Gutter & Downspout Calculator — ft, cost
How to use this calculator
Run a tape along every eave that drains to a gutter and add the totals — that combined run drives every other line item on the takeoff. Pick the gutter size that matches your roof catchment (5″ K-style for most homes, 6″ for hip roofs over 1,500 ft² or heavy-rain regions), then place one drop per 35 ft of run.
Field rules — 1 downspout per 35 ft, hangers at 24″ o.c. (tighten to 12″ under snow/ice), 2–3 elbows per drop (top offset + shoe), 10 ft sections. Add 10% offcut on long or multi-corner runs.
Saved Calculations
| Time | Run ft | Sections | DS | Hangers | Elbows | Cost |
|---|
How to Calculate Gutter & Downspout Materials
Measure every eave that drains to a gutter and add the runs together — that total length, plus your section size, drives the gutter-piece count. Most contractors run K-style aluminium in 10 ft sticks, set hangers at 24″ o.c. (tighten to 12″ under snow or ice-dam loads), and place one downspout per 35 ft of run. Drop-outlet and corner locations come from the roof plan, so size your roof area and pitch first to confirm drainage capacity before you order.
How to use
Enter the combined eave run in feet and inches, pick gutter size and style, then set downspout count, elbows per drop, corners and hanger spacing. Sections are rounded up to whole sticks — carry a 10% offcut allowance on long, multi-corner runs. The cost panel multiplies each line item by your local unit price for a quick material budget; pair it with the shingle calculator and metal roofing calculator when scoping a full re-roof, or the flat-roof membrane calculator for low-slope sections that drain to internal gutters.
Formulas
Gutter sections = ceil(run ÷ section length). Hangers = ceil(run × 12 ÷ spacing in inches) + 1. Elbows = downspouts × elbows per drop. Downspout sticks = downspouts × ceil(drop height ÷ 10 ft). Corners = inside + outside. End caps are added per open gutter end. Fascia board behind the gutter can be sized with the board-foot lumber calculator.
On install day
Three things estimators miss. Slope & outlet placement — gutters fall 1/4″ per 10 ft toward each drop, so a 40 ft single-slope run drops 1″ over the run; long eaves usually need a high point in the middle and two outlets, not a single 40 ft slope. Hidden hangers — quote screw-in hidden hangers with built-in screws (~$1.50–3 ea in 2025); spike-and-ferrule is cheaper but loosens in 5–10 yr cycles. Drop accessories — each downspout needs a drop outlet (cut into the gutter) plus 2–3 elbows and a band/strap per 10 ft drop run, per IRC R903.4. Sectional aluminium runs $4–8/lf installed DIY, $8–15/lf contractor; seamless 5″ K-style is $5–12/lf, 6″ $7–15/lf, copper $25–40/lf. Order ridge/edge metal with the shingle calculator on re-roofs so flashings tie in before the gutter goes back up.
FAQ
How many downspouts do I need? One downspout per 35 linear feet of gutter is a sound rule. Add more for heavy-rainfall regions, long single runs, or steep roofs that shed water fast.
What size gutter should I use? 5″ K-style handles most homes. Step up to 6″ when more than 1,500 ft² of roof drains to one run, on steep or hip roofs, or in high-rainfall regions.
Why round sections up? Gutter and downspout stock is sold in fixed lengths (10 ft here). Partial sticks still cost a full piece, and the offcut covers waste at corners, outlets and miters.
