Gutter Sizing Calculator: 5 Inch or 6 Inch

Answer these

Measurement method

Roof footprint area (sq ft)

Roof pitch (rise in 12)

Example: 4 for a 4-in-12 roof. See the pitch factor table on How We Test.

Local max rainfall intensity (inches per hour)

Look up your location at NOAA PFDS or ask your local building department.

All calculations use K-style gutter capacities as cited on /how-we-test/.

Your result

5-inch K-style gutters

2×3 in. rectangular downspouts

4 downspouts

3×4 in. rectangular downspouts

2 downspouts

Math trace

Pitch factor: 1.05 (for 4-in-12 roof) Effective drainage area: 2100.0000000000005 sq ft (2000.0000000000002 sq ft × 1.05 × 1 in/hr) K-style threshold: 5520 sq ft (5″) / 7960 sq ft (6″) Verdict: effective area ≤ 5,520 → 5-inch gutters

This tool provides initial sizing recommendations. Always have a licensed contractor verify dimensions and downspout placement for your specific roof layout. Gutter run lengths over 60 feet may require extra outlets — see our sizing guide.