Gutter Calculator
Enter your total roofline (eave) length to calculate how many linear feet of gutter you need and how many downspouts, based on a standard downspout spacing guideline.
How the Gutter formula works
Gutter length matches the roofline; downspouts are spaced along it:
Gutter needed = Roofline length × (1 + Waste%) Downspouts needed = ceil( Roofline length / Spacing per downspout )
A downspout roughly every 35-40 feet of gutter run is a commonly cited general guideline.
Step-by-step calculation
- Measure your total roofline (eave) length where gutter will be installed.
- Add a small waste allowance for seams and corners.
- Divide the roofline length by your chosen downspout spacing (commonly 35-40 ft) and round up for the number of downspouts.
Worked example
A roofline totaling 140 linear feet, with 5% waste, and downspouts every 35 ft: Gutter needed = 140 × 1.05 = 147 ft. Downspouts = ceil(140/35) = 4.
Frequently asked questions
Why does downspout spacing matter?
Downspouts that are spaced too far apart can cause gutters to overflow during heavy rain, since there's a limit to how much water a given gutter cross-section can carry to a single downspout — the 35-40 ft guideline is a general starting point, and heavier rainfall regions or larger roof areas may need closer spacing.
Does roof size (not just roofline length) affect how much gutter capacity I need?
Yes — a larger roof area sheds significantly more water during a storm than a small one with the same roofline length, so homes with large or steep roofs sometimes need wider gutters or closer downspout spacing than this general guideline suggests.