Free estimate — verify against local code before building
Gutter Size Calculator
Turn roof catchment and local rainfall into design flow, then compare it with editable gutter and downspout capacities.
What this calculator includes
Use contributing horizontal roof area and a locally verified design rainfall intensity to estimate runoff in gallons per minute. An editable roof factor and planning allowance stay visible, and the result compares that flow with user-entered capacities for three nominal gutter sizes and the proposed downspouts. This is a preliminary hydraulic screen, not the existing gutter inventory calculator and not approval of a product, outlet layout, or discharge design.
Next step in your project
Carry the water path to a verified outlet
Calculate fall, grade, sloped length, and pipe stock for the project-specified drainage run without assuming a universal slope.
Open Drainage Slope Calculator →How to use this gutter size calculator
- 01
Assign the contributing roof
Enter only the horizontal plan area draining to the gutter run being evaluated. Complex roofs need each plane assigned to its actual outlet path.
- 02
Verify local design rainfall
Use a rainfall intensity and storm duration accepted for the project location and design method, not a generic national default.
- 03
Enter documented capacities
Replace the example gutter and downspout capacities with values from an approved design method or the exact manufacturer and installation configuration.
- 04
Continue to the inventory
After qualified sizing and outlet review, use the Gutter & Downspout Calculator to count stock pieces, hangers, fittings, elbows, straps, and extensions.
Worked example
Example: 1,200 sq ft catchment
A 1,200-square-foot horizontal catchment at 3 inches per hour creates about 37.4 gpm before factors. A 1.1 roof factor and 10% planning allowance produce about 45.2 gpm, which is compared with the capacities entered for the gutter profile and proposed downspouts.
Practical buying and overage guidance
Do not buy from nominal size alone. Confirm profile capacity, gutter slope, outlet type and spacing, downspout geometry, hangers, expansion, overflow path, roof-edge condition, compatible metals, local rainfall method, final discharge, and supplier lead times. After sizing, create a separate stock-piece and fitting inventory.
Continue the project
Upgrade Roof Drainage
Plan roof area, gutter runs, downspouts, fittings, extensions, drainage stone, grading, excavation, supplies, and contractor pricing.
Open the project workflow →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 Atlas 14 precipitation frequency estimates (opens in a new tab)
Source for location- and duration-specific precipitation-frequency information in covered U.S. regions.
- SMACNA - technical standards (opens in a new tab)
Industry standards and manuals for sheet-metal and roof-drainage work; access may require purchase.
- International Code Council - model codes (opens in a new tab)
Model-code resources; the adopted local edition and amendments must be verified.
Frequently asked questions
How is roof runoff converted to gallons per minute?
One inch of rain over one square foot in one hour is about 0.01039 gallons per minute. The calculator multiplies horizontal catchment area, rainfall intensity, the entered roof factor, and the planning allowance.
Does a 6-inch gutter always carry more water than a 5-inch gutter?
Usually, but capacity also depends on profile, slope, outlet geometry, downspouts, seams, obstructions, installation, and the design method. Enter documented capacities rather than relying on nominal size alone.
Where do I find local rainfall intensity?
Use the source and duration required by the project authority or qualified designer. NOAA precipitation-frequency data can inform U.S. planning, but it does not by itself select the code design value or method.
Why can the result say professional design instead of a gutter size?
The entered design flow exceeds the largest comparison capacity. That is a deliberate stop, not permission to use the largest size; the roof may need more outlets, divided catchments, a different profile, or engineered drainage.
Does this calculator size underground drainage or the final outlet?
No. Downstream pipe, leaders, inlets, grading, erosion, detention, legal discharge, overflow, and foundation clearance are site-specific parts of the complete water path.