A gutter size calculator determines the right gutter dimensions for your roof based on drainage area and local rainfall intensity. Get size recommendation and downspout count.
Roof Drainage Area
Rainfall
How to Choose the Right Gutter Size
Gutter sizing is based on roof drainage area adjusted for pitch, multiplied by local rainfall intensity. The result determines the minimum flow capacity needed, which maps to standard gutter sizes.
Gutter Size Quick Reference
4-inch K-style: up to 800 sq ft at low rainfall. 5-inch K-style: up to 1,200-1,800 sq ft. 6-inch K-style: up to 2,000-3,000 sq ft. 6-inch half-round: more decorative, requires more downspouts than K-style of same size.
FAQ
What size gutters are standard?
5-inch K-style gutters are the most common for residential homes. They handle moderate rainfall for most roof sizes. 6-inch gutters are used for larger roofs or high-rainfall areas. 4-inch gutters are for small sheds or garage additions.
How do I know what size gutter to get?
Multiply your roof's drainage area (sq ft) by your local peak rainfall intensity (inches/hour). Divide by 5.56 to get the minimum gutter cross-section in square inches. A 5-inch K-style gutter has about 10 sq in of flow area.
How many downspouts do I need?
Standard rule: one downspout per 35-40 feet of gutter run. For high-rainfall areas, one per 20-30 feet. Each downspout drains approximately 600-1,200 sq ft of roof area depending on slope.
Should I get 5-inch or 6-inch gutters?
Use 5-inch for most homes under 2,500 sq ft of roof area with normal rainfall. Use 6-inch for larger roofs, steep pitches, or areas with heavy rainfall (over 2 inches per hour during peak storms).
Is this calculator free?
Yes, completely free with no signup required.