Gutter & Downspout Calculator — m, 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 (125 mm for most houses, 150 mm for hip roofs over 140 m² or high-rainfall regions per EN 12056-3), then place one drop per 10 m of run.
Field rules — 1 downspout per 10 m, brackets at 600 mm o.c. (400 mm in snow regions), 2–3 elbows per drop (top offset + shoe), 3 m sections. Add 10% offcut on long or multi-corner runs.
Saved Calculations
| Time | Run m | Sections | DS | Brackets | Elbows | Cost |
|---|
How to Calculate Gutter & Downspout Materials (Metric)
Add up every eave that drains to a gutter; that combined run, with your section length, sets the gutter-piece count. EN 12056-3 sizes gutters and rainwater pipes to the catchment area and rainfall intensity, so confirm the contributing roof area with the roof area and pitch calculator before fixing downspout count. Typical practice: 125 mm half-round or K-style in 3 m lengths, brackets at 600 mm o.c. (400 mm in snow regions), one downspout per ~10 m of run.
How to use
Enter the total eave run in metres (or switch to cm/mm), choose gutter size, then set downspout count, elbows per drop, corners and bracket spacing. Sections and downspout pipes round up to whole 3 m lengths; allow 10% extra on long or multi-corner runs. The cost panel applies your local unit price per item in your chosen currency. For a full re-roof estimate, combine with the metal roofing calculator, the shingle calculator, or the flat-roof membrane calculator for low-slope sections.
Formulas
Gutter sections = ceil(run ÷ section length, 3 m std). Brackets = ceil(run × 1000 ÷ spacing in mm) + 1. Elbows = downspouts × elbows per drop. Downspout pipes = downspouts × ceil(drop height ÷ 3 m). Corners = inside + outside. Fascia timber behind the gutter can be quantified with the lumber volume calculator.
On install day
Three things estimators miss. Fall & outlet placement — EN 12056-3 specifies 3–5 mm/m fall to each outlet, so a 12 m single-slope run drops 36–60 mm; long eaves usually need a high point in the middle with two outlets, not a single 12 m fall. Brackets — rafter-fix brackets ($1.50–3 ea in 2025 EUR) hold seamless gutter through snow load; clip-on fascia brackets are quicker but creep under ice. Drop accessories — each downspout needs a stop-end with outlet (cut into the gutter), 2–3 elbows and a pipe clip per 1.5 m of pipe. PVC gutter runs €6–12/m installed DIY, €12–22/m installed, zinc €25–45/m, copper €55–90/m. On re-roofs, take off ridge and edge metal with the shingle calculator so flashings tie in before the gutter goes back up.
FAQ
How many downspouts? Roughly one per 10 m of gutter run. Add more for high-rainfall regions, long single runs, or steep roofs.
What size gutter? 125 mm handles most houses. Step up to 150 mm for large catchment areas, steep or hip roofs, or heavy rainfall.
Why round up to whole lengths? Gutter and downspout stock is sold in fixed 3 m lengths. Partial pieces still cost a full length, and the offcut absorbs waste at corners, outlets and joints.
