Gutter Size Calculator
Gutters and downspouts are sized for the flow of water a roof sheds in a heavy storm. This calculator follows the building-code method: take the horizontal roof area draining into the gutter, multiply by a pitch factor that accounts for steep slopes catching more wind-driven rain, then multiply by the peak rainfall intensity for your location to get the required flow. Rainfall intensity is a regional value, so it is an editable input you take from NOAA Atlas 14. The result tells you the gallons per minute your gutter and downspout must carry.
Gutter sizing formula
Effective area = roof footprint area * pitch factor
Total flow (gal/min) = effective area * rainfall intensity * 0.0104
Flow per downspout = total flow / number of downspouts
Flow (cu ft/min) = total flow / 7.48052
The constant 0.0104 converts square feet times inches per hour into gallons per minute. The factor 7.48052 converts gallons to cubic feet. Distributing total flow across downspouts shows the load each outlet must handle.
Gutter sizing context
- Use the horizontal roof footprint, not the sloped surface area, then apply a pitch factor.
- Rainfall intensity should be the design storm value for your area from NOAA Atlas 14.
- More downspouts reduce the flow each must carry and the gutter cross-section required.
- Building and plumbing codes set pitch factors and minimum sizes; confirm your local code.
- Undersized gutters overflow in heavy rain, risking foundation and fascia damage.
Gutter size: frequently asked questions
How do I size a gutter?
Gutter sizing starts with the effective roof drainage area, which is the horizontal roof footprint draining to the gutter multiplied by a pitch factor for steeper roofs. Multiply that area by the local peak rainfall intensity to get the flow the gutter must carry, then choose a gutter and downspout rated for that flow.
What rainfall intensity should I use?
Use the peak 5-minute rainfall intensity for a chosen return period from NOAA Atlas 14 for your location. Rainfall intensity is a regional empirical value, so this calculator takes it as an editable input rather than assuming a figure that would be wrong outside your area.
What is the roof pitch factor?
A steeper roof catches more wind-driven rain than its flat footprint suggests, so building codes apply a multiplier. For example, a low slope uses a factor near 1.0 and a steep slope uses a larger factor. Enter the pitch factor from your local code; the default 1.0 represents a flat or nearly flat roof footprint.
How is required flow converted to gallons per minute?
Effective area in square feet times rainfall intensity in inches per hour gives a volume rate. Converting inches per hour over square feet to gallons per minute uses the factor 0.0104, derived from unit conversion (1 inch over 1 square foot per hour equals about 0.0104 gallons per minute).
Are these formulas standard?
Yes. The effective-area and flow method follows the approach in plumbing and building drainage codes. The unit-conversion constant is exact arithmetic. The only empirical input, rainfall intensity, is user-editable and should come from NOAA Atlas 14.
Official sources
- NOAA National Weather Service: Atlas 14 Precipitation Frequency Data Server.
- National Institute of Standards and Technology: Weights and measures unit definitions.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.