How this worksheet works
From eave runs to drainage pieces
Use this as a quantity check—not hydraulic sizing. Downspout count is estimated from total gutter length, maximum spacing, and the number of separate gutter runs.
How to use it
- Enter total gutter length and the number of separate runs.
- Set gutter stock length, maximum downspout spacing, vertical drop, downspout stock length, and waste.
- Review whole-piece quantities, then size the drainage system for local rainfall and roof catchment.
Useful for
- Straight-run gutter takeoffs
- Preliminary outlet and downspout counts
- Stock-length ordering
Assumptions
- Downspouts share a common drop length
- Each separate gutter run receives at least one downspout
- Waste applies to gutter and downspout linear quantities
Limitations
- Does not perform rainfall-intensity or catchment hydraulic sizing
- Elbows, offsets, end caps, miters, outlets, hangers, and leaders are not counted
- Roof geometry and discharge routing can require more downspouts
Common checks
Questions about this calculator
Is one downspout every 40 feet always enough?
No. Spacing is only a planning input. Required capacity depends on roof area, rainfall intensity, gutter size, outlet size, and layout.
Are fittings included?
No. Count elbows, outlets, end caps, miters, straps, hangers, and extensions from the final layout.