Free estimate — verify against local code before building
Rain Gutter and Downspout Calculator
Plan every gutter, fitting, downspout, and small-supply quantity, then audit how the amount was calculated.
What this calculator includes
Create a transparent gutter and downspout shopping takeoff from repeated roof runs or aggregate measurements. Enter roof geometry, pitch, your verified local rainfall intensity, gutter stock, fittings, hanger spacing, outlet layout, downspout height, discharge choice, and current prices. The estimate shows every geometry, allowance, and stock-rounding step. It does not certify hydraulic capacity or replace local-code, drainage, manufacturer, or professional review.
How to use this gutter & downspout calculator
- 01
Measure each drained roof section
For equal sections, enter the number of runs, eave length per run, and horizontal ridge-to-eave depth. For unequal or already measured work, enter combined eave length and horizontal plan drainage area. Split complex roofs at independent outlets and low points for final review.
- 02
Confirm pitch and local rainfall
Enter the roof rise per 12 inches of horizontal run or a project-approved custom factor. Look up and verify the rainfall duration and recurrence interval required by the local authority; the site does not infer rainfall from a ZIP code.
- 03
Build the shopping layout
Choose the actual gutter material, nominal size, stock length, and hanger spacing. Count corners and end caps, then use spacing mode for a preliminary outlet count or override it with the approved outlet/downspout layout.
- 04
Complete sizing and discharge review
Before purchase, verify gutter profile and slope, outlet and downspout capacity, expansion limits, fastening substrate, overflow path, discharge location, erosion, foundation clearance, underground connections, snow and ice exposure, and locally adopted requirements.
Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- NOAA/NWS Precipitation Frequency Data Server (opens in a new tab)
Location-specific precipitation-frequency estimates and intensity data; the calculator does not auto-localize this input.
- SMACNA Downspout and Gutter Sizing Calculator (opens in a new tab)
Recognized sizing workflow and cautions based on the Architectural Sheet Metal Manual; use it or an approved equivalent for final capacity sizing.
- 2024 International Residential Code, Chapter 9 - Roof Assemblies (opens in a new tab)
Roof drainage and secondary overflow context; locally adopted editions and amendments control.
- 2024 International Residential Code, P2912.5 - Gutters and Downspouts (opens in a new tab)
Material compatibility, watertight joints, continuous drainage, and discharge context for rainwater collection systems.
Frequently asked questions
Does this calculator tell me whether a 5-inch or 6-inch gutter is large enough?
No. The selected size describes the shopping list. Final capacity depends on local design rainfall, drained geometry, gutter profile and slope, outlet size, downspout shape and count, section length, walls and valleys, low points, overflow provisions, and the approved sizing method.
Where should the rainfall intensity come from?
Use the source and storm duration/recurrence interval required by the local authority or designer. NOAA's Precipitation Frequency Data Server is a primary U.S. source. The calculator keeps the value editable and visible because a generic national default is not a safe final design assumption.
Why are gutter pieces rounded separately for repeated runs?
A leftover from one eave is not assumed to span another eave. Each repeated run receives its own cut allowance and whole-stock rounding, which is more conservative than combining all linear feet into one idealized cut plan.
Are the hanger, elbow, strap, sealant, and screw counts exact?
They are transparent planning allowances. Manufacturer instructions, fascia and wall substrate, wind, snow and ice, expansion, actual offsets, joint design, and locally adopted requirements determine the final products, spacing, and fastening pattern.