Rain Gutter Calculator
Add each roof section that drains to a gutter, its pitch and your local rainfall intensity to find the right gutter size (5″ or 6″, K-style or half-round) and how many downspouts you need. All calculations run in your browser.
Roof sections (one per gutter run)
Use the peak short-duration (5-minute) rainfall intensity for your area, e.g. from NOAA Atlas 14 or your local building department.
How to use the Rain Gutter Calculator
- Add a roof section for each gutter run: the gutter (eave) length and the horizontal depth from eave to ridge.
- Pick the roof pitch of each section, e.g. 6/12.
- Enter your rainfall intensity (peak 5-minute rate) or choose a preset.
- Choose the gutter style and downspout size you plan to install.
- Press Calculate gutter size to see the recommended gutter, the number of downspouts and the per-section breakdown.
How gutter sizing works
- Plan area of each roof section = gutter length × horizontal depth from eave to ridge (measure the footprint, not the sloped surface).
- Design area = plan area × roof pitch factor.
- Downspouts = the larger of design area ÷ downspout capacity and gutter length ÷ 40 ft (at least one).
- Rain-adjusted area per downspout = design area × rainfall intensity (in/hr) ÷ number of downspouts. The smallest gutter whose capacity is at least this value is recommended; if even a 6″ gutter is too small, another downspout is added.
Example: a 40 ft run with a 15 ft deep roof at 6/12 pitch: 600 ft² × 1.10 = 660 ft² design area. With 2×3 downspouts (600 ft² each) that needs 2 downspouts. At 6 in/hr: 660 × 6 ÷ 2 = 1,980 ft², well within the 5,520 ft² capacity of a 5″ K-style gutter.
Capacity tables used
Gutter capacities are the widely published SMACNA-based figures for level residential gutters, expressed as design roof area at 1 in/hr; divide by your intensity to get the area each outlet can serve. Downspout capacity uses the 1 in² per 100 ft² rule of thumb.
| Gutter | At 1 in/hr | At 4 in/hr | At 6 in/hr | At 8 in/hr |
|---|---|---|---|---|
| 5″ K-style | 5,520 ft² | 1,380 ft² | 920 ft² | 690 ft² |
| 6″ K-style | 7,960 ft² | 1,990 ft² | 1,327 ft² | 995 ft² |
| 5″ half-round | 2,500 ft² | 625 ft² | 417 ft² | 313 ft² |
| 6″ half-round | 3,840 ft² | 960 ft² | 640 ft² | 480 ft² |
| Roof pitch | Pitch factor |
|---|---|
| Flat – 3/12 | 1.00 |
| 4/12 – 5/12 | 1.05 |
| 6/12 – 8/12 | 1.10 |
| 9/12 – 11/12 | 1.20 |
| 12/12 | 1.30 |
| Downspout | Opening (nominal) | Design roof area served |
|---|---|---|
| 2 × 3 in | 6 in² | about 600 ft² |
| 3 in round | 7.1 in² | about 700 ft² |
| 3 × 4 in | 12 in² | about 1,200 ft² |
| 4 in round | 12.6 in² | about 1,250 ft² |
Practical tips
- Slope gutters about ¼ in per 10 ft toward each downspout; a downspout at each end of a long run halves the flow in each half.
- Upsize where a roof valley dumps water into the gutter, and fit splash guards at inside corners.
- Discharge downspouts at least 4–6 ft from the foundation with extensions or splash blocks.
- 6″ gutters are usually paired with 3×4 downspouts, which also clog less often with leaves.
Need the sloped roof surface for shingles? Use the Roofing Calculator.
Frequently asked questions
Do I need 5-inch or 6-inch gutters?
5-inch K-style gutters suit most homes. Choose 6-inch gutters for large or steep roof sections, long runs, regions with intense downpours, or when you want fewer downspouts. The calculator checks the design roof area each downspout serves against the capacity of each size at your rainfall intensity.
How many downspouts do I need?
A common rule is one square inch of downspout opening per 100 sq ft of roof: a 2×3 in downspout serves about 600 sq ft and a 3×4 in about 1,200 sq ft. Also place one at least every 30–40 ft of gutter. The calculator uses whichever gives more downspouts, and adds more if the gutter itself would overflow.
What is the roof pitch factor?
Wind drives rain onto steep roofs at an angle, so a steep roof catches more rain than its flat footprint. SMACNA-based sizing multiplies the plan area by 1.00 for flat to 3/12, 1.05 for 4/12–5/12, 1.10 for 6/12–8/12, 1.20 for 9/12–11/12 and 1.30 for 12/12.
What rainfall intensity should I use?
Use the peak short-duration (5-minute) rainfall intensity for your location, which is much higher than hourly or daily totals. In the US you can look it up in NOAA Atlas 14 (Precipitation Frequency Data Server); many areas fall between about 4 and 8 in/hr.
Are K-style gutters better than half-round?
For the same width, K-style gutters hold much more water than half-round gutters because of their flat bottom and deeper profile, so a 5-inch K-style handles more roof area than a 6-inch half-round. Half-round gutters are chosen mainly for appearance on historic or traditional homes.
Is this calculation code compliant?
It gives a planning estimate based on widely published SMACNA-style tables. Plumbing and building codes (such as the IPC) and manufacturers may use different methods, so confirm with your local building department or a gutter professional for important projects.