What Size Gutters Do I Need Calculator
Most homes do fine with 5-inch K-style gutters (good for about 5,500 sq ft of roof); go 6-inch for bigger roofs or heavy rain. This calculator sizes your gutters by roof area and counts how many downspouts each run needs — the same math as a how many downspouts do I need calculator.
Size for the worst storm, not the average one — overflow rots fascia boards and soaks foundations.
Estimates only. Valleys, dormers, and gutter slope change real capacity. A gutter installer should confirm before you buy.
How gutter sizing works
Gutters are sized by the roof area draining into them, not the size of the house. A 5-inch K-style gutter handles about 5,520 square feet of roof under moderate rainfall — enough for most single-family homes. Steep roofs shed water faster, so the calculator bumps the effective area up 10% for pitches over 8/12.
Downspouts are the other half of the system: one per 40 feet of gutter, never stretched past 50 feet. A long run with a single downspout in the middle drains half the distance each way, which is why counting outlets matters as much as gutter width.
5 in K-style ≈ 5,520 sq ft roof · 6 in K-style ≈ 7,960 sq ft · 1 downspout per 40 ft
Related tools
Keep exploring: price the roof above those gutters with the Roof Replacement Cost Calculator, or manage where the water goes next with the French Drain Cost Calculator.
What Size Gutters Do I Need Calculator FAQ
What size gutters do I need for my house?
Most single-family homes do fine with 5-inch K-style gutters, which handle roughly 5,500 sq ft of roof area. Step up to 6-inch gutters for roofs above that, steep pitches, or areas with heavy rainfall. Enter your roof area in the gutter-size mode above for a direct answer.
What is the difference between 5-inch and 6-inch gutters?
A 6-inch K-style gutter holds about 40% more water — roughly 7,960 sq ft of roof vs 5,520 for 5-inch. It is only one inch deeper, so the difference is barely visible from the ground. Pair 6-inch gutters with larger 3x4 inch downspouts.
How many downspouts do I need?
Plan one downspout per 40 feet of gutter run, and never stretch a single downspout past 50 feet. A 120-foot run needs 3 downspouts. Use the downspout mode above — it is the same math as a how many downspouts do I need calculator.
What size downspouts go with 5-inch gutters?
Standard 2x3 inch downspouts pair with 5-inch gutters. Use 3x4 inch downspouts with 6-inch gutters, or with 5-inch gutters in heavy-rain areas for extra capacity.
K-style vs half-round gutters — which is better?
K-style gutters hold roughly twice the water of half-round gutters the same width, which is why they dominate residential installs. Half-round is mostly chosen for historic or traditional looks.
How much slope do gutters need?
About 1/16 inch per foot toward the downspout — roughly 1 inch of drop over 16 feet. Too little slope causes standing water; too much looks crooked from the street.
How often should I clean my gutters?
At least twice a year — spring and late fall — and more often under mature trees. Clogged gutters overflow no matter how well they were sized.