Gutter Size Calculator
Enter your roof's drainage area and local peak rainfall intensity to determine whether you need 5-inch or 6-inch gutters and how many downspouts are required.
Roof & Drainage Inputs
sq ft
ft
:12
Recommended Gutter Size
5" K-Style (most common residential)
Design Flow49.1 GPM
Gutter Capacity56 GPM
Downspouts Needed2
Downspout Spacing20.0 ft apart
Slope Correction1.05× (pitch 6:12)
Effective Drain Area1,575 sq ft
Gutter Size Options
4" K-Style28 GPM ✗
5" K-Style (most common residential)RECOMMENDED56 GPM ✓
6" K-Style (large roofs)87 GPM ✓
7" K-Style (commercial)124 GPM ✓
5" Half-Round30 GPM ✗
6" Half-Round60 GPM ✓
Downspout Recommendations
Quantity needed2 downspouts
Recommended size3"×4" rectangular (46 GPM each)
Max spacing20 ft (every 20 ft)
Gutter slope1/16" per foot minimum, 1/8" preferred
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How to Use This Calculator
Gutter sizing is determined by two factors: how much roof area drains to each gutter run, and how hard it rains in your area:
- Drainage Area per Gutter Run — The roof area (sq ft) that drains to each individual gutter. For a gable roof, this is typically half the total roof area. A dormer or large addition may create a separate drainage zone.
- Roof Pitch — Steeper roofs concentrate rainfall faster, so add a pitch adjustment factor: multiply drainage area by 1.05 for 6/12 pitch, 1.10 for 8/12, 1.20 for 12/12.
- Peak Rainfall Intensity — The 5-minute peak rain rate in inches per hour for your location. US averages: Pacific Northwest 2–3 in/hr, Midwest 4 in/hr, Southeast 6–8 in/hr. Check NOAA's Precipitation Frequency Atlas for your zip code.
The calculator outputs recommended gutter width, downspout size, and minimum number of downspouts per run.
The Formula
Adjusted Area = Drainage Area × Pitch Factor
Required Flow = Adjusted Area × Rainfall Intensity / 96 (gallons/min)
K-Style Gutter Capacity (gal/min at 1/16" slope):
4-inch: 20 gpm
5-inch: 32 gpm
6-inch: 52 gpm
Downspout Capacity:
2×3-inch rectangular: 17 gpm each
3×4-inch rectangular: 28 gpm each
4-inch round: 31 gpm each
Downspouts Needed = Required Flow ÷ Downspout Capacity (round up)
Required Flow = Adjusted Area × Rainfall Intensity / 96 (gallons/min)
K-Style Gutter Capacity (gal/min at 1/16" slope):
4-inch: 20 gpm
5-inch: 32 gpm
6-inch: 52 gpm
Downspout Capacity:
2×3-inch rectangular: 17 gpm each
3×4-inch rectangular: 28 gpm each
4-inch round: 31 gpm each
Downspouts Needed = Required Flow ÷ Downspout Capacity (round up)
Example: 1,200 sq ft Ranch in the Southeast
Example: 2,400 sq ft total roof, two sides draining, moderate 5/12 pitch, Southeast (6 in/hr rainfall)
Each Side Drainage Area1,200 sq ft
Pitch Factor (5/12)1.05
Adjusted Area1,260 sq ft
Peak Rainfall6 in/hr
Required Flow1,260 × 6 / 96 = 78.75 gpm
Gutter Size Needed6-inch (52 gpm per run — need 2 runs or oversized)
Downspouts Needed3 per side (3 × 28 = 84 gpm with 3×4 spouts)
In high-rainfall areas, undersized 5-inch gutters overflow during heavy downpours. For Southeast homes with large roof areas, 6-inch gutters and multiple downspouts prevent water damage at the foundation.
Frequently Asked Questions
6-inch gutters are better for roofs over 1,200 sq ft per side or in areas with heavy rainfall. They handle 60% more flow than 5-inch gutters and clog less frequently. The cost difference is small — about $1–2 per linear foot installed.
Maximum 40 feet per downspout for 5-inch gutters, 50 feet for 6-inch in moderate rainfall. In high-rainfall areas, reduce these maximums by 20–30%. Long runs without adequate downspouts overflow in the middle during storms.
NOAA's Precipitation Frequency Atlas (NOAA Atlas 14) at hdsc.nws.noaa.gov provides 5-minute peak rainfall data for any US location. Use the 100-year, 5-minute return period value for sizing. This calculator includes typical values for major US regions.
Oversized gutters hold debris and standing water more easily, which can accelerate rust and rot. They also look bulky on small homes. Size to your actual drainage area — don't massively oversize just because it seems safer.
Aluminum seamless gutters cost $4–$9 per linear foot installed (5-inch). 6-inch aluminum runs $6–$12 per linear foot. Copper costs $25–$50 per linear foot. For a typical 150-foot home perimeter, budget $600–$1,350 for aluminum 5-inch gutters installed.