
| Dallas, TX | 3.69 in/hr | 625 sq ft | 960 sq ft | 5 inch |
| Houston, TX | 4.85 in/hr | 500 sq ft | 768 sq ft | 6 inch |
| Miami, FL | 5.22 in/hr | 416 sq ft | 640 sq ft | 6 inch |
| New Orleans, LA | 5.42 in/hr | 416 sq ft | 640 sq ft | 6 inch |
Rainfall changes over short distances, so look up your own address rather than the nearest city. On NOAA's Precipitation Frequency Data Server, click your location and read the 60-minute row in the 100-year column. That figure in inches is your rate in inches per hour. Then enter it in the gutter size calculator.
In Los Angeles and Phoenix the code table even allows a 4 inch gutter on this roof section. Where the pick is close to the limit, as in Miami and New Orleans, a larger roof section tips it to the next size up.
How to measure the roof area
Each gutter only carries the roof that drains into it. For a simple gable roof, that is one side: the eave length times the flat distance from the eave edge to the ridge. Include the overhangs. A 40 × 28 ft house with 1 ft overhangs has a 42 ft eave and 15 ft from the eave edge to the ridge, so each gutter drains 42 × 15 = 630 sq ft. Measure from the ground or from a plan; the pitch does not change the number.
Add roof that spills onto this one. Where an upper roof drains onto a lower roof, the lower gutter carries both. For downspouts and drains, the code also adds half the area of any wall that sheds rain onto the roof (Section 1106.4), such as a second-story wall above a porch roof.
Don't forget the downspouts
A big gutter with too few downspouts still overflows. At 3 inches per hour, the 2015 code allows 1,840 sq ft of roof per 2 × 3 inch downspout and 4,400 sq ft per 3 × 4 inch downspout. Divide the roof area by the number of downspouts on the run to check each one.
What the code table doesn't cover
- K-style gutters. The 2015 table rates half-round gutters only. If you're buying K-style, use the table as a guide and check the maker's capacity rating.
- Newer code editions. The 2024 code rates gutters by flow in gallons per minute instead of roof area. The gutter size calculator page compares the two; the 2015 table calls for the larger gutter. Your building department can tell you which edition it uses.
Frequently asked questions
Are 5 inch gutters big enough?
For most roof sections outside the heaviest-rain areas, yes. At 3 inches per hour, the plumbing code lets a 5 inch half-round gutter drain up to 834 sq ft of roof at 1/16 inch of slope per foot. Where the rate is over 4 inches per hour, as in Houston, Miami and New Orleans, a 5 inch gutter is limited to 500 sq ft or less (416 sq ft above 5 inches per hour), so a 6 inch is often needed.
When do I need 6 inch gutters?
When the roof area draining into one gutter is more than a 5 inch gutter is allowed at your rainfall rate. That happens with large or long roof sections, with an upper roof draining onto a lower one, and in places with intense storms such as Houston, Miami and New Orleans.
How do I find my rainfall rate for gutters?
Use NOAA's Precipitation Frequency Data Server. Click your location and read the 60-minute row in the 100-year column. The 2015 International Plumbing Code sizes gutters on that 100-year, 1-hour rate.
Do 6 inch gutters need bigger downspouts?
Not automatically. Downspout size depends on the roof area each one drains and your rainfall rate, not on the gutter width. At 3 inches per hour, the 2015 code tables allow 1,840 sq ft of roof per 2 × 3 inch downspout and 4,400 sq ft per 3 × 4 inch downspout. If the gutter drains more than one downspout can carry, use the larger downspout or add another.
Calculators for this
Gutter Size
Do you need 5 inch or 6 inch gutters?
Gutter
Enter your eave lengths to get 10 ft gutter sections, downspouts, hangers, end caps and elbows, plus the right slope for each gutter run.
Roofing Shingle
Estimate roof area from your house footprint and pitch, then get squares, shingle bundles, nails, underlayment, drip edge and cost.
Published 2026-10-31. Sources: International Plumbing Code 2015, Chapter 11: Section 1106, Tables 1106.2(2) and 1106.6; NOAA Atlas 14, Precipitation Frequency Data Server. See how we calculate.