Roofing
Gutter Calculator
Work out gutter and downspout quantities from roof edge length and the area draining to each run.
Result
One downspout per 35 feet of gutter is a common rule of thumb. Steep roofs, large catchment areas and high-rainfall regions need more. The gutter size calculator works it out properly.
Get a Free Business AuditWhat moves the number
- Linear feet of eave, which sets the gutter quantity.
- Drainage area per downspout, which decides how many are needed.
- Hangers, which are spaced by code and by product rather than by eye.
Where this one goes wrong
Counting gutter and forgetting downspouts. A correctly sized gutter with too few downspouts overflows in heavy rain, and the customer experiences that as a gutter failure.
Questions
About this calculator
How many downspouts does a run need?
It depends on the roof area draining into it and the local rainfall intensity. A long run with one outlet at the end will overflow before the gutter is at fault.
What hanger spacing should I use?
The manufacturer's figure, tightened in areas with snow load. Hangers too far apart is the most common cause of a gutter that sags and then separates.
Does gutter material change the quantity?
Not the length, but it changes the joints and the hangers. Seamless runs avoid joints entirely, which is a labour and a leak consideration rather than a quantity one.
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How to use this calculator
- Total eave length (ft) (ft). Enter your own figure; the page starts at 140 ft.
- Corners. Enter your own figure; the page starts at 4.
- Hanger spacing (inches) (in). Enter your own figure; the page starts at 24 in.
- Downspout drop height (ft) (ft). Enter your own figure; the page starts at 20 ft.
- Gutter cost per linear ft ($). Enter your own figure; the page starts at $9.
- Downspout cost per linear ft ($). Enter your own figure; the page starts at $7.
The result updates as you type, so you can try a figure, see what it does to downspouts needed, 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 |
|---|---|
| Total eave length (ft) | 140 ft |
| Corners | 4 |
| Hanger spacing (inches) | 24 in |
| Downspout drop height (ft) | 20 ft |
| Gutter cost per linear ft | $9 |
| Downspout cost per linear ft | $7 |
What comes out
| Result | Value |
|---|---|
| Downspouts needed main result | 4 |
| Gutter (linear ft) | 140 |
| Hangers | 70 |
| Material cost | $1,868 |
140 ft of gutter needs 4 downspouts and 70 hangers, using 80 ft of downspout, about $1,868 in materials.
How sensitive the answer is
Holding everything else steady and changing only gutter cost per linear ft, this is what happens to downspouts needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Gutter cost per linear ft ($) | Downspouts needed |
|---|---|
| $4.5 | 4 |
| $6.75 | 4 |
| $9 example above | 4 |
| $13.5 | 4 |
| $18 | 4 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Total eave length (ft) (ft)
- Measured in ft, starting at 140 ft.
- Corners
- A number, between 0 and 60, starting at 4.
- Hanger spacing (inches) (in)
- Measured in in, between 0 and 60, starting at 24 in.
- Downspout drop height (ft) (ft)
- Measured in ft, starting at 20 ft.
- Gutter cost per linear ft ($)
- A money amount, starting at $9.
- Downspout cost per linear ft ($)
- A money amount, starting at $7.
What the results tell you
| Result | What it is |
|---|---|
| Downspouts needed | The headline result. A number. |
| Gutter (linear ft) | A number. |
| Hangers | A number. |
| Material cost | A money figure. |
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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