How to use the gutter and downspout calculator
- Enter the total gutter length and the roof area that drains into it, measured flat as on a floor plan.
- Set the rainfall rate you want to plan for, the longest run per downspout and the roof area per downspout from your product's guidance.
- Enter the wall height, section length, hanger spacing and elbows per downspout.
- Read the number of downspouts, the water flow, and the sections, hangers, pipe and elbows.
Formula
Downspouts by run = gutter length / longest run per downspout, rounded up
Downspouts by area = roof area / roof area per downspout, rounded up; use the larger of the two
Flow in gallons per minute = roof area x rainfall in inches per hour x 0.01039
Sections = length / section length, rounded up; hangers = length x 12 / spacing, rounded up, plus 1
One inch of rain per hour on one square foot is 144 cubic inches per hour, or 144 / 231 gallons per hour, which is about 0.01039 gallons per minute.
120 feet of gutter on 1,800 square feet of roof
With 120 feet of gutter, a longest run of 40 feet and 600 square feet per downspout, you need 3 downspouts by run length and 3 by area. At 4 inches of rain per hour the roof sends 1,800 x 4 x 0.01039 = 74.8 gallons per minute, or 24.9 for each of 3 downspouts.
With 10 foot sections and hangers every 24 inches the job takes 12 sections and 61 hangers. A 10 foot wall needs 30 feet of downspout and, at 2 elbows each, 6 elbows. If the roof area were 3,000 square feet, the area rule would call for 5 downspouts.
Why roof area is measured flat
Rain falls straight down, so what the roof collects depends on its horizontal footprint including the overhang, not the sloped surface. A steeper roof sheds water faster but does not collect more of it. If you know the footprint of the house and the overhang, use that area. For the sloped surface area, use the roofing calculator and the roof pitch calculator.
Valleys and dormers that send water to one gutter add to the load on that run, so split the roof by where each section drains if you want a closer answer.
Numbers to check against your product
The rainfall rate, run length, roof area per downspout and hanger spacing are example values. Local rainfall design figures differ a great deal, and gutters and downspouts come in different sizes with different ratings. Use what the maker and your local code say.
Corners, end caps, sealant, fasteners and splash blocks are not counted. For other exterior materials, try the siding calculator.
Assumptions and limits
- Rainfall, run length, roof area per downspout, hanger spacing and elbows are example values.
- Water flow is the simple roof area times rainfall conversion, with no allowance for wind, gutter slope or clogging.
- At least one downspout is always shown.
- This is a planning estimate, not an engineering or code check.
Frequently asked questions
How many downspouts do I need?
Divide the gutter length by the longest run one downspout should serve, and the roof area by the area one downspout can handle, round both up and use the larger count.
How do I convert rainfall to gallons per minute?
Multiply the roof area in square feet by the rainfall in inches per hour and by 0.01039. For 1,800 square feet at 4 inches per hour that is 74.8 gallons per minute.
How many gutter hangers do I need?
Multiply the length in feet by 12, divide by the spacing in inches, round up and add one. For 120 feet at 24 inches that is 61.
Does roof pitch change how much water the gutters get?
No. The flow depends on the flat footprint of the roof, since rain falls vertically, so a steeper roof does not collect more.