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, or solve downspout count and spacing per run.
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. A separate multi-plane mode lets up to six roof planes, each with its own pitch factor, feed into as many as six labeled gutter runs, and an editable downspout solver turns any run's design flow into a downspout count and maximum even spacing. 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 — use the optional multi-plane rows when different planes feed different runs or carry different pitches.
- 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
Solve downspout count and spacing
Enter or accept the editable per-downspout capacity, then use the solver's downspout count and maximum even spacing as a planning starting point for outlet layout, not a final design.
- 05
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.
Scope and safety note
Preliminary planning screen only. It does not certify hydraulic capacity, code compliance, universal rainfall intensity, outlet spacing, overflow, underground drainage, or lawful discharge. Have the complete roof-to-outlet water path verified for the site and selected products.
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 and test check recorded July 15, 2026. What this review covers
Formula version 2026-07-17.1
No named qualified reviewer is recorded for this calculator. Treat the result as planning support and obtain the professional or code review identified in its safety note.
- 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 - the I-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. In multi-plane mode, each row's area (or length times width) is first multiplied by that row's own pitch factor, and rows sharing a run label are summed before the same conversion is applied to that run's total.
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.
Are the downspout solver's default capacity and elbow/bracket counts authoritative?
No. The default per-downspout capacity uses a common rule of thumb of about 600 sq ft of roof per downspout at a 1 in/hr design rainfall, converted with the same gallons-per-minute formula used elsewhere on this page, and the elbow and bracket counts use common install assumptions (a typical wall-offset elbow pair and a minimum bracket count). All three are editable planning values, not manufacturer specs or code minimums — replace them with documented capacities and the real wall-offset geometry when available.
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.