Key takeaways
- Flow in gallons per minute = rainfall in inches per hour × area × 0.0104.
- Your choice of rainfall figure can nearly triple the answer for the same roof.
- The code table doesn't list K-style gutters, the common shape, so ask the maker for capacity.
Enter your roof area measured level, not along the slope, and a rainfall figure. Then choose a downspout size to see how many you'd need.
How the calculator works
Gallons per minute = rainfall × area × 0.0104
This is Equation 11-1 in the 2021 International Plumbing Code (IPC). A wall that sheds water onto the roof adds half its area. We compare the result with the IPC's tables:
| Downspout | Capacity |
|---|---|
| 2 in round or 2 × 2 in | 30 gpm |
| 3 in round or 2 × 4 in | 92 gpm |
| 4 in round or 3 × 4 1/4 in | 192 gpm |
| Semicircular gutter, 1/8 in per foot | Capacity |
|---|---|
| 4 in | 39 gpm |
| 5 in | 74 gpm |
| 6 in | 110 gpm |
| 8 in | 247 gpm |
| 10 in | 472 gpm |
The rainfall choice changes everything
Gutter guides rarely explain this. For 1,000 square feet in Washington, DC, using NOAA's rainfall data:
| Rainfall basis | Rainfall | Flow | Smallest semicircular gutter |
|---|---|---|---|
| 100-year, 60-minute storm | 3.18 in/hr | 33.1 gpm | 4 in |
| 10-year, 5-minute storm | 6.77 in/hr | 70.4 gpm | 5 in |
| 100-year, 5-minute storm | 9.00 in/hr | 93.6 gpm | 6 in |
Same roof, nearly three times the flow. Plumbing tables often work from an hourly rate, while sheet metal guidance such as SMACNA's calculator uses 5-minute storms, which suits how quickly a short gutter fills. We read NOAA's unlabeled data file, so check your location's figures directly.
Finding your rainfall
- Open NOAA's Precipitation Frequency Data Server and choose your location.
- Choose intensity in inches per hour.
- Read the storm duration and return period you want to design for.
NOAA's coverage isn't complete. A query we tried for Portland, Oregon returned no data, and NOAA says a new atlas is on the way. Check with your local building department if you're stuck.
K-style gutters
The IPC table doesn't list K-style gutters, the common US shape. The only K-style figure we found is from a 1999 magazine article, so we don't give a size for them. Use the flow figure above, and ask the gutter maker or installer for their product's capacity at your slope. We also couldn't confirm that local codes apply these IPC tables to one- and two-family homes.
Water at the ground
The 2021 IRC Section R401.3 calls for grade to fall at least 6 inches within 10 feet of foundation walls, or drains or swales where barriers prevent that. Send water to a safe place, and not toward your foundation or a neighbor's property. Drainage law varies by state, so ask locally about any dispute.
Frequently asked questions
What size gutter do I need?
Work out your flow first. For semicircular gutters, compare it with the IPC table. For K-style gutters, ask the maker for capacity at your slope.
How many downspouts do I need?
Divide peak flow by the downspout's capacity, then round up. Long runs and layout can call for more.
Why use level area and not the sloped roof?
Rain falls vertically, so the level area is what the equation uses. Steep roofs and wind can change real conditions, and some guidance adds a pitch factor.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Plumbing Code, Chapter 11
- NOAA: Precipitation Frequency Data Server
- SMACNA: Downspout and gutter sizing calculator
- 2021 International Residential Code, Chapter 4, Section R401.3