Gutter and downspout calculator
Roof plan area, local rainfall intensity and gutter length: you get the number of downspouts, their size, gutter size and rainwater flow.
Downspouts
1 unit
- Downspouts1 unit
- Downspout diameter63 mm Ø
- Half-round gutter size (1% slope)150 mm Ø
- Rainwater flow2.78 l/s
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How it's calculated (visible formula)
equivalent area = plan area × intensity ÷ 100 mm/h; downspouts = ceil(gutter length ÷ 20 m); size by area served
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Measure roof plan area draining to each gutter.
- Find your design rainfall intensity.
- Correct the area for that intensity.
- Split between downspouts (one per 30–40 ft of gutter).
- Size downspouts and gutters from the tables.
Worked example: 1,076 sq ft plan area, 4 in/h (100 mm/h), 66 ft of gutter
- Equivalent area: 100 m²
- 1 downspout (20 m ÷ 20)
- 100 m² per downspout
→ A 63 mm (2½ in) round downspout and a 6 in (150 mm) half-round gutter.
Common mistakes
- Using sloped roof area instead of plan area.
- Not correcting for local rainfall intensity.
- Downspouts more than 40 ft apart.
Pro tips
- Slope gutters about ¼ in per 10 ft toward the outlet.
- Add guards where trees drop leaves.
- Discharge downspouts away from the foundation.
- Add an overflow on concealed gutters.
From calculation to quote
Add labor, overhead and your margin, and generate the professional PDF.
FAQs
- What rainfall intensity should I use?
- Your local 5-minute or design storm (IPC Figure 1106.1 in the US).
- How many downspouts?
- Roughly one per 30–40 ft of gutter and per 600–800 sq ft of roof.
- What slope for gutters?
- About ¼ in per 10 ft (0.2–1%).
- Vinyl, aluminum or copper gutters?
- Vinyl is cheapest; aluminum is the common middle; copper lasts longest.
- How far apart are gutter hangers?
- Every 24–36 in, closer in snow country.
- Can one roof drain onto another?
- Only if the lower roof and gutter are sized for the extra flow.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.