5" vs 6" K-Style Gutters: Real Capacity Difference

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What IPC Table 1106.6 Actually Lists

Horizontal gutters are sized so flow does not exceed Table 1106.6 (IPC 2021 §1106.6). At 1/8 inch per foot — the slope this cluster assumes — the single-dimension (semicircle) rows that match residential talk are:

110 ÷ 74 ≈ 1.49. The extra inch is not a cosmetic upsell; it is about half again the tabulated flow at the same slope. Steeper gutter pitch (the 1/4 and 1/2 in/ft rectangular rows in the same table) carries more, but a typical residential K-style hang is closer to the shallow column than to a 1/2 in/ft trough.

That is why there is no separate “chart” page in this cluster: the calculator already maps any plan area and rainfall onto those two capacities, and the table itself is two rows you care about for K-style work. Reprinting the whole IPC gutter table as a third URL would not add a reference the calculator and this comparison do not already give you.

K-Style vs the Semicircle Footnote

Table 1106.6 footnote a: dimensions are width by depth for rectangular shapes; single dimensions are diameters of a semicircle. A 5" K-style is not a 5" half-round. K-style has a flat back, a molded front, and a different hydraulic section. Industry practice still sizes residential K-style against those nominal 5" / 6" rows because that is the published code table, and because claiming a 5" K-style “really holds 6" of water” is how you overflow a fascia.

This comparison treats 5" K-style as not more than 74 gpm and 6" K-style as not more than 110 gpm at 1/8 in/ft. If a manufacturer publishes a tested K-style capacity, use the tested number. Until then, do not assume the ogee profile beats the semicircle line.

When the 6" Upgrade Is Worth It

Run the calculator. The upgrade is worth it when any of these are true:

• Per-section flow is over 74 gpm after you have at least two downspouts and a sane run length. Adding a third 2×3 on a short eave can be uglier and leakier than hanging 6" with two 3×4 outlets.
• You would need more than three 2×3 downspouts to keep 5" legal. The calculator steps to 6" K-style and 3×4 leaders at that point on purpose.
• Local rainfall is 5 in/hr or higher on a roof that is not tiny. A 1,500 sq ft eave at 6/12 and 6 in/hr is already 103 gpm before you split it — 5" only works if you carve that eave into small pieces.
• You are already in the air. Rehanging 5" next year because the first storm proved the math costs more than the extra inch of metal today.

Stay on 5" when per-section flow is comfortably under 74 gpm, the fascia is already punched for 5" hangers, and you can put two outlets on a simple eave without a 50-ft dead run. A ranch with 800 sq ft per slope in a 3 in/hr climate does not need 6" for capacity.

Downspouts Travel With the Trough

A 6" gutter feeding a single 2×3 is a fat trough into a skinny leader. IPC Table 1106.3: 2" / 2×2 = 30 gpm; 3" / 2×4 = 92 gpm. When you step to 6" K-style, step the outlets to 3×4 unless you are adding enough 2×3s that each one stays under 30 gpm. The calculator pairs them that way.

Outlet location still matters. Capacity math assumes the flow in each section is Q divided by the number of downspouts. Cluster three outlets on one corner and the far bay is still a 40-ft river. Spread them. Keep runs in the 35–40 ft neighborhood even when gpm would allow fewer pipes.

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