Gutter Slope Calculator
Gutters should slope at least ¼ inch for every 10 feet toward a downspout. On a 40-foot run, that means a 1-inch total drop from the high end to the low end. Enter your run length and downspout layout below to get the exact start and end heights.
Gutter run
ft
¼″ per 10 ft is the common minimum.
Downspout sizing (optional)
sq ft
Total drop
1"
High end sits 1" above the outlet over a 40 ft slope run.
Slope run
40 ft
Downspouts (by spacing)
2
Show the math
slope run = 40 ft
drop = (40 ÷ 10) × 0.25 = 1"
downspouts ≈ one per 35 ft of run = 2
How to use this calculator
- Enter the gutter run length in feet.
- Choose a slope — ¼ inch per 10 feet is the common minimum.
- Select where the downspouts are (one end, both ends, or the middle).
- Optionally enter the roof area draining to size downspouts.
- Read the total drop and the high and low end heights.
Formula
total drop (in) = (run length ÷ 10) × slope per 10 ft
with downspouts at both ends, each slope run = length ÷ 2
downspouts ≈ one per 30–40 ft of run
Worked example
A 40 ft run at ¼ inch per 10 ft:
drop = (40 ÷ 10) × 0.25 = 1 inch total
so the far end sits 1 inch higher than the downspout end
Second example
A 60 ft run with a downspout at each end:
slope from the middle: each side = 60 ÷ 2 = 30 ft
drop each side = (30 ÷ 10) × 0.25 = 0.75 inch
the centre sits 0.75 inch above each outlet
Reference tables
| Run length | ¼" per 10 ft | ½" per 10 ft |
|---|---|---|
| 10 ft | 0.25" | 0.5" |
| 20 ft | 0.5" | 1.0" |
| 30 ft | 0.75" | 1.5" |
| 40 ft | 1.0" | 2.0" |
| 50 ft | 1.25" | 2.5" |
| 60 ft | 1.5" | 3.0" |
For long runs, slope from a high mid-point down to a downspout at each end.
| Run length | Downspouts |
|---|---|
| Up to 30 ft | 1 |
| 35–40 ft | 1–2 |
| 40–75 ft | 2 |
| 75–110 ft | 3 |
Rule of thumb: one downspout per 30–40 ft of run. At 35–40 ft, 1–2 depending on rainfall. Or size by roof area (2×3 in ≈ 600 sq ft; 3×4 in ≈ 1,200 sq ft).
Tips & buying advice
- A slope of ¼ inch per 10 feet keeps water moving without making the gutter look crooked.
- On runs longer than about 35–40 feet, put the high point in the middle and slope down to a downspout at each end.
- More slope isn't better. Too much looks off from the street, and in heavy rain the water can shoot right past the outlet.
- Add a downspout about every 30–40 feet, or size them by roof area if you get a lot of heavy rain.
Frequently asked questions
What is the correct slope for gutters?
At least ¼ inch of drop for every 10 feet of gutter, sloping toward the downspout. Some installers go up to ½ inch per 10 feet so it drains faster. Either way, the gutter should always tilt toward its outlet.
Can gutters have too much slope?
Yes. Too much slope is easy to spot against the roofline, and in a downpour the water can rush right past the downspout opening. Stick to about ¼–½ inch per 10 feet unless the manufacturer says otherwise.
How many downspouts do I need?
A good rule is one downspout for every 30–40 feet of gutter. For big roofs or heavy rain, size them by roof area instead. A 2×3-inch downspout handles roughly 600 sq ft of roof, and a 3×4-inch about 1,200 sq ft.
How do I measure gutter slope?
Measure the length of the run, multiply by the slope, and mark the high and low ends on the fascia. For example, a 20-foot run at ¼ inch per 10 feet drops ½ inch from end to end. Snap a chalk line between the two marks and hang the gutter along it.
Should long gutters slope both ways?
Yes. On runs longer than about 35–40 feet, set the high point in the middle and slope down to a downspout at each end. That cuts the drop on each side in half and keeps the gutter looking close to level.
How far apart should gutter downspouts be?
No more than about 30–40 feet apart. Long runs drain best with a downspout at each end and the high point in the middle. If you live somewhere with heavy rain, add more and size them by roof area.
What is the minimum slope for a gutter?
A quarter inch of drop for every 10 feet of run. That's just enough to keep water moving toward the downspout, and it works out to a 1-inch drop over 40 feet. Keep it under about ½ inch per 10 feet so the gutter doesn't look crooked.
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Sources
- PNNL Building America Solution Center: Gutters and Downspouts — slope and downspout spacing guidance — accessed October 8, 2026
- Englert: Gutter installation — pitch and sizing guidance — accessed October 8, 2026
Built and maintained by Sikander Mushtaq. Formulas checked against gutter manufacturer installation guides, SMACNA downspout sizing. See our methodology and sources, or learn about YardageMath.
Last updated:
Estimates only. Confirm quantities with your supplier or contractor. Read the full disclaimer.