How the gutter calculator works
A gutter system is a kit of parts: straight lengths, joints, end caps, outlets, downspouts, elbows and a hanger every few hand-spans. Most shopping-list errors come from counting the gutter and forgetting everything that holds it up and drains it. Separate runs never share an outlet or a bracket, so the calculator rounds every count per run and then multiplies by the number of runs.
Downspouts = runs × ⌈run length ÷ max length per downspout⌉ · Hangers = runs × (⌈run length ÷ spacing⌉ + 1)Each run is divided into stock lengths, 4 m in much of Europe and 10 ft in the US, which also gives the number of joints. Hangers are spaced evenly along each run, with one extra at its far end. The fall is the length each outlet drains multiplied by the slope. Every downspout gets three elbows: two to swing the pipe from the outlet back to the wall and one at the bottom to throw water away from the foundation. Seamless gutters formed on site have no joints, so ignore that line if you order them.
Worked example
A gable house has two 12 m eaves, 24 m in total, with the gutter 5 m above ground. Each run needs three 4 m lengths, six in all, joined by four connectors and closed by four end caps. At one downspout per 10 m, a 12 m run needs ⌈1.2⌉ = 2 outlets, one at each end, so the house needs four, not the three that 24 ÷ 10 would suggest: a downspout on the front cannot drain the back. Brackets every 60 cm give 12 ÷ 0.6 = 20 plus one per run: 21 per run, 42 in all. Each outlet drains 6 m, so at 2 mm per metre the gutter falls 12 mm from the middle to each end. The four downspouts need 20 m of pipe and twelve elbows.
Common gutter and downspout sizes
| System | Gutter size | Matching downspout | Typical use |
|---|---|---|---|
| US K-style | 5 in | 2 × 3 in | Most houses |
| US K-style large | 6 in | 3 × 4 in | Big or steep roofs, heavy rain |
| Half-round (Europe) | 100 mm | 68 mm round | Sheds, small roofs |
| Half-round (Europe) | 112–125 mm | 68–80 mm round | Houses |
| Box or deep profile | 150 mm and up | 80–100 mm | Large roofs, commercial |
Bigger roofs and steeper pitches shed more water per metre of eave. Manufacturers publish the roof area each outlet can drain for your rainfall intensity; use those tables, or local plumbing rules, for anything larger than a typical house.
Setting the slope
A gutter that is dead level holds standing water, rots fascia boards and breeds mosquitoes; one that falls too steeply looks crooked from the street. Aim for about 2–5 mm per metre (¼–½ in per 10 ft). Snap a chalk line from the high end to the outlet, and on runs longer than about 12 m (40 ft) slope both halves down to outlets at each end instead of one long fall that would drop too far below the roof edge.
Frequent mistakes
- Too few downspouts. One outlet on a long run overflows in a downpour however clean the gutter is.
- Brackets too far apart. Wide spacing lets the gutter sag under snow, ice or wet leaves.
- Discharging at the wall. Lead water at least 1–1.5 m (3–5 ft) away from the foundation or into a drain or soakaway approved locally.
- Working off a ladder alone. Use a stabiliser or scaffold tower and keep clear of overhead power lines.
Frequently asked questions
How many downspouts do I need for 40 feet of gutter?
Plan one downspout for every 30–40 ft of gutter, so a 40 ft run needs at least one, and two (one at each end) is better in heavy-rain areas.
How much slope should a gutter have?
About ¼ in per 10 ft (2 mm per metre) is the usual minimum, up to ½ in per 10 ft (4–5 mm per metre). A 12 m run at 2 mm/m drops 24 mm.
How far apart should gutter hangers be?
Every 24 in (60 cm) is the common spacing for hidden hangers; up to 36 in (90 cm) in mild climates, and closer where heavy snow sits on the roof.
How many gutter sections do I need for a 12 m wall?
With 4 m lengths you need three, joined by two connectors, plus two end caps and at least one outlet.
What size gutter do I need, 5 or 6 inch?
A 5 in K-style gutter suits most houses. Choose 6 in for large roof planes, steep pitches or regions with intense rain, paired with 3 × 4 in downspouts.
Last reviewed: 2026-09-27 · How we calculate