Understanding your result
The headline is the number of downspouts for the run, with the downspout size and whether the gutter is big enough. The stats show the design area, the gutter’s load as a percentage of its capacity, the area each downspout drains and the spacing. The breakdown applies the pitch allowance, checks the gutter, and counts downspouts by capacity and by spacing.
The two numbers to look at are the gutter load (over 100 percent means a bigger gutter or more downspouts) and which rule set the downspout count. If spacing set it, the downspouts have spare capacity; if capacity set it, a bigger downspout may halve the count.
The run length and the parts list come from the gutter length calculator; the slope toward each downspout from the gutter slope calculator; and what to do with the water at the bottom from the downspout extensions calculator and the rain barrel calculator.
How we calculate this
The gutter capacities are the figures for level K-style gutters widely published from the SMACNA method, expressed at 1 inch per hour so they can be scaled to any rainfall: a gutter that carries the runoff from 5,520 square feet at 1 inch per hour carries a sixth of it at 6 inches per hour. The downspout rule of 100 square feet per square inch is the gutter trade’s rule of thumb for a typical design storm; it is an input, so it can be lowered where storms are intense.
The roof area is the plan area draining to the run, not the sloped area: rain falls vertically, and the pitch allowance handles the wind. The gutter and downspout sizing table sets the capacities out by rainfall.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Gutter capacity at 1 in/hr | 5,520 sq ft (5 in K), 7,960 (6 in K), 2,500 (5 in half-round), 3,840 (6 in half-round) | Capacity figures for level gutters as published from the SMACNA Architectural Sheet Metal Manual method |
| Pitch allowance | 1.00 / 1.05 / 1.10 / 1.20 / 1.30 | SMACNA roof pitch factors |
| Rainfall intensity | 6 in/hr | A mid-range 5-minute, 10-year intensity; take yours from NOAA Atlas 14 |
| Roof area per sq in of downspout | 100 sq ft | Gutter trade rule of thumb (2 × 3 ≈ 600 sq ft, 3 × 4 ≈ 1,200 sq ft) |
| Maximum spacing | 40 ft of gutter per downspout | Common installer practice (30 to 40 ft) |
Assumptions last reviewed October 7, 2026.
The calculator does not model gutter outlets smaller than the downspout, valleys that dump concentrated water at one point, or debris; a valley discharging onto a run is worth an extra downspout near it. The guide to gutter sizing by roof area walks through the method for a whole house.
Two worked examples
A 1,000 square foot roof face on a 40-foot run
Plan area 1,000 square feet at 6/12, 6 inches per hour, 5-inch K-style, 2 × 3 downspouts, 40-foot run.
- Design area: 1,000 × 1.10 = 1,100 sq ft
- Gutter capacity: 5,520 ÷ 6 = 920 sq ft: too small (120 percent)
- Downspouts: by capacity 1,100 ÷ 600 = 1.83, so 2; by spacing 1; 2 downspouts
Two downspouts at the ends with the high point in the middle halve the water each half carries, which brings each half within the 5-inch gutter’s capacity; a 6-inch gutter is the cleaner fix.
A 6-inch gutter with 3 × 4 downspouts in heavy rain
Plan area 700 square feet at 6/12, 8 inches per hour, 6-inch K-style, 3 × 4 downspouts, 60-foot run.
- Design area: 700 × 1.10 = 770 sq ft
- Gutter capacity: 7,960 ÷ 8 = 995 sq ft: adequate (77 percent)
- Downspouts: by capacity 770 ÷ 1,200 = 0.64, so 1; by spacing 60 ÷ 40 = 2; 2 downspouts
Spacing set the count here, not capacity: a 60-foot run needs a downspout at each end whatever the roof.
Where to find your inputs
Roof area draining to the run. The plan area of the roof face above the gutter, overhang included; on a gable roof, half the footprint per side.
Pitch. From the pitch calculator.
Rainfall intensity. From NOAA Atlas 14 for your location: 5-minute duration, 10-year return period, in inches per hour.
Gutter and downspout. From the existing system or the quote.
Common mistakes
- Sizing by the house, not the run. Each run carries only the roof face above it.
- Ignoring the pitch. Steep roofs catch more rain.
- One downspout on a long run. Spacing matters as much as capacity.
- Valleys over a run. A valley concentrates water; put a downspout near it.
- Small outlets. A 3 × 4 downspout on a 2 × 3 outlet carries only what the outlet passes.
- Discharging at the foundation. Carry the water 4 to 6 feet away.
Questions people ask
- How many downspouts does my house need?
- At least one per gutter run, one per 30 to 40 feet of gutter, and enough capacity for the roof area: by the trade rule of about 100 square feet of roof per square inch of downspout, a 2 × 3 inch downspout (6 square inches) drains about 600 square feet and a 3 × 4 (12 square inches) about 1,200. A 1,000 square foot roof face at 6/12 needs two 2 × 3s or one 3 × 4.
- What size gutter do I need?
- Compare the design roof area with the gutter's capacity at your rainfall. Level K-style gutters carry about 5,520 square feet of roof at 1 inch per hour (5-inch) and 7,960 (6-inch); divide by your rainfall intensity. At 6 inches per hour a 5-inch gutter handles 920 square feet and a 6-inch 1,327.
- What is the pitch allowance?
- Steep roofs catch more wind-driven rain than their plan area suggests, so the sheet-metal industry's method (SMACNA) multiplies the plan area by 1.00 up to 3/12, 1.05 at 4/12 and 5/12, 1.10 from 6/12 to 8/12, 1.20 from 9/12 to 11/12 and 1.30 at 12/12 and steeper.
- Where do I find my rainfall intensity?
- NOAA Atlas 14 gives precipitation frequency estimates for any US location; gutter design uses the 5-minute duration at a 10-year return period, converted to inches per hour. Plumbing codes use the 60-minute, 100-year figure for roof drains; it is a different design storm.
- Should I use 2 × 3 or 3 × 4 downspouts?
- 3 × 4 downspouts carry twice the water and clog less with leaves, at slightly more cost; they need a gutter outlet to match, which 5-inch gutters can take. Where a run would need three or more 2 × 3s, two 3 × 4s are usually the better choice.
- Where should downspouts go?
- At the ends of runs or at the low point of each slope, with the gutter sloping toward them; on long runs, one at each end with the high point in the middle halves the distance water travels. Keep them away from walkways and discharge 4 to 6 feet from the foundation.