Free Tool
Gutter & Downspout Sizing Calculator
Enter roof area and local rainfall intensity to estimate the right gutter and downspout size.
This is the flat plan area of roof draining to one downspout — not the whole roof if it has multiple gutter runs, and not the sloped surface area.
Roof pitch
Gutter slope
Enter your roof area and local rainfall intensity above to see your results.
This is a planning estimate based on published sizing references, not a stamped engineering design — confirm against your local plumbing code before ordering, especially for a commercial or unusually large roof.
Why drainage sizing uses a different pitch adjustment
A steeper roof sheds water faster and, with wind, catches rain at more of an angle — both increase how much water a gutter actually has to handle, beyond what the roof’s flat footprint suggests. Drainage design accounts for this with a more conservative adjustment factor than a roofing-material coverage calculation would use, since overestimating a gutter’s capacity risks real overflow, while overestimating material is just extra cost.
Finding your local rainfall intensity
Gutter and downspout capacity is rated at a 1 inch/hour baseline, then adjusted for your area’s actual design rainfall rate. This varies enormously by region — coastal and southeastern areas often see rates several times higher than the interior of the country. Your local plumbing code or building department can specify the rate to design for, or you can look up rainfall intensity-duration-frequency data for your area (commonly referenced as NOAA Atlas 14) directly.
Reference: gutter capacity at 1 in/hr
The underlying table this calculator uses, before adjusting for your local rainfall rate — divide any of these by your intensity (in/hr) to see that size’s actual capacity.
| Gutter | Level | 1/8″/ft | 1/4″/ft |
|---|---|---|---|
| 3″ | 680 sq ft | 960 sq ft | 1,360 sq ft |
| 4″ | 1,440 sq ft | 2,040 sq ft | 2,880 sq ft |
| 5″ | 2,500 sq ft | 3,520 sq ft | 5,000 sq ft |
| 6″ | 3,840 sq ft | 5,440 sq ft | 7,680 sq ft |
| 7″ | 5,520 sq ft | 7,800 sq ft | 11,040 sq ft |
| 8″ | 7,960 sq ft | 11,200 sq ft | 14,400 sq ft |
| 10″ | 14,400 sq ft | 20,400 sq ft | 28,800 sq ft |
Worked example
A 1,500 sq ft roof section (6-8/12 pitch), 2 in/hr local rainfall, 1/8″ per ft gutter slope:
- 1. Design area: 1,500 × 1.10 = 1,650 sq ft
- 2. A 5″ gutter at 1/8″ slope handles 3,520 sq ft at 1 in/hr, or 3,520 / 2 = 1,760 sq ft at 2 in/hr — enough, so 5″ is the recommended size
- 3. Required downspout area: (1,650 × 2) / 1,200 ≈ 2.75 sq in → a 3″ downspout (7.07 sq in) covers it
Frequently asked questions
What is "design area" for gutter sizing?
Your roof's flat plan area, adjusted upward for pitch. Steeper roofs shed rain faster and with more wind-driven force, so drainage design uses a more conservative adjustment than a roofing-material coverage calculation would — that's why this figure differs from a roof's actual sloped surface area.
Why do I need my local rainfall intensity?
Gutter and downspout capacity is rated at a 1 inch/hour baseline, then divided by your area's actual design rainfall rate — a location with heavier storms needs a bigger gutter for the same roof. Check your local plumbing code or building department, or look up your area's rainfall intensity-duration-frequency data (commonly referenced as NOAA Atlas 14).
What if my roof needs more than one downspout?
Enter the roof area draining to a single downspout, not your whole roof — a run of gutter shouldn't exceed about 50 linear feet between downspouts or expansion joints anyway. Split a larger roof into sections and size each one separately.
Is a 5-inch or 6-inch gutter better?
Neither is universally "better" — 5-inch is the most common size on typical residential roofs and is sized correctly by this calculator for plenty of them, but a bigger roof, steeper pitch, or heavier local rainfall can genuinely need 6-inch or larger. Let the calculation drive the choice rather than picking a size by habit.
Should I round up to the next gutter or downspout size?
This calculator already recommends the smallest standard size that meets the calculated requirement. Rounding up further isn't necessary from a capacity standpoint, though some installers prefer the next size up on roofs with heavy debris or leaf load, since it reduces clogging risk.