Global Inputs
Assumed Capacities (library-driven, editable)
Gutter capacity is maximum recommended flow per continuous run (GPM). Downspout capacity is per downspout (GPM). Switch libraries or edit values below.
K-Style 4″
K-Style 5″
K-Style 6″
K-Style 7″
Half-Round 5″
Half-Round 6″
Half-Round 7″
Box 4″
Box 5″
Box 6″
Downspout 2×3″
Downspout 3×4″
Downspout Round 4″
Downspout Round 5″
Downspout Mix Optimizer
Allowed Downspout Types
Costs are relative (materials + labor index). Use them to bias the optimizer toward preferred types.
Roof Sections (per continuous gutter run)
| # | Run Name | Run Length (ft or m) | Plan Area (ft² or m²) | Pitch (rise/run) | Profile | Actions |
|---|
Example rows are pre-loaded below. Edit or remove as needed.
Results
| Run | Q Required | Recommended Gutter | Downspouts | Total # | Spacing | Headroom |
|---|---|---|---|---|---|---|
| Total | — | — | — | — | — | |
Formula Used
Rational Method: The peak runoff is estimated as Q = C × i × A where C is the runoff coefficient (dimensionless), i is rainfall intensity, and A is contributing area.
- US units: Qgpm = 0.0104 × C × iin/hr × Aft². (Derived from 1 in/hr over 1 acre ≈ 1.008 cfs; 1 cfs = 448.83 gpm.)
- Metric: QL/s = (C × imm/hr × Am²)/3.6; convert to gpm via 1 L/s ≈ 15.8503 gpm.
- Sloped area option: If enabled, the plan area is multiplied by pitch factor = √(1 + (rise/run)²).
- Safety factor: The calculated flow is multiplied by your chosen multiplier to provide headroom for short-term peaks and partial blockages.
Selection Logic: The smallest gutter size whose assumed capacity (GPM) is ≥ required flow is chosen. With the optimizer OFF, a single downspout type is used. With the optimizer ON, a mix of allowed types is selected to meet capacity per your goal.
These are planning approximations for preliminary sizing. Always verify with local code, manufacturer charts, and site-specific constraints (valleys, scuppers, leaf guards, outlet positions).
How to Use
- Pick units, set intensity, C, safety, and whether to use sloped area.
- Use the apply preset link to select or add rainfall presets; they persist in your browser.
- Select a capacity library to populate default gutter/downspout capacities, or import your own JSON from supplier charts.
- Enable the optimizer and choose allowed downspout types and an optimization goal to let the tool mix sizes intelligently.
- Enter roof runs, calculate, then export CSV or PDF.
FAQs
Use your jurisdiction’s design storm (return period and duration) from IDF/IFD curves. The preset manager lets you save local values so your team can reuse them.
The rational method traditionally uses plan (horizontal) area. This tool can multiply by pitch to approximate sloped surface effects when desired.
Capacities vary by profile shape, slope, outlets, and debris screens. Use library JSON to match your supplier’s data. The defaults are conservative planning values.
Space them evenly along each run, biasing toward valley outlets and low points. For long eaves, consider both corners or a mid-run drop to shorten spacing.
Yes. Turn on the optimizer and choose allowed sizes. The solver finds a combination that meets flow using either the fewest downspouts or lowest cost.