What Size Gutters Do I Need?
Free gutter sizing calculator using NRCA contractor standards. Get the right size (5", 6", or 7") for your home based on roof footprint, pitch, regional rainfall, and tree coverage.
Use the free sizing calculator
Gutter Sizing Calculator
Get the recommended gutter size (5", 6", or 7") for your home based on roof square footage, pitch, regional rainfall intensity, and tree coverage. Built using NRCA and SMACNA contractor sizing standards.
The math behind the recommendation
Capacity comparison: 5" vs 6" vs 7" gutters
| Size | Max Capacity | Roof Area Drained | Best For | Cost vs 5" |
|---|---|---|---|---|
| 5" | 11 GPM | ~1,200 sq ft (1"/hr rain) | Standard residential, low-rainfall regions | Baseline |
| 6" | 16 GPM | ~1,750 sq ft (1"/hr rain) | Florida/Gulf Coast, steep roofs, heavy trees | +$2-3/ft |
| 7" | 22 GPM | ~2,400 sq ft (1"/hr rain) | Commercial, extreme rainfall, very large homes | +$5-7/ft |
What size gutters are standard in Florida?
6-inch seamless aluminum is the recommended size for most Florida homes due to peak rainfall intensity of 9 or more inches per hour during summer thunderstorms and hurricanes. Florida homes built before 2010 typically have 5-inch gutters that overflow during heavy rain. Newer construction commonly specs 6-inch standard. About 70% of new Northeast Florida installs are 6-inch.
How to calculate gutter size yourself
The NRCA (National Roofing Contractors Association) formula for sizing gutters:
- Measure roof footprint in square feet (length x width of the area each gutter run drains).
- Multiply by pitch factor to get effective drainage area: 1.0 for flat (1:12 to 4:12), 1.2 for medium (5:12 to 8:12), 1.4 for steep (9:12+).
- Multiply by maximum local rainfall intensity in inches per hour. NWS 5-minute rainfall intensity data: Florida 9", Southeast 7", Midwest/NE 6", Mountain West 5", Southwest 4".
- Divide by 96.23 to get required gallons per minute (GPM).
- Pick smallest gutter that handles the load: 5" (11 GPM), 6" (16 GPM), 7" (22 GPM).
Example: 2,000 sq ft Jacksonville home, medium pitch, moderate trees, 2 stories. Effective area = 2,000 x 1.2 = 2,400 sq ft. Required flow = (2,400 x 9) / 96.23 = 224 GPM. Adjusted for trees and stories = 280 GPM. Split across 5 downspout zones = 56 GPM per zone. Recommendation: 6-inch gutters with 5 strategically placed 3x4 downspouts.
How many downspouts do you need?
Rule of thumb: one downspout per 30 to 40 feet of gutter run, with a maximum of 50 feet between downspouts. A typical 150-foot residential gutter system needs 4 to 5 downspouts. Florida and heavy-rainfall regions benefit from one per 30 feet to handle peak flow.
Downspout sizing matters too: 2x3 inch handles 600 sq ft of roof; 3x4 inch handles 1,200 sq ft. For 6-inch gutters, always pair with 3x4 downspouts.
When undersized gutters cost you money
Undersized gutters overflow during heavy rain. Overflow water sheets down fascia, soaks into soffits, runs along the foundation, and saturates landscaping. Over time this causes:
- Fascia rot ($800 to $3,000 to repair per side of house)
- Soffit damage and pest entry ($600 to $2,500)
- Foundation cracking from soil saturation ($4,500 to $25,000)
- Basement or crawlspace moisture and mold ($2,000 to $15,000)
- Landscape erosion and mulch washout (recurring)
Upgrading from 5-inch to 6-inch on a 150-foot home costs about $300 to $450. The break-even on damage prevention is typically 18 months for at-risk Florida homes.
Common questions about gutter sizing
What size gutters do I need?
Most U.S. homes need 5-inch K-style aluminum (11 GPM capacity). Florida, Gulf Coast, steep roofs, large catchment areas (2,000+ sq ft), or heavy tree coverage need 6-inch (16 GPM). 7-inch is oversized for commercial or extreme rainfall.
What is the difference between 5 inch and 6 inch gutters?
5-inch K-style: 11 GPM, paired with 2x3 downspouts. 6-inch K-style: 16 GPM, paired with 3x4 downspouts. 6-inch carries 45% more water and clogs less often. Cost difference is $2-3 per linear foot installed.
How do I calculate gutter size?
Required GPM = (roof sq ft x pitch factor x rainfall intensity in inches per hour) / 96.23. Match GPM to gutter capacity: 5" handles 11 GPM, 6" handles 16, 7" handles 22.
Are 6 inch gutters worth the extra cost?
Yes for Florida and heavy-rainfall regions, homes with steep roofs or large catchment areas, homes with significant tree coverage, and homes that have overflowed during storms. Cost difference is small ($300-450 on a 150-foot home).
What size gutters are standard in Florida?
6-inch seamless aluminum is the recommended standard for most Florida homes due to 9+ inch per hour peak rainfall during summer thunderstorms and hurricanes. Pre-2010 Florida homes commonly have 5-inch that overflow.
What is the maximum capacity of a 5 inch gutter?
11 gallons per minute under ideal conditions (NRCA standard). Capacity drops 30-50% with debris, improper slope, or undersized downspouts. 5" drains roughly 1,200 sq ft at 1 inch per hour rainfall.
How many downspouts does my house need?
One per 30-40 feet of gutter run, max 50 feet between downspouts. A 150-foot system needs 4-5 downspouts. Florida and heavy-rain regions benefit from one per 30 feet.
Related guides and tools
National gutter cost calculator Jacksonville gutter cost calculator Foundation drainage cost calculator Seamless aluminum gutters Gutter cleaning and repair LeafBlaster Pro gutter guards Hurricane prep checklist Free on-site sizing assessmentFree On-Site Sizing and Quote
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