Gutter Size Calculator
Enter your roof area to find the recommended gutter size and downspout count, based on NRCA and This Old House sizing guidance.
Enter Roof Details
Formulas used
Gutter size recommendation
5-inch gutter: roof area ≤ 1,000 sq ft
6-inch gutter: roof area > 1,000 sq ft This simplified cutoff is the plain-language threshold given by NRCA / This Old House gutter sizing guidance (5-inch gutters handle roughly 600–750 sq ft, 6-inch gutters roughly 1,200–1,500 sq ft, both under a 1-inch-per-hour rainfall baseline). See the reference table below for the more granular capacity ranges.
True sloped roof area (reference only)
Pitch factor = √(1 + (rise ÷ 12)²)
True roof area = Plan area × Pitch factor Shown for reference — the gutter-size thresholds above are stated by the source guide in plain (plan/footprint) roof-area terms, not pitch-adjusted terms, so this figure does not change the size recommendation.
Downspout count
Downspouts = ceil(Roof area ÷ 400 sq ft) A single 2″×3″ (or 3″ round) downspout drains approximately 400 square feet of roof area, per NRCA / This Old House gutter sizing guidance.
Gutter & Downspout Sizing Reference
| Gutter Size | Roof Area Capacity (1 in/hr) | Notes |
|---|---|---|
| 5-inch | 600–750 sq ft (up to ~1,000 sq ft per simplified code guidance) | Standard for most homes |
| 6-inch | 1,200–1,500 sq ft (1,000–2,000+ sq ft per simplified guidance) | Roughly double 5-inch capacity; recommended for low-pitch or larger roofs |
| Downspout Guidance | Detail |
|---|---|
| Capacity per downspout | ~400 sq ft of roof area per 2″×3″ rectangular or 3″ round downspout |
| Spacing | Every 30–40 ft of gutter run |
Sources: National Roofing Contractors Association (nrca.net) and This Old House (thisoldhouse.com/gutters), as summarized in our Gutter and Downspout Sizing Guide.
Related Tools
- Roofing Calculator — full material estimator with squares and bundles
- Roof Area Calculator — true sloped area from footprint and pitch
- Roof Pitch Calculator — pitch to degrees converter
- Gutter and Downspout Sizing Guide — full explanation and sourcing
Frequently Asked Questions
For roof areas up to about 1,000 square feet, 5-inch gutters are standard and typically handle 600–750 square feet under a 1-inch-per-hour rainfall rate. For larger roofs (roughly 1,200–1,500+ square feet), 6-inch gutters — which roughly double the capacity of 5-inch gutters — are recommended. (Source: NRCA, This Old House.)
Plan for roughly one downspout per 400 square feet of roof area, spaced every 30–40 feet of gutter run. Homes with roof areas under 800 square feet can typically use one downspout per side. (Source: NRCA, This Old House gutter sizing guide.)
Steeper roofs concentrate water flow faster and can push more water into the gutters at once. This calculator shows your true sloped roof area for reference, but the gutter-size thresholds cited from NRCA and This Old House are stated in plain (flat/plan) roof-area terms, not pitch-adjusted terms — so the size recommendation below is driven by plan area, not the sloped figure.
The sizing guide notes that some regions — coastal areas, or those prone to frequent thunderstorms — may warrant sizing for a 2-inch-per-hour rainfall rate instead of the standard 1-inch-per-hour baseline, but it does not publish a specific numeric multiplier for that adjustment. Standard practice in that case is to size one gutter/downspout tier larger than the 1-in/hr recommendation, or consult a local contractor familiar with your jurisdiction's rainfall design standard.
Use the horizontal (plan/footprint) area of the roof section draining into a given gutter run — not the sloped surface area. The NRCA/This Old House thresholds used here (600–750 sq ft for 5-inch, 1,200–1,500 sq ft for 6-inch) are expressed in these plan-area terms.