Can I use this without the roof calculator?
Yes. Switch to manual values and enter eave length and roof area directly.
Gutter and Downspout Calculator
Gutter linear footage and downspout count from eave length and roof area.
You'll need — ft of gutter and — downspouts.
Roof data source
Pull eave length and roof area from the roof calculator, or override manually.
Chained roof data
Run the roof calculator first to prefill eave length and roof area.
Gutter and downspout inputs
Drop height, waste percent, downspout coverage and hanger spacing.
You'll need — ft of gutter and — downspouts.
You'll need
Gutter and downspout estimate
Warnings
- INFO No roof data is saved in the active project yet, so manual values are being used.
- ERROR Enter an eave/fascia length greater than zero.
- ERROR Enter a roof area greater than zero.
Detail values
- Eave length used
- — ft
- Roof area used
- — sq ft
- Gutter hangers
- — pieces
- Downspout pipe needed
- — ft
Assumptions
- Downspout coverage: 600 sq ft per downspout -- an estimating rule of thumb, not a rainfall-based drainage sizing
- Hanger spacing: 24 in on centre -- a manufacturer figure, not code; tighten to 16-18 in under snow load
- Estimated waste: 5% for an average project -- applied to the gutter length only; the downspout sections already carry their own offcut
Included: gutter, gutter hangers, downspouts, downspout pipe.
Not included: labor, delivery, permits, disposal of waste material, downspout elbows, outlets, end caps, sealant and fasteners.
Formula details
- Gutter length: fasciaLength x (1 + waste)
- Downspout count: ceil(roofArea / downspoutCoverage)
The 600 sq ft/downspout default is an estimating rule of thumb for a 2x3 in downspout with a 5 in K-style gutter. No standards body publishes it, and it is NOT a drainage calculation: sizing conductors and leaders to code is IPC 1106, which works from the site's 100-year hourly rainfall rate and the horizontally projected roof area, and this screen never asks for a rainfall rate. Set the coverage to whatever your own sizing calls for.
- Gutter hangers: ceil(eaveLength / hangerSpacing)
Hanger spacing is a manufacturer instruction, not a code requirement, and there is no single figure to cite. 24 in on centre is what most hanger packaging prints; 16-18 in is common practice where snow and ice load the gutter. Set it to what your hanger's instructions say. Counts hangers along the eave only -- no end hanger per run, because the number of runs is not asked for here.
- Downspout pipe: downspoutCount x dropHeight, bought in 10 ft sections
Downspout is stocked in 10 ft sections and crimps end to end, so the drops are planned as one joinable run (units/stock-lengths.ts). Assumes every drop falls the same height. Elbows and the offset from fascia to wall are not counted.
Material List
About this calculator
Pull eave length and roof area from the active project's roof result, or override manually.
FAQ
Where does the 600 sq ft downspout default come from?
It's the standard trade rule of thumb pairing a 2x3 in rectangular downspout with a 5 in K-style gutter, consistent with SMACNA-aligned sizing guidance. It's editable if your downspout size differs.
Does this size the gutter itself?
This estimates linear footage and downspout count, not gutter capacity/sizing by rainfall intensity -- that's a separate SMACNA-level engineering calculation outside this tool's scope.