Roof area per downspout = gutter capacity ÷ rainfall intensity. A 5 in K-style gutter carries the runoff of 5,520 sq ft of roof in a 1 in/h rain, so at 6.78 in/h (Washington, DC) it serves 814 sq ft between downspouts, and a 6 in K-style gutter serves 1,174 sq ft. A 40 ft run under a 16 ft deep roof at 6/12 has 704 sq ft to drain and needs 2 downspouts of the 2 × 3 in size.
How the gutter size is calculated
Gutters are sized by the roof area they drain, the pitch of that roof, and the heaviest rain you should expect. Fine Homebuilding's method has three steps: multiply the roof's level area by a pitch factor to get the watershed, find the local rainfall intensity, and divide the gutter's capacity at 1 in/h of rain by that intensity to get the most roof one downspout can serve.
Drainage area = roof plan area × pitch factor · Roof per downspout = gutter capacity ÷ rainfall intensity (capped by the downspout's own limit) · Downspouts = drainage area ÷ that, rounded up
Worked example with the defaults: 40 ft × 16 ft of level roof is 640 sq ft. A 6/12 roof has a pitch factor of 1.1, so the drainage area is 704 sq ft. In Washington, DC the 5-minute, 10-year rainfall is 6.78 in/h (NOAA Atlas 14). A 5 in K-style gutter reaches 5,520 ÷ 6.78 = 814 sq ft, but a 2 × 3 in downspout drains only 600 sq ft, so 704 ÷ 600 = 1.17 rounds up to 2 downspouts. A 3 × 4 in downspout drains 1,200 sq ft, so one downspout would do the job and the gutter limit of 814 sq ft governs.
Gutter size chart: 5 in or 6 in
These are the published capacities at 1 in/h of rain, and what each serves at a few rainfall intensities:
| Gutter | At 1 in/h | 4 in/h | 6 in/h | 8 in/h | 10 in/h |
|---|---|---|---|---|---|
| 5 in K-style | 5,520 sq ft | 1,380 sq ft | 920 sq ft | 690 sq ft | 552 sq ft |
| 6 in K-style | 7,960 sq ft | 1,990 sq ft | 1,327 sq ft | 995 sq ft | 796 sq ft |
| 5 in half-round | 2,500 sq ft | 625 sq ft | 417 sq ft | 313 sq ft | 250 sq ft |
| 6 in half-round | 3,840 sq ft | 960 sq ft | 640 sq ft | 480 sq ft | 384 sq ft |
Capacities for 5 and 6 in K-style come from Fine Homebuilding and This Old House, which also give the half-round figures. For a small roof in a moderate climate a 5 in gutter with enough downspouts is enough. A 6 in gutter takes more water between downspouts, which suits long runs, big roof planes and heavy-rain regions. Compare the downspout count in the table above before paying for the larger size: sometimes one more downspout does the same job.
Downspout size and count
This Old House puts the rule of thumb at about 1 sq in of downspout cross-section for every 100 sq ft of roof. That gives 600 sq ft for a 2 × 3 in downspout, 1,200 sq ft for 3 × 4 in, 706 sq ft for 3 in round and 1,255 sq ft for 4 in round. Runs of 60 ft or more should have at least two outlets, and the calculator never returns fewer than that. Put downspouts at the low end of each run, and see how to set the fall in the gutter slope calculator.
Rainfall intensity by city or ZIP code
The rain that matters is a short, hard burst, not the yearly total. The calculator uses the 5-minute rainfall expected once in 10 years, from NOAA Atlas 14, for 36 US cities, in in/h: Denver 5.06, Chicago 7.43, Atlanta 7.76, New York 7.88, Dallas 8.40, Houston 10.10, Miami 10.80. Pick the closest city, or read your own ZIP code's value from NOAA's Precipitation Frequency Data Server and type it into the intensity field: it overrides the city. Places outside the Atlas 14 volumes, including the Pacific Northwest, are not in the list.
What to work out next
The gutters hang on the fascia beside the soffit: the soffit calculator counts panels and channel for the same eaves. The roof plane area comes from the roof square calculator, and the water needs somewhere to go: a low spot near the foundation may call for the sump pump size calculator. Replacing the whole roof? See the roof replacement cost calculator, and find every roofing tool under roofing and siding.