Gutter Size Calculator
Size gutters and downspouts for your roof.
Default 3 in/hr
Default 12 gpm
Results update as you type.
How it works
Gutters and downspouts must carry away all the rain a roof sheds, and undersized drainage causes overflow, foundation damage, and erosion. This calculator sizes your system. You enter the roof's drainage area, the local rainfall intensity for a heavy storm, and the flow capacity of one downspout, and it returns the peak collection flow rate and how many downspouts you need. The core fact is that each square foot of roof produces about 0.623 gallons of runoff per inch of rainfall, so a big roof in an intense downpour generates a surprising amount of water quickly. Rainfall intensity is the key local variable: rainy regions design for higher peak rates, often 4 to 7 inches per hour, while drier areas use less. The roof drainage area should be the horizontal footprint the gutter serves, not the sloped surface, and steep roofs may warrant a modest increase. Once you know the peak flow, dividing by a single downspout's capacity tells you how many outlets to install, spacing them evenly along the gutter runs. Larger gutters and downspouts each handle more flow. Enter your roof area, rainfall intensity, and downspout capacity to size your gutters.
Flow (gph) = roof area × rainfall × 0.623; downspouts = flow ÷ capacity
Each square foot of roof sheds about 0.623 gallons per inch of rain, so the collection rate equals the roof area times the rainfall intensity times 0.623. Dividing that flow (converted to gallons per minute) by the capacity of a single downspout, then rounding up, gives the number of downspouts required.
Source:Roof drainage estimate
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter roof drainage area.
- 2
Adjust the options
Adjust rainfall intensity and capacity per downspout if they apply — sensible defaults are already set.
- 3
Read your result
Collection flow updates instantly as you type — no button to press.
Frequently asked questions
How many downspouts do I need?
Calculate the peak flow from your roof area and local rainfall intensity, then divide by the capacity of one downspout. A 2000-square-foot roof in a 3-inch-per-hour storm sheds about 62 gallons per minute, needing roughly six average downspouts. Spreading downspouts evenly along the gutters ensures water drains before it can overflow anywhere.
What rainfall intensity should I use?
Use a heavy-storm figure for your region rather than an average. Many areas design gutters for a 5-year or 10-year peak, commonly between 3 and 7 inches per hour depending on climate. Rainy and storm-prone regions use higher values. Local building codes or weather data give the design rainfall intensity appropriate for your location.
Should I use the roof's sloped area or footprint?
Use the horizontal footprint, the flat area the roof covers as seen from above, since that is what determines how much rain lands on it. For steep roofs, wind can drive extra rain onto the surface, so some guidelines add a small percentage for pitch. For most homes, the footprint area gives a sound and safe estimate.
Do bigger gutters reduce the number of downspouts?
Larger gutters hold and carry more water, which helps, but downspouts are usually the limiting factor because they set how fast water actually leaves the gutter. Upsizing downspouts, for example from 2-by-3-inch to 3-by-4-inch, roughly doubles capacity and can reduce the number needed. Balancing gutter size and downspout size and count gives the best drainage.
What happens if gutters are undersized?
Undersized gutters or too few downspouts overflow during heavy rain, sending water down exterior walls and pooling around the foundation. Over time this causes basement leaks, foundation settling, soil erosion, and damaged landscaping. Properly sizing the system so it handles peak storms protects the house and is far cheaper than repairing water damage later.