Gutter and Downspout Calculator (Rain Runoff)
Enter roof area and rainfall to size gutter downspouts.
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Gutter and Downspout Calculator (Rain Runoff) by TheCalcTools
How the Gutter and Downspout Calculator (Rain Runoff) works
Runoff depends on the roof’s horizontal footprint and the rainfall intensity. One inch of rain on one square foot is 0.623 gallons.
Flow in gallons per minute = area × inches per hour × 0.0104.
Downspouts are sized with a rule of thumb for the roof area each can drain: about 600 sq ft for 2 × 3 inch and 1,200 sq ft for 3 × 4 inch.
The formula
Worked example
Using the default values shown in the calculator:
| Roof area (sq ft, horizontal footprint) | 1800 |
|---|---|
| Design rainfall intensity (in per hour) | 4 |
| Pitch factor for steep roofs | 1 |
| Downspout size | 2 × 3 in (about 600 sq ft of roof) |
| Rain event to collect (in) | 1 |
- 1,800 sq ft × 4 in/h × 0.0104 = 74.9 gpm at peak.
- With 2 × 3 inch downspouts you need 3. A 1 inch rain collects 1,121 gallons.
Result:
| Peak runoff (gallons per minute) | 74.9 |
|---|---|
| Downspouts needed | 3 |
| Runoff per hour at that rainfall (gallons) | 4,486 |
| Water from the rain event (gallons) | 1,121 |
| Roof area drained per downspout (sq ft) | 600 |
Frequently asked questions
What rainfall intensity should I use?
Use the rate for a 10 year storm in your area from your local rainfall data. Many designs use 4 to 6 inches per hour.
Do I need more downspouts than the rule says?
Add one if the run of gutter is over 40 feet, or if the roof has valleys that concentrate flow.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.