Downspout Size Calculator
Enter the roof's plan area, its pitch and the heaviest rainfall you expect in an hour. The calculator gives the number of 2 × 3 in and 3 × 4 in downspouts, the peak flow and the area each one can drain.
- Downspouts needed
- Peak flow
- Area per downspout
Use the plan area, the footprint seen from above. The pitch factor adds for steeper roofs.
Each downspout can only drain so much roof. At 5 in of rain an hour, a 2 × 3 in downspout drains about 600 sq ft and a 3 × 4 in about 1,200 sq ft. A 1,500 sq ft low-pitch roof works out at 1,575 sq ft, so it needs three 2 × 3 in or two 3 × 4 in downspouts.
Downspout chart
Low pitch, 5 in an hour. Search for a roof.
| Roof (sq ft) | 2 × 3 in | 3 × 4 in | Peak flow (GPM) |
|---|---|---|---|
| 500 | 1 | 1 | 27 |
| 800 | 2 | 1 | 44 |
| 1000 | 2 | 1 | 55 |
| 1500 | 3 | 2 | 82 |
| 2000 | 4 | 2 | 109 |
| 2500 | 5 | 3 | 136 |
| 3000 | 6 | 3 | 164 |
| 4000 | 7 | 4 | 218 |
Number needed.
Roof area and downspouts
Low pitch, 5 in an hour. Slide the roof area.
The house shows the 2 × 3 in downspouts along the gutter.
How to size downspouts
Roof to downspouts. Step through it.
Area
Plan area.
Downspouts at the ends of a gutter run let it drain both ways and halve the load on each.
2 × 3, 3 × 4 or round
Downspout sizes. Compare them.
Standard.
Oversized.
Classic.
Larger downspouts clog less with leaves as well as carrying more water.
Before you install
Downspout checks. Tick each item.
Tick each item as you go.
Run the water at least 4 to 6 ft away from the foundation.
What you get
Downspout sizing. Each tile explains one result.
2 × 3 in.
3 × 4 in.
Peak GPM.
Drained.
Guide.
Inside corners collect water from two roof faces, so they are a good place for a downspout.
Frequently Asked Questions
No question matches that search.
How many downspouts do I need?
Divide the roof area by about six hundred square feet for each two by three inch downspout, adjusted for rainfall.
How much roof does one downspout drain?
About six hundred square feet for two by three inch and twelve hundred for three by four inch.
Should I use three by four inch downspouts?
Yes, on large or steep roofs and where leaves clog smaller ones.
How far apart should downspouts be?
No more than about forty feet of gutter between them.
Does roof pitch matter?
Yes. Steeper roofs catch more rain and need more capacity.
What rainfall should I design for?
The heaviest rain in an hour for your area, often about five inches.
Is a downspout the same as a downpipe?
Yes. Downspout is the US word, downpipe the UK word.
Water away, fast
Fit enough downspouts for your roof and the gutters never overflow, even in the heaviest storms.
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