Gutter Sizing Calculator — Gutter Size and Downspout Count
Determine the correct gutter size and number of downspouts for your roof drainage area based on rainfall intensity and roof area.
How to Use This Calculator
- Enter the gutter run length and roof width (how deep the drainage area is)
- Select your roof pitch
- Choose rainfall intensity for your region
- Select gutter style — K-style is most common for residential
- Click Calculate to get recommended gutter size and downspout count
How the Calculation Works
// Adjusted drainage area:
drainage_sqft = roof_length × roof_width × pitch_multiplier
// Required flow rate (GPM):
flow_gpm = drainage_sqft × rainfall_in_per_hr / 96.23
// K-style capacity by size:
// 4" = 20 GPM | 5" = 40 GPM | 6" = 55 GPM
// Half-round:
// 4" = 16 GPM | 5" = 32 GPM | 6" = 45 GPM
num_downspouts = ⌈gutter_length / 35⌉ Gutter capacity is sized to handle the peak flow from the drainage area during a design storm. Flow rate depends on the roof area (adjusted for pitch) and local rainfall intensity. Once flow is known, gutter capacity tables (based on SMACNA standards) determine the correct gutter size.
Variables:
- drainage_sqft: Horizontal roof area draining to this gutter run, adjusted for pitch
- rainfall_in_per_hr: Design rainfall intensity for your region (typically 2–7 in/hr)
- gutter_length: Total length of the gutter run in feet
Pro Tips
- 5-inch K-style gutters are the standard for most residential homes
- 6-inch gutters are recommended for larger roofs or areas with heavy rainfall
- One downspout per 30–40 linear feet of gutter is a solid rule of thumb
- Slope gutters 1/4 inch per 10 feet toward the downspout for proper drainage
- Half-round gutters have a traditional look and are easier to clean than K-style
Recommended Products
Amerimax 5" K-Style Aluminum Gutter — 10 ft
White pre-formed aluminum K-style gutter section.
View on Amazon →Spectra Metal 6" K-Style Gutter — 10 ft
Heavy-gauge 6" gutter for high-rainfall regions.
View on Amazon →LeafFilter Micro-Mesh Gutter Guard — 3 ft (4-pack)
Stainless steel micro-mesh keeps debris out of gutters.
View on Amazon →Amerimax 3"x4" Aluminum Downspout — 10 ft
Standard rectangular downspout for K-style gutter systems.
View on Amazon →Frequently Asked Questions
What size gutters do I need for my house?
Most homes use 5-inch K-style gutters. For large roofs (over 5,000 sq ft) or high-rainfall areas, 6-inch gutters are recommended. Sheds and garages can use 4-inch.
How many downspouts do I need?
One downspout per 30–40 linear feet of gutter, or one per 600–800 sq ft of drainage area — whichever requires more.
What is the correct gutter slope?
Gutters should slope 1/4 inch per 10 feet of run toward the downspout. Too little causes pooling; too much causes overflow at the downspout.
K-style vs half-round gutters — which is better?
K-style holds more water and is more common. Half-round is traditional, easier to clean, but has lower flow capacity per size, so may require more downspouts.