Roofing
Gutter Size Calculator
Size gutter and downspouts from the roof area draining to them and the rainfall intensity they have to handle.
Result
A simplified sizing guide based on drainage area, pitch and rainfall intensity. Local rainfall data and plumbing code drainage tables should govern the final specification on anything unusual.
Get a Free Business AuditWhat moves the number
- Drainage area served by each run, which is what the sizing is based on.
- Roof pitch, since a steeper roof delivers water to the gutter faster.
- Local rainfall intensity, which varies enormously and is the design input.
Where this one goes wrong
Sizing on roof area alone. A steep roof delivers the same volume of water in less time, so pitch has to be part of the calculation or the gutter overflows during exactly the storms it exists for.
Questions
About this calculator
When is five inch gutter not enough?
On large or steep roof areas, or in regions with high rainfall intensity. The standard size is a default rather than a universal answer, and oversizing is far cheaper than a callback in a storm.
Why does pitch affect gutter sizing?
Because a steeper roof discharges the same rainfall over a shorter period. The peak flow into the gutter is higher even though the total volume is the same.
Is downspout size or count more important?
Both, and count usually more. Two adequate outlets outperform one oversized one, because the gutter has to carry water to the outlet before the outlet can take it.
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How to use this calculator
- Roof area draining to this gutter (sq ft) (sq ft). Enter your own figure; the page starts at 1,400 sq ft.
- Roof pitch. Enter your own figure; the page starts at 6/12 (common).
- Peak rainfall intensity (inches per hour). Enter your own figure; the page starts at 4.
- Gutter run length (ft) (ft). Enter your own figure; the page starts at 40 ft.
The result updates as you type, so you can try a figure, see what it does to recommended gutter size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Roof area draining to this gutter (sq ft) | 1,400 sq ft |
| Roof pitch | 6/12 (common) |
| Peak rainfall intensity (inches per hour) | 4 |
| Gutter run length (ft) | 40 ft |
What comes out
| Result | Value |
|---|---|
| Recommended gutter size main result | 6-inch K-style with extra downspouts |
| Effective drainage area | 1,540.0 |
| Peak flow | 64.0 |
| Downspouts recommended | 3 |
An effective drainage area of 1,540.0 sq ft at 4 in/hr produces about 64.0 gallons per minute. Use 6-inch K-style with extra downspouts with 3 downspouts.
How sensitive the answer is
Holding everything else steady and changing only roof area draining to this gutter (sq ft), this is what happens to recommended gutter size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Roof area draining to this gutter (sq ft) (sq ft) | Recommended gutter size |
|---|---|
| 700 sq ft | 6-inch K-style with extra downspouts |
| 1,050 sq ft | 6-inch K-style with extra downspouts |
| 1,400 sq ft example above | 6-inch K-style with extra downspouts |
| 2,100 sq ft | 6-inch K-style with extra downspouts |
| 2,800 sq ft | 6-inch K-style with extra downspouts |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Roof area draining to this gutter (sq ft) (sq ft)
- Measured in sq ft, starting at 1,400 sq ft.
- Roof pitch
- Choose from 1/12 (very low), 2/12, 3/12, 4/12, 5/12, 6/12 (common), 7/12, 8/12, 9/12, 10/12, 12/12 (45°), 14/12 (steep).
- Peak rainfall intensity (inches per hour)
- A number, between 0 and 12, starting at 4.
- Gutter run length (ft) (ft)
- Measured in ft, starting at 40 ft.
What the results tell you
| Result | What it is |
|---|---|
| Recommended gutter size | The headline result. A category or planning selection, rather than a measured quantity. |
| Effective drainage area | A decimal figure. |
| Peak flow | A decimal figure. |
| Downspouts recommended | A number. |
What this cannot see
Material quantities assume a clean rectangular plane and the waste factor you enter. Valleys, hips, dormers, penetrations and a steep pitch all raise real waste, and a tear-off frequently uncovers decking that was not in the quote.
Keep going
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