Gutter Calculator
Size gutters and downspouts by roof area, pitch, and rainfall intensity using the SMACNA method, and get a gutter, downspout, and hanger material takeoff.
Horizontal area draining to this gutter run.
Steeper roofs add effective drainage area (wind-driven rain).
Check NOAA for your local 100-yr, 5-min design intensity. Default 6 in/hr (moderate-heavy).
Total length of this gutter run, for material takeoff.
For corner-piece takeoff.
Gutter & downspout sizing
~every 20 ft of run
8 × 10 ft sections
- Hangers
- 32hangers
- Corners
- 4
- Effective drainage area
- 2,200sq ft
- Rainfall intensity
- 6 in/hr
5" K-style covers ~95% of residential installs. Step up to 6" for large roof areas, steep pitches, or heavy-rain regions.
Downspout sizing matters: 2×3 pairs with 5" gutters; 3×4 pairs with 6" and handles much more water.
Add downspouts (don't just upsize gutters) when one run can't keep up — more outlets drain faster.
Check NOAA's rainfall intensity for your area (100-yr, 5-min duration) for code-accurate sizing.
This uses a simplified SMACNA rainfall-intensity method — verify against the full SMACNA tables, your local code, and the authority having jurisdiction.
How this calculator works
This gutter calculator does two jobs: it sizes the gutter and downspouts for a roof using the SMACNA rainfall-intensity method, and it gives you a material takeoff — gutter linear feet, downspout count, and hangers. It is intentionally ungated: no email, no PDF, just the sizing and the reasoning behind it.
How sizing works
Start with the roof area draining to the run. A pitch factor scales it up for wind-driven rain on steeper roofs, producing the effective drainage area. That area is then adjusted for your local rainfall intensity — double the intensity and you effectively double the water a gutter must carry. The result is compared against typical single-run capacities to recommend 5-inch or 6-inch K-style, and the matching 2×3 or 3×4 downspout.
Add outlets, don't just upsize
When one run cannot keep up, the better fix is usually more downspouts, not a bigger gutter. More outlets drain the gutter faster and keep water from backing up mid-run. The calculator sizes downspout count by both drainage demand and a 30–40 foot spacing cap, and it warns you when even a 6-inch run is over capacity — the signal to split the roof into multiple runs or move to an engineered design.
About the numbers
The capacity thresholds here are simplified, planning-grade approximations normalized to a 6 in/hr reference. They are excellent for specifying a residential job quickly, but for code-stamped work you should consult the full SMACNA Architectural Sheet Metal Manual tables for your gutter slope and the local design intensity, and verify against your authority having jurisdiction.
Frequently asked questions
- What size gutters do I need?
- For most homes, 5-inch K-style gutters handle the job — they cover about 95% of residential installs. Step up to 6-inch K-style when the roof draining to one run is large, the pitch is steep, or you are in a heavy-rain region. The calculator decides by combining your drainage area, a pitch factor, and the rainfall intensity, then comparing against typical single-run capacities.
- How many downspouts do I need?
- A rough rule is one downspout per 600–1,200 sq ft of roof depending on downspout size and rainfall, but spacing also matters: aim for an outlet at least every 30–40 feet of gutter run. The calculator takes the larger of the two requirements — drainage demand and spacing — so a long run gets enough outlets even if the area alone would not require them.
- What's the difference between 5-inch and 6-inch gutters?
- A 6-inch K-style gutter holds noticeably more water than a 5-inch and, importantly, pairs with a larger 3×4 downspout instead of a 2×3. The bigger downspout is where most of the extra capacity comes from — a 3×4 drains roughly twice the area of a 2×3. Choose 6-inch for big or steep roofs and heavy-rain climates.
- How do I size gutters for heavy rain?
- Rainfall intensity drives the whole calculation. A roof that is fine with 5-inch gutters at 4 in/hr may need 6-inch at 8 in/hr, because the water volume scales directly with intensity. Look up your local design intensity from NOAA (the 100-year, 5-minute value) and enter it — do not assume a national average.
- How far apart should downspouts be?
- Keep outlets within about 30–40 feet of each other along the gutter run. Water has to travel to a downspout, and a long run between outlets overwhelms the gutter in the middle even if the total downspout count looks adequate. The calculator caps spacing and tells you the resulting interval.
- Does roof pitch affect gutter size?
- Yes. A steeper roof catches more wind-driven rain and effectively delivers more water to the gutter than its flat footprint suggests. The SMACNA method applies a pitch adjustment factor — roughly 1.0 for low slopes up to about 1.2 for steep roofs — which this calculator includes when you pick your pitch range.