Rain Gutter Calculator: Gutter Sections, Downspouts, Hangers, Fittings and Cost
Make a full gutter materials list from your eave length: gutter sections with waste, downspouts and pipe, elbows, hangers, end caps, outlets, connectors and corners, the slope to build in, a roof-area downspout check and an optional cost estimate.
How it works
Downspouts = max(⌈eave length ÷ spacing⌉, number of runs, ⌈roof area ÷ area per downspout⌉) · Sections = ⌈eave length × (1 + waste) ÷ section length⌉ · Hangers per run = ⌈(run length − 24 in) ÷ hanger spacing⌉ + 1 (first and last 12 in from the ends) · Elbows = downspouts × elbows each · End caps = 2 × runs · Connectors = ⌈eave length ÷ section length⌉ − runs · Downspout pipe = downspouts × height · Fall = 1/4 in per 10 ft of run
The gutter length is the total eave you are protecting. Downspouts go about every 30–40 ft of gutter, with at least one on every separate run; if you enter the roof area, the calculator also checks the common sizing rule of about 600 sq ft of roof per 2×3 in downspout (1,200 sq ft per 3×4 in) and adds downspouts when the roof is larger than the spacing allows. Sectional gutter comes in 10 ft or 16 ft lengths (3 m or 4 m in metric markets), so the length plus a small waste allowance is divided by the section length and rounded up; a connector joins each pair of sections in a run. Seamless gutter is formed on site in one piece per run. Each run needs a left and a right end cap, each downspout a drop outlet, and usually three elbows: two at the top to bring the pipe back to the wall under the eave overhang and one at the bottom to turn water away. Hangers go every 24 in, closer (12–18 in) where snow and ice load the gutter, with the first and last about 12 in from the ends of each run, which works out to about one hanger per 2 ft. Downspout pipe is sold in 10 ft lengths and strapped to the wall twice per length. Slope each run about 1/4 in per 10 ft toward its downspout. Runs over 40 ft are pitched from a high point in the middle to a downspout at each end. Quantities are planning estimates; manufacturers’ instructions and local codes govern.
Worked example
With the defaults, 120 ft of eave in 2 runs with a downspout every 35 ft needs ⌈120 ÷ 35⌉ = 4 downspouts. With 5% waste, 120 × 1.05 = 126 ft of gutter is 13 sections of 10 ft. Hangers every 24 in, starting 12 in from each end: (720 − 24) ÷ 24 = 29 spaces + 1 = 30 per run, 60 in all. Elbows: 4 × 3 = 12. End caps: 4, drop outlets: 4, connectors: 12 − 2 = 10. With a 10 ft drop the downspouts take 4 lengths of 10 ft pipe and 8 straps. Each 60 ft run is over 40 ft, so it is pitched from the middle: 1/4 in per 10 ft over 30 ft = 0.75 in of fall.
Sources: Lowe’s gutter installation guide (hangers every 24 in or 18 in for snow, 1/4 in per 10 ft slope, 2 straps per 10 ft downspout) · RONA: Install a PVC gutter (downspouts at 40 ft intervals, runs over 40 ft pitched from the center, 2 elbows per downspout) · Home Depot gutter installation guide (quantity chart: 50 ft run = 5 lengths, 4 joiners, 25 hooks, 3 elbows per downspout) · BuildingClub: Gutter and downspout calculator (sections, elbows 2–3 per downspout, end caps 2 per run)
Frequently asked questions
How far apart should downspouts be?
One downspout every 30–40 ft of gutter is the common rule, with at least one per continuous run and one at each end of a long run. The roof area matters too: a 2×3 in downspout drains about 600 sq ft of roof and a 3×4 in about 1,200 sq ft by the usual sizing rule.
How many gutter hangers do I need?
One every 24 in is standard, with the first and last about 12 in from the ends of each run, so about one hanger per 2 ft of gutter: 25 for a 50 ft run. In snow and ice country use 18 in or even 12 in spacing, and follow the spacing in your gutter manufacturer’s instructions.
How many elbows does each downspout need?
Usually three: two at the top to offset the pipe from the gutter outlet back to the wall under the eave overhang, and one at the bottom to send water away from the foundation. A gutter close to the wall may need only two.
How much should gutters slope?
About 1/4 in of fall per 10 ft of run toward the downspout. Long runs, over about 40 ft, are usually pitched from a high point in the middle down to a downspout at each end.
Should I choose seamless or sectional gutters?
Seamless gutters are formed on site in one piece per run, so they have no joints to leak but need a contractor with a forming machine. Sectional gutters come in 10 ft or 16 ft lengths that a DIYer can install, joined with connectors at each seam.
What size gutter do I need?
5 in K-style gutters with 2×3 in downspouts suit most houses. Use 6 in gutters with 3×4 in downspouts for large or steep roofs, long runs or regions with intense storms. Check the sizing with the gutter capacity calculator.
Related calculators
Estimate for planning your project. Add a margin for waste and confirm with your supplier’s specifications and local building code.