Gutter capacity & flow reference
How much roof each gutter size can drain at a given rainfall intensity.
Calculator
At 6 in/hr, a 5" K-style gutter drains about 2,500 ft² of roof and a 6" K-style about 3,840 ft². These are typical published planning values that scale inversely with rainfall intensity — confirm your local rainfall and follow local code.
This is the lookup behind the size calculator: how many square feet of roof each gutter size can drain, at the rainfall intensity you set. Dial the intensity up and every capacity drops — hard rain is what overwhelms a gutter.
Use it to sanity-check a size, compare profiles, or see how much margin you have before an outlet overflows. The numbers come straight from the signature drainage dataset.
Formula
max_area(size) = base_at_6in/hr × 6 ÷ rainfall_in/hr
Each size has a base capacity at 6 in/hr. Capacity scales inversely with intensity: halve the rain and a size drains twice the roof; double the rain and it drains half.
Worked example
At 6 in/hr:
- 5" K-style ≈ 2,500 ft²
- 6" K-style ≈ 3,840 ft²
- 7" K-style ≈ 5,520 ft²
- 5" half-round ≈ 1,920 ft²
- 6" half-round ≈ 2,500 ft²
Notice a 5" half-round (1,920) carries less than a 5" K-style (2,500) — that is why half-round often needs to go one size larger. And a 6" K-style handles roughly 50% more roof than a 5".
Reading the numbers
- They are per outlet-fed run. A run with two downspouts effectively splits the load — the table is the capacity a single well-drained run of that size can carry.
- Half-round runs shallower. Its semicircular section holds less water than a K-style of the same nominal width, so it drains less roof.
- Debris cuts real capacity. Leaves and grit narrow the effective channel — leave margin, or plan on guards in tree cover.
- Labeled planning values. Confirm your local rainfall and follow local code — this is not a certified hydraulic design.
Reference table
| Gutter size | 2 in/hr | 4 in/hr | 6 in/hr | 8 in/hr |
|---|---|---|---|---|
| 5" K-style | 7,500 | 3,750 | 2,500 | 1,875 |
| 6" K-style | 11,520 | 5,760 | 3,840 | 2,880 |
| 7" K-style | 16,560 | 8,280 | 5,520 | 4,140 |
| 5" half-round | 5,760 | 2,880 | 1,920 | 1,440 |
| 6" half-round | 7,500 | 3,750 | 2,500 | 1,875 |
Max roof area (ft²) a size can drain, by rainfall intensity. Labeled SMACNA-style planning values that scale inversely with intensity (base at 6 in/hr × 6 ÷ intensity). See the full dataset.