How to use the gutter calculator
- Add up the length of every roof edge that will have a gutter.
- Enter the number of separate gutter runs. Each run has two ends and at least one downspout.
- Enter the length of the sections you will buy.
- Enter the maximum distance you want between downspouts and the hanger spacing.
Downspout and hanger spacing depend on roof area, local rainfall, snow load and the gutter system. The defaults are planning values; follow the gutter manufacturer's instructions and local practice. Measure every roof edge you plan to gutter, including the short returns at corners, which Decoratoradvice finds are easy to leave out.
Gutter formulas
Sections = total length ÷ section length, rounded up
Downspouts = total length ÷ downspout spacing, rounded up (at least one per run)
Hangers = total length ÷ hanger spacing, rounded up
End caps = runs × 2
Worked example
Two runs totalling 120 ft, 10 ft sections, downspouts no more than 35 ft apart, hangers every 24 in:
Sections: 120 ÷ 10 = 12
Downspouts: 120 ÷ 35 = 3.4, so 4
Hangers: 1,440 in ÷ 24 in = 60
End caps: 2 × 2 = 4
Parts not counted here
Corners (inside and outside), outlets, downspout elbows, downspout pipe length (roughly the wall height for each downspout), sealant and screws. List them by walking round the house with a sketch.
Tips
- Slope gutters slightly toward each outlet; follow the manufacturer's recommended fall.
- Place downspouts where water can drain away from the foundation.
- Seamless gutters are formed on site to length; ask the installer how they count downspouts and hangers.
- Check roof edges and pitch with the roof pitch calculator and the overall roof size with the roofing calculator.
Common mistakes
- Counting only the front of the house. Add every edge that sheds water.
- Too few downspouts on long runs. Long runs carry more water to each outlet.
- Forgetting corners and end caps. They are sold separately.
Frequently asked questions
How many downspouts do I need?
At least one per run, and enough that no stretch of gutter exceeds the spacing you choose. For 120 ft at 35 ft spacing, 4.
How far apart should gutter hangers be?
Follow the gutter manufacturer. Closer spacing is stronger, which matters where snow and ice load the gutters.
How many sections of gutter do I need?
Total length ÷ section length, rounded up. Seamless gutters are cut to each run instead.
Do I need gutters on every side?
Gutters go wherever roof water drips toward walls, walkways or doors. A small roof plane over a lawn may not need one.
How do I measure gutter length?
Measure the eave at the fascia, or from the ground along the house wall plus the overhang at each end.
Do metal roofs need different gutters?
Snow and water can slide off metal roofs faster; some systems add snow guards. Plan panels with the metal roofing calculator and follow the manufacturer for gutters.
Key takeaways
- Sections = total length ÷ section length.
- Downspouts: at least one per run, plus enough to keep within your spacing.
- Hangers = total length ÷ hanger spacing.
- Count corners, outlets and elbows separately.