Gutter Size Calculator

About the Gutter Size Calculator

Choosing the right gutter size depends on the roof area draining into the gutter and the maximum rainfall intensity in your area. Under-sized gutters overflow during heavy rain, causing the same damage as having no gutters at all. Over-sized gutters cost more and may look disproportionate on smaller homes.

This gutter size calculator uses the standard formula: effective roof area multiplied by the maximum rainfall intensity (in inches per hour). A 5-inch K-style gutter handles about 5,520 square feet of effective roof area at 1 inch/hour rainfall. A 6-inch K-style gutter handles about 7,960 square feet.

The effective roof area is larger than the footprint because the roof slope captures wind-driven rain on the vertical surface. The calculator applies a slope adjustment factor to give you the effective area, then recommends the gutter size.

When This Page Helps

A gutter that's too small will overflow in every rainstorm. This calculator uses your roof dimensions and local rainfall intensity to recommend 5-inch or 6-inch gutters backed by engineering data.

How to Use the Inputs

1. Enter the roof drainage length (eave to ridge horizontal distance).
2. Enter the gutter run length (linear feet of gutter collecting the water).
3. Enter the roof pitch for the slope adjustment factor.
4. Enter the local maximum rainfall intensity (inches per hour, typically 2–7).
5. Review the recommended gutter size.

Example Calculation

Tips & Best Practices

• When in doubt, go one size larger — the cost difference is small but the performance improvement is significant.
• Match downspout size to gutter size: 3×4 for 5-inch, 4×5 for 6-inch.
• In areas with rainfall intensity above 6 in/hr, consider additional downspouts rather than larger gutters.
• Half-round gutters hold 20–30% less water than K-style of the same width.
• Gutter guards reduce effective capacity by 15–25% — factor this into your sizing.
• Check your local rainfall data at weather.gov for accurate intensity figures.

Gutter Sizing Standards

The Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) publishes gutter sizing standards based on the maximum rainfall rate, effective roof area, and gutter slope. These standards are referenced by most building codes and roofing specifications.

Rainfall Intensity Maps

The US Weather Bureau publishes rainfall intensity data for every county. The 10-year, 5-minute duration intensity is the standard used for gutter sizing. This represents the heaviest short-duration rainfall expected once every 10 years. Your local building department can provide the applicable intensity.

Gutter Slope Effects

A steeper gutter slope moves water faster, increasing capacity. A 1/2-inch per 10-foot slope handles about 20% more water than a 1/4-inch per 10-foot slope. However, steeper slopes are more visible from the ground. Balance capacity with aesthetics.

Frequently Asked Questions

• What is rainfall intensity?

  Rainfall intensity is the maximum rate of rainfall measured in inches per hour for a specific return period (usually 10-year or 25-year storm events). It varies by location: 2–3 in/hr in the Pacific Northwest, 4–6 in/hr in the Southeast, 6–8 in/hr in the Gulf Coast.

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

  5-inch K-style gutters handle about 5,520 sq ft of roof area at 1 in/hr rainfall. 6-inch K-style gutters handle about 7,960 sq ft — 44% more capacity. The cost difference is typically 15–25% more for 6-inch. For most homes, 5-inch is adequate; 6-inch is better for large roofs or heavy rainfall areas.

• Do I need 6-inch gutters?

  Consider 6-inch gutters if: your roof area per gutter run exceeds 800 sq ft, your rainfall intensity exceeds 4 in/hr, you have a steep roof (7:12 or more), or you plan to install gutter guards (which reduce effective capacity). Monitoring trends in this area over successive periods will highlight improvement opportunities and confirm whether changes are producing the desired effect.

• What is effective roof area?

  Effective roof area accounts for the roof slope, which catches wind-driven rain on its vertical surface. The formula adds half the roof's height to the horizontal depth. A steeper roof has a larger effective area than a low-slope roof of the same footprint.

• Can I mix 5-inch and 6-inch gutters?

  Yes. You can use 6-inch gutters on the sides of the house with large roof areas and 5-inch on smaller sections like porches or garages. Just use appropriately sized downspouts for each gutter section.

• What about commercial gutters?

  Commercial buildings often use 7-inch or 8-inch gutters, or box-style (industrial) gutters. These are engineered for specific drainage requirements and are not covered by standard residential sizing guidelines.