Half-round gutters are usually paired with round downspouts (3 or 4 inch) and hung with brackets rather than hidden hangers.
How the gutter calculator works
GutterCalc multiplies the drainage area (gutter length × roof depth) by the pitch factor and the rainfall intensity, then picks the smallest gutter of your style whose capacity is at least that adjusted area. Downspouts = the larger of the gutter length ÷ the maximum run per downspout and the roof area ÷ the downspout capacity, rounded up. Hangers = length ÷ spacing + 1. Slope = ¼ inch (or your figure) per 10 feet over the run to each downspout.
Gutter capacity (adjusted sq ft)
| Gutter | Capacity |
|---|---|
| 5″ K-style | 5,520 |
| 6″ K-style | 7,960 |
| 5″ half-round | 2,500 |
| 6″ half-round | 3,840 |
Pitch factor
| Roof pitch | Factor |
|---|---|
| Flat to 3/12 | 1 |
| 4/12 – 5/12 | 1.05 |
| 6/12 – 8/12 | 1.1 |
| 9/12 – 11/12 | 1.2 |
| 12/12 and steeper | 1.3 |
Downspout capacity (rule of thumb)
| Downspout | Opening | Roof area |
|---|---|---|
| 2 × 3″ rectangular | 6 sq in | ≈ 600 sq ft |
| 3 × 4″ rectangular | 12 sq in | ≈ 1,200 sq ft |
| 3″ round | 7.1 sq in | ≈ 700 sq ft |
| 4″ round | 12.6 sq in | ≈ 1,250 sq ft |
Capacities, pitch factors and the 1 sq in per 100 sq ft downspout rule are widely published rules of thumb for residential gutters, not a code calculation; local codes (for example the plumbing code's storm drainage tables) and your installer may size differently. Results are estimates.
Frequently asked questions
Do half-round gutters carry less water?
Yes, roughly half as much as K-style of the same nominal width.
What downspout goes with half-round gutters?
Usually 3-inch round with 5-inch gutters and 4-inch round with 6-inch gutters.
Are half-round gutters harder to install?
They need brackets on the fascia or roof, but the sizing and slope are the same as for K-style.
Is GutterCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.