How to measure for gutters
Gutters run along the eaves, the roof edges that shed water, so a gable roof usually needs gutters on its two long sides and a hip roof on all four. Add up those edges for the linear feet, count the separate runs, and give every run at least one downspout. The calculator adds 2 ft to each downspout for the elbows and the outlet at the bottom.
| House | Gable roof (2 long sides) | Hip roof (all 4 sides) |
|---|---|---|
| 40 × 25 ft | 80 ft | 130 ft |
| 50 × 30 ft | 100 ft | 160 ft |
| 60 × 30 ft | 120 ft | 180 ft |
| 60 × 40 ft | 120 ft | 200 ft |
| 70 × 35 ft | 140 ft | 210 ft |
Overhangs make each eave slightly longer than the wall below it, so measure along the fascia when you can. A free public standard for downspout spacing was not found, so spacing is your input: use the gutter maker's or installer's figure. If you leave it blank, the calculator puts one downspout on each run.
What size gutter do you need?
A gutter has to carry the rain that falls on the roof area draining into it. The flow is the roof's plan area (measured flat, from above) times the rainfall intensity:
Runoff (gallons per minute) = plan area (sq ft) × rainfall (in/hr) × 0.0104800 sq ft × 3 in/hr × 0.0104 = 25.0 gpm
The International Plumbing Code sizes gutters this way, using the 100-year, 1-hour rainfall rate, and gives capacities in its gutter table: at a slope of 1/8 inch per foot, about 39 gpm for a 4-inch semicircular gutter, 74 gpm for 5-inch and 110 gpm for 6-inch.[2] Local rainfall rates come from NOAA's precipitation frequency data server.[3] Homes are normally built under the residential code, which has no gutter-sizing table, so treat the plumbing code figures as a reference point when you compare sizes in a quote.
| Plan area draining to the gutter | 2 in/hr | 4 in/hr | 6 in/hr |
|---|---|---|---|
| 400 sq ft | 8.3 | 16.6 | 25.0 |
| 600 sq ft | 12.5 | 25.0 | 37.4 |
| 800 sq ft | 16.6 | 33.3 | 49.9 |
| 1,000 sq ft | 20.8 | 41.6 | 62.4 |
| 1,200 sq ft | 25.0 | 49.9 | 74.9 |
When the code requires gutters
The model residential code requires a controlled way to dispose of roof water only where expansive or collapsible soils are known to exist, and then the water has to discharge at least 5 ft from the foundation or into an approved drainage system.[1] Elsewhere gutters are optional under that code, though your local building department may add rules.
How long gutters last
The builders' life-expectancy study gives about 20 years for aluminum and galvanized steel gutters and more than 50 years for copper; aluminum downspouts about 30 years and copper downspouts 100 years or more. NAHB calls these figures a general guideline only.[4]
Worked example and paying for it
For the 50 × 30 ft house: 100 ft of gutter, 2 downspouts × (10 + 2) ft = 24 ft of downspout. The cost is 100 × your gutter price per foot + 24 × your downspout price per foot, plus guards and removal. The Census Bureau's housing survey of home improvements has no separate gutter category, so there is no national median cost to compare.[5]
A gutter job is usually small enough for savings; borrowing against the house for it alone rarely makes sense, and the CFPB warns that a loan secured by your home puts the home at risk if payments are missed.[6] Home equity interest is tax deductible only when the money buys, builds or substantially improves the home; the IRS treats work that only keeps the home in good condition as a repair, not an improvement.[7] If gutters are part of a larger exterior project, the home improvement loan calculator compares ways to pay, and the roof replacement cost calculator, vinyl siding cost calculator and asphalt driveway cost calculator size the rest.
Downspout placementPut downspouts at the corners where the ground slopes away from the house, and send the outlet toward that slope. Long runs may need more than one; ask the installer how they sized it.
Frequently asked questions
What does gutter cost depend on?
Four quantities and your prices: the linear feet of eave, the number and length of downspouts, any guards, and removal of old gutters. The calculator works out the quantities; enter the per-foot prices from your quote to see the total.
How many downspouts do I need?
At least one per gutter run. Longer runs and roofs in heavy-rain areas need more; the spacing should come from the gutter maker or your installer, since no free public standard sets it. Enter that spacing and the calculator counts them.
What size gutters do I need?
Enough capacity for the roof area and your local rainfall. The plumbing code's table puts a 5-inch semicircular gutter at about 74 gpm and a 6-inch at about 110 gpm at a 1/8-in-per-foot slope.[2] Compare that with your runoff from the formula above.
Are gutters required by code?
Under the model residential code, only where expansive or collapsible soils are known to exist; the water must then discharge at least 5 ft from the foundation.[1] Check local rules.
How long do gutters last?
About 20 years for aluminum or galvanized steel and more than 50 years for copper, according to the NAHB life-expectancy study, which it presents as a guideline only.[4]
Sources
- 2021 International Residential Code, Chapter 8: Roof-Ceiling Construction (Section R801.3, Roof drainage)
- 2021 International Plumbing Code, Chapter 11: Storm Drainage (Section 1106.6, Size of roof gutters)
- NOAA Atlas 14 Precipitation Frequency Data Server
- Study of Life Expectancy of Home Components
- American Housing Survey 2023, Table C-16-OO: Home Improvement Costs, Owner-Occupied Units (Table Creator estimates)
- Using home equity to meet financial needs (Older Americans Housing Guide)
- Publication 936 (2025), Home Mortgage Interest Deduction