Gutter and downspout calculator
How many downspouts, gutter sections and hangers a roof needs, from its gutter length and roof area.
Downspouts
- 10 ft gutter sections
- Hangers
How to use it
Enter the total length of gutter around the house in feet, and the roof area that drains into it. You can get the roof area from the roofing calculator: use the roof surface area, not the house footprint. Choose the downspout size and hanger spacing.
How it's calculated
The calculator works out downspouts two ways and uses whichever needs more:
- By gutter length: no more than 40 ft of gutter per downspout. Gutter length ÷ 40, rounded up.
- By roof area: each downspout needs about 1 square inch of opening for every 100 sq ft of roof. A 2 × 3 in downspout (6 sq in) drains about 600 sq ft; a 3 × 4 in (12 sq in) drains about 1,200 sq ft.
Then:
- Gutter sections = gutter length ÷ 10, rounded up, for 10 ft sections.
- Hangers = gutter length in inches ÷ hanger spacing, rounded up.
| Roof area | 2 × 3 downspouts | 3 × 4 downspouts |
|---|---|---|
| 1,200 sq ft | 2 | 1 |
| 1,800 sq ft | 3 | 2 |
| 2,400 sq ft | 4 | 2 |
Gutter length can push these numbers up: 120 ft of gutter needs at least 3 downspouts whatever the roof area.
Placement tips
- Put a downspout at each end of a long run and near inside corners, where water collects.
- Steep roofs shed water faster. Lean toward 3 × 4 downspouts or more of them on roofs steeper than about 9/12.
- Hangers closer together (18 in) in areas with heavy snow or ice, which add weight.
- Slope the gutter about ¼ in for every 10 ft toward each downspout so water doesn't pool.
These are general rules of thumb. Local codes and the gutter maker's instructions take priority.