Gutter Size Calculator: Drainage Area and Flow

How to use the gutter size calculator

1. Enter the plan length and width of the roof area that drains to this gutter run — the horizontal footprint, not the sloped surface area.
2. Enter the pitch (inches of rise in 12 inches of run). The pitch factor is the same This Old House factor used on the gutter installation cost calculator: 1.00 below 4:12, 1.05 at 4:12, 1.10 at 6:12, 1.20 at 9:12 and 1.30 at 12:12.
3. Enter the design rainfall in inches per hour. The default 1 in/h is a CalcTheJob starting point; use your local 100-year, 1-hour rate where the plumbing code requires it.

Compare the IPC flow (or the conservative pitch-factor figure) with the manufacturer’s or SMACNA published capacity for the gutter profile you will actually install. Do not use the commonly copied “5,520 / 7,960 sq ft” 5-inch and 6-inch K-style figures — those numbers have been traced to a mis-mapped half-round chart.

IPC flow conversion and pitch factor

International Plumbing Code Equation 11-1 converts rainfall on a roof to flow: Q (gpm) = 0.0104 × roof area (sq ft) × rainfall intensity (in/h). One inch of rain per hour on one square foot is about 0.0104 gallons per minute. IPC Equation 11-1 uses that horizontal plan area. This page also multiplies plan area by the This Old House pitch factor and reports a higher effective-area flow. That pitch factor is a CalcTheJob conservative add-on, not part of the IPC method.

A worked example: 40×20 ft roof at 6:12

A 40 × 20 ft plan area is 800 sq ft. By IPC Equation 11-1 at 1 inch per hour the flow is 0.0104 × 800 × 1 = 8.32 gpm. At 6:12 the This Old House pitch factor is 1.10, so the conservative effective area is 880 sq ft and the add-on flow is 0.0104 × 880 × 1 = 9.15 gpm. Raise the rainfall to your local design rate to scale both figures.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• IPC Equation 11-1 flow is Q (gpm) = 0.0104 × plan (horizontal) roof area (sq ft) × design rainfall (in/h). The 0.0104 factor is 1 in/h over 1 sq ft ≈ 0.0104 gpm.
• A second, conservative figure multiplies plan area by the This Old House pitch factor already used on the gutter installation cost calculator (1.00 below 4:12, 1.05 at 4:12, 1.10 at 6:12, 1.20 at 9:12, 1.30 at 12:12). That pitch factor is a CalcTheJob add-on, not part of the IPC equation.
• The default 1 in/h rainfall is a CalcTheJob starting point. Use the local 100-year, 1-hour rate where the plumbing code requires it.
• This page does not recommend a 5-inch or 6-inch K-style section from an unverified capacity chart (including the commonly copied 5,520 / 7,960 sq ft figures). Downspout count and slope drop stay on the gutter installation cost calculator.

Sources

1. International Plumbing Code, Maryland adoption via UpCodes, IPC 2024 §1106.2.1 Rainfall Rate Conversion Method (Equation 11-1). Requires converting roof rainfall to a gallon-per-minute flow with Equation 11-1, where R is rainfall intensity in inches per hour and A is roof area in square feet.
2. Harth, Storm Drainage Sizing: Roof Drains, Leaders and Secondary Drains (IPC Chapter 11). States Equation 11-1 in text as gpm = 0.0104 × rainfall (in/h) × roof area (sq ft), which is the coefficient used here.
3. This Old House, Gutter Installation Cost. Source of the pitch factor already used on the gutter installation cost calculator and reused here for effective drainage area.

Last reviewed November 12, 2026. The manufacturer’s or SMACNA published capacity for the gutter you install, and the local rainfall rate, win.