Gutter Size Calculator: 5-Inch vs 6-Inch
See whether 5-inch or 6-inch K-style gutters can carry your roof — the drainage area is adjusted for roof pitch and your local rainfall intensity, then compared to industry capacity.
Calculator
700 sq ft draining at ×1.10 pitch factor and 1.5 in/hr is an adjusted 1,155 sq ft → 5" K-style. Look up your local rainfall intensity (NOAA) for your ZIP.
Most American homes wear 5-inch K-style gutter, and it is plenty for an average roof in average rain. Step up to 6-inch K-style when the roof section is large, steep, or in a region that gets hard downpours — the wider trough holds about 40% more water and pairs with larger 3×4 in downspouts. The decision is not guesswork: it comes from three numbers you can measure or look up.
The first is the plan (footprint) area that drains into this particular gutter run — use the flat area, not the pitched surface area, because rain falls vertically. The second is a pitch factor: a steeper roof hurls water at the gutter faster, so the effective area is scaled up (1.0 for nearly flat, up to 1.30 for a 12/12 and steeper). The third is your local rainfall intensity in inches per hour, which you look up once from NOAA precipitation-frequency data for your ZIP. Multiply the three and compare the result to the published capacity of each profile.
Formula
Adjusted drainage area, then compared to K-style capacity at 1 in/hr:
adjusted_area = plan_area × pitch_factor × rainfall_in_hr\n\n5" K-style handles up to 5,520 sq ft (adjusted)\n6" K-style handles up to 7,960 sq ft (adjusted)
Because the capacities are quoted at 1 in/hr, multiplying the area by your real inches-per-hour puts both sides in the same units. If the adjusted area exceeds even the 6-inch figure, the run needs to be split or given extra downspouts rather than a bigger trough.
Worked example
A 700 sq ft roof section, a 6/12 pitch (factor 1.10), in a climate that peaks near 1.5 in/hr:
adjusted = 700 × 1.10 × 1.5 = 1,155 sq ft\n1,155 ≤ 5,520 → 5" K-style is fine
The adjusted area is far below the 5-inch limit, so standard 5-inch K-style carries this run comfortably. You would only reach for 6-inch on a much larger, steeper, or wetter roof.
Reading the result honestly
These capacities are steady-state figures for a properly sloped, clean gutter; a clogged trough or a sagging run overflows well before the number says it should, so keep the pitch right (a quarter inch per 10 ft) and the leaves out. The rainfall intensity is the one input worth getting right — a Gulf-Coast cloudburst can run several inches per hour while a dry inland town rarely tops one, and that ratio changes the answer more than the roof size does. When the adjusted area lands close to a limit, size the outlets on the downspout capacity tool and add an outlet rather than assuming the bigger gutter alone will cope. Results are planning estimates, not a hydraulic guarantee.
Reference table
K-style gutter capacity is the roof drainage area a properly sloped run can carry at 1 in/hr of rainfall. Divide by your local rainfall intensity to size a real roof.
| K-style gutter | Drainage area @ 1 in/hr | At 2 in/hr | At 4 in/hr |
|---|---|---|---|
| 5-inch K-style | 5,520 sq ft | 2,760 sq ft | 1,380 sq ft |
| 6-inch K-style | 7,960 sq ft | 3,980 sq ft | 1,990 sq ft |
Steeper roofs throw more water at the gutter, so the plan area is multiplied by a pitch factor before comparing to capacity:
| Roof pitch | Drainage factor |
|---|---|
| Flat to 3/12 (×1.00) | ×1 |
| 4/12–5/12 (×1.05) | ×1.05 |
| 6/12–8/12 (×1.10) | ×1.1 |
| 9/12–11/12 (×1.20) | ×1.2 |
| 12/12 and steeper (×1.30) | ×1.3 |