Gutter Slope Calculator

Gutters should slope at least ¼ inch per 10 feet toward a downspout. A 40-foot run at that pitch needs a 1-inch total drop from the high end to the low end. Enter your run length and downspout layout below for exact start and end heights.

Gutter run

Run length (ft)

Slope

¼″ per 10 ft is the common minimum.

Downspout position

Downspout sizing (optional)

Roof area draining (sq ft)

sq ft

Total drop

High end sits 1" above the outlet over a 40 ft slope run.

Slope run

40 ft

Downspouts (by spacing)

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 below the downspout end

Gutter slope calculator diagram showing the high end, low end and total drop

Reference tables

Gutter drop by run length

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.

Tips & buying advice

A minimum of ¼ inch of fall per 10 feet keeps water moving without the gutter looking crooked.
For runs over about 35–40 feet, peak in the middle and slope to a downspout at each end.
Too much slope looks off and lets water overshoot the outlet in heavy rain — more is not better.
Add a downspout roughly every 30–40 feet, or size by roof area for heavy rainfall.

Frequently asked questions

What is the correct slope for gutters?

A minimum of ¼ inch of fall per 10 feet of run toward the downspout. Some installers use up to ½ inch per 10 feet for faster drainage. The gutter should always tilt toward its outlet.

Can gutters have too much slope?

Yes. Excessive slope is visually obvious against the roofline and can let water race past the downspout opening in a downpour. Stick to about ¼–½ inch per 10 feet unless a manufacturer says otherwise.

How many downspouts do I need?

As a rule of thumb, one downspout per 30–40 feet of gutter. For heavy rainfall or large roofs, size by drainage area — 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 run length, multiply by the slope rate, and mark the high and low ends. For example, a 20-foot run at ¼ inch per 10 feet drops ½ inch end to end. Snap a chalk line between the marks.

Should long gutters slope both ways?

Yes. For runs longer than about 35–40 feet, set the high point in the middle and slope down to a downspout at each end. That halves the required drop and keeps the gutter closer to level.

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: October 7, 2026

Estimates only. Confirm quantities with your supplier or contractor. Read the full disclaimer.