What the calculator assumes
It counts parts for K-style metal gutters that are sold in sections. A run is one stretch of gutter with an end cap at each end. Every run you enter is the same length. If yours are different lengths, work out each one on its own and add up the parts.
How it works
- Gutter sections = run length ÷ section length, rounded up
- Hangers = run length ÷ hanger spacing, rounded up
- Downspouts = run length ÷ gutter per downspout, rounded up. Each one needs an outlet.
- Elbows = downspouts × elbows per downspout
- Downspout pipe = downspout height ÷ 10, rounded up, for each downspout
- Bands = 2 for each piece of pipe
- End caps = 2 for each run, one left and one right
- Seamers = sections − 1, plus 1 for each corner
- Drop = 1/4 inch for every 10 feet of gutter that drains to one downspout
Example: a house with a 40 foot gutter run on the front and another on the back. Sections are 40 ÷ 10 = 4 for each run, so 8. Hangers 24 inches apart are 480 ÷ 24 = 20 for each run, so 40. Downspouts are 40 ÷ 40 = 1 for each run, so 2 downspouts and 2 outlets. Elbows are 2 × 3 = 6. The ground is 10 feet down, so each downspout takes 1 piece of pipe and 2 bands, which makes 2 pieces and 4 bands. End caps are 2 × 2 = 4. Seamers are 4 − 1 = 3 for each run, so 6. Each run drops 40 ÷ 10 × 1/4 = 1 inch toward its downspout.
A 42 foot run would take 5 sections and 2 downspouts, because both numbers round up.
How many downspouts
The Amerimax planning chart counts one outlet for every 40 feet of gutter. Its advice page recommends one downspout for every 20 feet, and more for a large roof or heavy rain. Treat 40 as the most one downspout should handle, and change the number above to 20 if you want to follow the recommendation.
Size matters too. Amerimax lists a 2x3 inch downspout at 186 gallons per minute and a 3x4 inch downspout at 434. It suggests bigger downspouts, or more of them, for wide roofs, heavy downpours and gutters that overflow.
The drawing puts each downspout at the outer end of the stretch it drains. That is one layout that works. Put yours where the water can run away from the house.
Slope and hangers
A gutter is not hung level. Amerimax says to slope it at least 1/4 inch for every 10 feet, down toward the downspout. On a run with a downspout at each end, the high point is in the middle. Amerimax calls for one hanger every 24 inches, or every 18 inches where snow and ice are heavy.
What it leaves out
- Sizing. This is a parts list. It does not pick a gutter or downspout size for your roof area or rainfall.
- Sealant and screws. Metal gutters need sealant at the seams and end caps. Amerimax puts it at about 1/2 ounce for each seam.
- Something at the bottom of each downspout, such as a splash block or an extension, to carry water away from the foundation.
- Seamless gutters. Those are formed on site from one piece of metal and cut to length, so a run has no seamers.
Common questions
How many downspouts do I need?
The Amerimax planning chart uses one for every 40 feet of gutter, and its advice page recommends one for every 20 feet. Every run needs at least one. Add more for a large roof or heavy rain.
How far apart should gutter hangers be?
Amerimax calls for one hanger every 24 inches, or every 18 inches where snow and ice are heavy. A 40 foot run takes 20 hangers at 24 inches.
How much slope does a gutter need?
At least 1/4 inch for every 10 feet, sloped toward the downspout, according to Amerimax. A 40 foot run that drains to one end drops 1 inch.
How many elbows do I need for a downspout?
Usually 3. Two at the top carry the pipe from the gutter back to the wall, and one at the bottom turns the water away from the house. The Amerimax planning chart counts 3 for each downspout.
How long is a section of gutter?
Most gutter sections are sold 10 feet long. Amerimax says some are also sold in 16 foot, 21 foot and longer lengths. Seamless gutters are formed on site in one piece.
What size downspout do I need?
This calculator does not size them. As a guide, Amerimax lists a 2x3 inch downspout at 186 gallons per minute and a 3x4 inch downspout at 434. It suggests bigger or more downspouts for wide roofs, heavy downpours and gutters that overflow.
Tools for this job
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- Chalk line. Snaps the sloped line on the fascia that the gutter follows.
- Line level. Hangs on a string to set the fall toward the downspout.
- Aviation snips. Cut gutter sections and downspouts to length.
- Downspout crimper. Narrows the end of a downspout so it slides into the next piece.
- Rivet tool. Joins downspouts and elbows with rivets.
- Gutter sealant. Seals the seams, end caps and outlets.
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Sources
- Amerimax, Metal Gutter System planning chart and installation sheet
- Amerimax, Installing Metal Gutter Systems
- Amerimax How-To Center: gutter questions and answers
Facts on this page were last checked against these sources in October 2026.