How to use the gutter calculator
- Add one row per gutter run. A run is a single straight edge of roof that ends at a corner or a gable. If the front and back of the house match, enter one row and set How many to 2.
- Enter the height to the gutter. This is the downspout length, measured from the ground to the bottom of the eave.
- Pick a section length. Stores usually sell 10 ft lengths; some carry 16 or 20 ft.
- Count your corners. Each corner joins two runs, so it replaces two end caps.
- Add prices to get a materials total, or hand the lengths to the gutter installation cost calculator for a professional budget.
The gutter formulas
Sections per run = run length ÷ section length, rounded up
Hangers per run = run length ÷ hanger spacing (rounded down) + 1
Downspouts per run = run length ÷ most gutter per downspout, rounded up (at least 1)
Downspout pipe = downspouts × height to the gutter
Fall = length drained by one downspout ÷ 10 × fall per 10 ft
Worked example
A simple gable house with two 40 ft eaves, 10 ft to the gutter
- Gutter = 40 + 40 = 80 ft, or 4 + 4 = 8 sections of 10 ft.
- Hangers every 24 in: 40 ft = 480 in ÷ 24 = 20, + 1 = 21 per run, 42 hangers.
- Downspouts: 40 ÷ 40 = 1 per run, 2 downspouts and 2 × 10 ft = 20 ft of pipe.
- Elbows: 3 per downspout = 6 elbows. End caps: 2 per run = 4.
- Fall: 40 ft ÷ 10 × 1/4 in = 1 in lower at the downspout end.
Gutter parts at a glance
| Part | What it does | How many |
|---|---|---|
| Gutter section | The channel itself | Run length ÷ section length |
| Hanger | Holds the gutter to the fascia | About one every 24 in |
| End cap | Closes the end of a run | 2 per run, fewer with corners |
| Outlet (drop outlet) | Hole where water enters the downspout | 1 per downspout |
| Elbow | Bends the downspout to the wall and away at the bottom | 2 to 3 per downspout |
| Downspout pipe | Carries water to the ground | Height to the gutter, per downspout |
| Connector | Joins two sections | 1 per seam |
Where to put downspouts
Put downspouts at corners where possible: they are less visible and easy to fix to the wall. Lead each one at least 3 to 5 ft away from the foundation with an extension, splash block or underground drain. Avoid draining onto walkways and driveways where water can freeze.
Common mistakes to avoid
- Measuring the wall instead of the eave. The gutter follows the roof edge, which overhangs the wall and is longer at the corners.
- Too few downspouts. Long runs with one downspout overflow in heavy rain. Stick to 30 to 40 ft per downspout.
- Hanging the gutter level. Without fall, water sits in the gutter and rots the fascia.
- Setting the gutter too high. The back of the gutter should sit under the drip edge so water from the shingles lands in it.
- Forgetting the elbows. Every downspout needs at least two to reach the wall from the outlet and one to turn away at the bottom.
Frequently asked questions
How many downspouts do I need?
A common rule is one downspout for every 30 to 40 feet of gutter, and at least one per run. Very long runs work best with a downspout at each end and the gutter sloping both ways from the middle. Large or steep roofs in heavy-rain areas may need more, or larger 3 × 4 in downspouts.
How far apart should gutter hangers be?
Most manufacturers ask for a hanger about every 24 inches. In areas with heavy snow or ice, 16 to 18 inches is safer. Always put a hanger within a few inches of each end and each seam.
How much slope should a gutter have?
About 1/4 inch of fall for every 10 feet of gutter, toward the downspout. A 40 ft run with one downspout ends 1 inch lower at the outlet than at the far end. Too little slope leaves standing water; too much is visible from the street.
Should I buy 10 ft sections or seamless gutters?
Sectional gutters from the store come in 10 to 20 ft lengths that you join with connectors and sealant, which suits DIY. Seamless gutters are formed on site in one piece per run by a contractor and have fewer places to leak. This calculator counts sections; ignore the seam count if you go seamless.
How do I measure for gutters?
Measure the length of each roof edge (eave) that will get a gutter, on the fascia board, not the house wall. Note every corner and where each downspout can go, ideally at a corner where water can be led away from the foundation.
What size gutter do I need?
5 inch K-style gutters with 2 × 3 inch downspouts suit most homes. Larger roofs, steep roofs and areas with intense rain often use 6 inch gutters with 3 × 4 inch downspouts. A local installer can confirm what works for your rainfall.