What size gutter do I need?

Gutter size depends on roof drainage area and peak rainfall intensity. For most residential homes under 1,000 sq ft of drainage area in moderate rainfall (4 in/hr), 5-inch K-style gutters work. For larger drainage areas or heavy rainfall regions, use 6-inch gutters. This calculator determines exact sizing based on your roof and local weather data.

What is the difference between 5-inch and 6-inch gutters?

5-inch K-style gutters handle drainage areas up to about 1,400 sq ft in moderate rainfall. 6-inch gutters handle up to about 2,500 sq ft. The 6-inch option is worth the small extra cost if your roof drains more than 1,200 sq ft per gutter run or you're in a high-rainfall area.

How many downspouts do I need?

Provide one downspout for every 30–40 linear feet of gutter, with a minimum of one downspout per gutter run. Also provide one downspout per 600–800 sq ft of roof drainage area. Never run a single gutter more than 40 feet between downspouts or the gutter will overflow at the center during heavy rain.

What slope should gutters be installed at?

Gutters should slope toward downspouts at 1/4 inch per 10 feet (or 1/16 inch per foot) of run. This is enough to drain water without being visible from the street. For long runs over 40 feet, crown the gutter at the midpoint and slope both ends toward a downspout.

What material is best for gutters?

Aluminum seamless gutters are the best value — lightweight, rust-proof, available in any color, and long-lasting (20–30 years). Copper is premium and lasts 50+ years but costs 4–8x more. Vinyl is inexpensive but cracks in cold climates. Steel is strong but can rust if paint chips.

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

Roof Drainage Area (sq ft)?

sq ft

Rainfall Intensity?

Gutter Run Length (ft)?

Roof Pitch (rise:12)?

: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)

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.

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Sources & References

The formulas, code requirements, and reference data used in this calculator are sourced from the following authoritative organizations and publications. Local codes vary — always verify with your jurisdiction's building department.

International Residential Code (IRC) 2024 — Chapter 9: Roof Assemblies — ICC
NRCA Roofing Manual (Steep-Slope & Low-Slope) — National Roofing Contractors Association
Asphalt Shingle Installation Manual — ARMA (Asphalt Roofing Manufacturers Association)
International Plumbing Code (IPC) Chapter 11 — Storm Drainage — ICC
SMACNA Architectural Sheet Metal Manual — SMACNA
Asphalt Roofing Manufacturers Association — Drainage Guidelines — ARMA

Disclaimer: Calculator results are estimates intended for planning and educational purposes only. They do not replace professional engineering analysis, building permit review, or contractor estimates. Always consult a licensed roofing contractor or structural engineer for final specifications.