Gutter Size Calculator
Measure the part of the roof that drains into one gutter and look up your rainfall rate. You'll get the smallest gutter that can carry it, whether 5" or 6" is enough, and how many downspouts you need.
- Plumbing code tables
- Feet or metric
- Downspout check
2 Gutter size
Updates as you type| 5" gutter | Big enough (up to 834 sq ft) |
|---|---|
| 6" gutter | Big enough (up to 1,280 sq ft) |
| Gutter slope used | 1/16" per foot |
| Roof area per downspout | 600 sq ft with 1 downspout |
| 2 × 3 in rectangular downspout | Big enough (up to 1,840 sq ft each) |
| 3 in round downspout | Big enough (up to 2,930 sq ft each) |
| 3 × 4 in rectangular downspout | Big enough (up to 4,400 sq ft each) |
| 4 in round downspout | Big enough (up to 6,130 sq ft each) |
1/16" per foot is the flattest slope in the code table. If your gutter is hung flatter than that, the table doesn't cover it, so treat this size as the minimum and lean toward the bigger size when it's close.
The code table rates half-round gutters. For K-style gutters, check the maker's capacity rating. Your local code may also use a different edition, so check with your building department.
Quick answer
Size a gutter from the roof area that drains into it and your 100-year, 1-hour rainfall rate. At 3 inches per hour and the flattest slope in the plumbing code table, a 5" half-round gutter handles up to 834 sq ft of roof and a 6" handles up to 1,280 sq ft. A 40 × 15 ft roof section (600 sq ft) needs a 5" gutter.
Smallest gutter by roof area and rainfall
| Roof area | 2 in/hr | 3 in/hr | 4 in/hr | 5 in/hr | 6 in/hr |
|---|---|---|---|---|---|
| 300 sq ft | 3" | 4" | 4" | 5" | 5" |
| 500 sq ft | 4" | 5" | 5" | 5" | 6" |
| 750 sq ft | 5" | 5" | 6" | 6" | 7" |
| 1,000 sq ft | 5" | 6" | 7" | 7" | 8" |
| 1,250 sq ft | 5" | 6" | 7" | 8" | 8" |
| 1,500 sq ft | 6" | 7" | 8" | 8" | 10" |
| 2,000 sq ft | 7" | 8" | 10" | 10" | 10" |
Half-round gutter sizes at 1/16" per foot of slope, from IPC 2015 Table 1106.6. Roof area is the flat footprint draining into one gutter. Use the calculator above for your exact size.
How gutters are sized
The International Plumbing Code sizes gutters from three things:
- Roof area that drains into the gutter, measured flat (the code calls it horizontally projected area). A steep roof catches no more rain than a flat one with the same footprint.
- Rainfall rate: the 1-hour rainfall with a 1-in-100 chance of happening in any year where you live. The 2015 IPC sizes storm drainage on this 100-year hourly rate (Section 1106.1).
- Gutter slope, because water runs faster in a steeper gutter.
Table 1106.6 in the 2015 code lists the largest roof area each half-round gutter can drain. For the example, the area is 40 × 15 = 600 sq ft. At 3 inches per hour and 1/16" per foot of slope, a 5" gutter takes up to 834 sq ft, so a 5" gutter is enough.
Find your rainfall rate
NOAA's Precipitation Frequency Data Server gives the rate for most of the US. Click your location, then read the row for 60-minute duration in the 100-year column. That number in inches is your rate in inches per hour. The code also allows rates from approved local weather data, so check with your building department.
5" vs 6" gutters by rainfall rate
Largest roof area per gutter at 1/16" per foot of slope, from IPC 2015 Table 1106.6:
| Rainfall rate | 4" gutter | 5" gutter | 6" gutter |
|---|---|---|---|
| 1 in/hr | 1,440 sq ft | 2,500 sq ft | 3,840 sq ft |
| 2 in/hr | 720 sq ft | 1,250 sq ft | 1,920 sq ft |
| 3 in/hr | 480 sq ft | 834 sq ft | 1,280 sq ft |
| 4 in/hr | 360 sq ft | 625 sq ft | 960 sq ft |
| 5 in/hr | 288 sq ft | 500 sq ft | 768 sq ft |
| 6 in/hr | 240 sq ft | 416 sq ft | 640 sq ft |
A steeper gutter carries more. At ⅛" per foot, a 5" gutter drains 1,172 sq ft at 3 inches per hour, against 834 sq ft at 1/16". 1/16" per foot is the flattest slope in the table. For a gutter hung that flat or flatter, use this column and treat the size as a minimum.
Downspouts
Each downspout also has a limit. From IPC 2015 Tables 1106.2(1) and 1106.2(2), at 3 inches per hour:
- 2 × 3 in rectangular: up to 1,840 sq ft of roof
- 3 in round: up to 2,930 sq ft of roof
- 3 × 4 in rectangular: up to 4,400 sq ft of roof
- 4 in round: up to 6,130 sq ft of roof
To size each downspout, divide the roof area by the number of downspouts on the run. The gutter calculator counts the sections, hangers and elbows once you know the size.
What the table doesn't cover
- K-style gutters. The 2015 code table rates half-round (semicircular) gutters only. For K-style gutters, check the maker's capacity rating.
- Walls above the roof. For downspouts and drain piping, IPC 1106.4 adds half the area of any wall that sheds rain onto the roof, such as a second-story wall above a porch roof. Adding it for the gutter too is the safe choice.
- Newer code editions. The 2024 IPC rates gutters by flow in gallons per minute instead of roof area, and its figures are higher. At ⅛" per foot it rates a 5" half-round gutter at 74 gpm and a 6" at 110 gpm; the 2015 table works out to about 37 and 57 gpm (3,520 and 5,440 sq ft × 0.0104 gpm per sq ft at 1 inch per hour). This calculator uses the 2015 tables, which call for the larger gutter. Your building department can tell you which edition it enforces.
Frequently asked questions
Do I need 5 inch or 6 inch gutters?
It depends on roof area and rainfall. At 3 inches per hour and 1/16" per foot of slope, the plumbing code lets a 5" half-round gutter drain up to 834 sq ft of roof and a 6" up to 1,280 sq ft. At 4 inches per hour those drop to 625 and 960 sq ft.
What rainfall rate should I use for gutters?
The 100-year, 1-hour rate for your location. The 2015 International Plumbing Code sizes storm drainage on this rate (Section 1106.1). NOAA's Precipitation Frequency Data Server shows it: use the 60-minute row in the 100-year column.
Do I measure the roof along the slope?
No. Gutter tables use the horizontally projected area, the footprint you would see from above. A steep roof sheds the same rain as a flat roof of the same footprint.
How much roof can one downspout handle?
At 3 inches per hour, the plumbing code allows 1,840 sq ft for a 2 × 3 inch downspout and 4,400 sq ft for a 3 × 4 inch one. Halve those at 6 inches per hour.
Are K-style gutters rated in the code?
No. The 2015 IPC gutter table covers half-round gutters only. For K-style gutters, check the maker's capacity rating.
Last reviewed 2026-10-27. See how we calculate for our sources and assumptions.