Roofing planning tool

Gutter Size Calculator

Planning arithmetic only — not a gutter/downspout design or code result. This tool converts the tributary catchment area, runoff coefficient, design rainfall intensity, and per-unit capacities you enter into a peak design flow and a whole capacity-unit count. It does not look up or approve rainfall, survey roof geometry, choose a runoff coefficient, verify manufacturer capacity, or design gutter profile/slope/length, outlets, downspout spacing, elbows, restrictions, fasteners, debris/ice allowance, maintenance, primary/secondary overflow, ponding or structural rain load, underground piping, detention/infiltration, grading, erosion control, or discharge. Verify local code and stormwater/property rules, approved discharge location, foundation and neighboring-property impacts, manufacturer data, and the complete route to a lawful receiving point. Use a qualified drainage/building professional and the authority having jurisdiction for consequential work.

Explicit drainage arithmetic

Compare entered gutter-system capacities

Unit systemChanges all displayed and submitted area, rainfall, and capacity units together; it does not convert entered values.

Catchment entry methodUse a verified tributary area or itemize only explicit plan rectangles. No slope or roof geometry is inferred.

Verified tributary catchment area (ft²)Enter the verified horizontal tributary area; no roof dimensions are inferred.

These are the horizontal tributary catchment areas feeding the capacity point I am evaluating. The calculator does not infer roof planes, pitch, valleys, walls, or routing.Required. Do not apply a pitch factor or guess tributary areas; obtain qualified field review when they are unknown.

Explicit runoff coefficient C (0–1)Enter the coefficient from your identified project criterion. No coefficient is supplied or recommended.

Explicit rainfall intensity (in/hr)Enter a project-specific intensity in in/hr from an identified source. The tool performs no rainfall lookup and supplies no default.

Rainfall intensity source note (optional)Record the source, location criterion, duration, and return period used for the entered in/hr value.

I entered the design intensity and units from a source appropriate to the project location, storm duration/recurrence criterion, and applicable local requirements. This calculator does not look up or approve rainfall.Required provenance reminder. Record the source and criteria above; the calculator does not verify the source or intensity.

Candidate capacities

Enter each independently verified per-unit capacity for the exact component or system conditions. No product, gutter size, or installed count is recommended.

Candidate capacity 1

Stable candidate IDRequired unique ID used to preserve this candidate in the calculation record.

Candidate labelRequired descriptive label for the independently verified capacity.

Candidate capacity typeClassifies the entered capacity only; it does not select or approve a component.

Verified capacity per unit (gpm)Enter the independently sourced per-unit capacity in gpm for the exact conditions being evaluated.

Capacity source note (optional)Record the manufacturer table, test, calculation, conditions, or other identified source.

Required: at least one and up to 50 independently sourced candidates.

Plan responsibly

Before You Build

This project may be subject to local permit, zoning, site, or structural requirements.

Permit and drainage reviewConfirm permits, inspections, and drainage review required by the authority having jurisdiction.
Property boundariesVerify that routing and discharge do not create prohibited impacts across a property boundary.
Easements and discharge rightsConfirm easements, drainage rights, and lawful discharge locations before selecting an outlet route.
Local rainfall and amendmentsUse locally accepted rainfall data, runoff criteria, amendments, and approved product capacities.

Entered-capacity arithmetic only: This calculator does not design gutter geometry, downspout routing, overflow, underground drainage, detention, erosion control, or lawful discharge.

Verify every input: Rainfall intensity, runoff coefficient, catchment area, and candidate capacity must come from accepted project evidence; none is inferred.

View code references

Model-code reference

2024 IPC, Section Chapter 11

Storm drainage: Model-code storm-drainage provisions do not turn entered flow and capacity arithmetic into a complete roof-drainage design.

Source: ICC — 2024 IPC Chapter 11 Storm Drainage · Reviewed 2026-08-24

Local requirements may differ. A model-code reference does not confirm the code edition, amendments, zoning rules, or permit requirements adopted where the project is located.

Model-code references

2024 IPC, Section Chapter 11: Storm drainage. Source: ICC — 2024 IPC Chapter 11 Storm Drainage. Reviewed 2026-08-24.

Local requirements may differ. Verify the adopted code, amendments, zoning rules, and permit requirements with the local authority.

Methodology

How we calculate this

Uses only explicit user-entered tributary catchment, coefficient, rainfall intensity, and candidate capacity. Candidates are independent; no geometry, rainfall, coefficient, or product capacity is inferred.

US Q = C × A_ft² × I_in/hr × 4/385; metric Q = C × A_m² × I_mm/hr ÷ 60; count = exact ceil(Q / entered capacity)

Entered evidence only

Enter a verified capacity for exact component/system conditions. The calculator supplies no candidate capacities and reports only capacity units needed at the entered per-unit capacity.

Professional boundary

Worked examples