Sectional gutter comes in straight lengths joined with connectors, and seamless gutter is formed on site from coil stock. Either way, the slope comes from where the hangers are set, so the drop has to be planned before any hanger goes up.
How to use this calculator
- Measure from the high end of the gutter to the downspout in feet. Count only the section that drains to that downspout.
- Leave the slope at 0.125 inch per 10 ft to start. That default comes from a gutter maker's installation instructions. If your gutter system's manual gives a different slope, enter that instead.
- Read the total drop, which is how far the downspout end sits below the high end. The drop per foot shows how much each foot of gutter falls.
- Mark the high end on the fascia at the height the top of the gutter should reach. Measure down from that mark at the downspout end by the total drop, and make another mark there.
- Snap a chalk line between the marks and fasten each hanger so the top of the gutter follows it. Then pour a little water into the high end and check that it runs toward the downspout.
- If the result note says the run is longer than a downspout should drain, split the gutter and run the calculator again for each part.
Worked examples
Front porch gutter
Gutter run to the downspout: 18 ft · Slope: 0.125 in / 10 ft
- Total drop
- 0.23 in
- Drop per length
- 0.013 in per ft
Long back wall
Gutter run to the downspout: 46 ft · Slope: 0.125 in / 10 ft
- Total drop
- 0.58 in
- Drop per length
- 0.013 in per ft
Shed roof edge
Gutter run to the downspout: 12 ft · Slope: 0.25 in / 10 ft
- Total drop
- 0.30 in
- Drop per length
- 0.025 in per ft
Garage side run
Gutter run to the downspout: 26 ft · Slope: 0.25 in / 10 ft
- Total drop
- 0.65 in
- Drop per length
- 0.025 in per ft
Total drop over common gutter runs at each slope
| Gutter run | Slope: 0.125 in / 10 ft | Slope: 0.25 in / 10 ft |
|---|---|---|
| 20 ft | 0.25 | 0.50 |
| 30 ft | 0.38 | 0.75 |
| 40 ft | 0.50 | 1.00 |
| 50 ft | 0.63 | 1.25 |
Each cell: Total drop (in). Inputs not in the table use the calculator's starting values.
How the calculation works
The calculator divides the run by the length the slope is quoted over, then multiplies by the slope. The result is the total drop, in inches. The drop per foot is the slope spread evenly along the gutter, so it stays the same however long the run is.
The default slope, 0.125 inch per 10 ft, comes from a gutter maker's installation instructions. Use the figure in your own manual if it differs.
Setting the slope with a chalk line
Start by marking the high end, the far end of the run, at the height the top of the gutter should reach. Measure down from that mark at the downspout end by the total drop, and mark that spot too.
Snap a chalk line between the marks and fasten each hanger so the top of the gutter follows it. The marks sit on the fascia, so that board must be sound. If it needs replacing, work out the lumber with the board feet calculator before you mark the line.
Too little slope leaves water standing in the gutter after rain. Too much can look crooked from the street, even when it drains well, so follow the drop the calculator gives.
Long runs and downspout spacing
A gutter maker's instructions allow at most 40 ft of gutter per downspout. A longer run needs a downspout at each end, or a downspout in the middle with the gutter sloping from both ends toward it. Run the calculator for each section, since each has its own high end and drop.
That figure is a limit, not a target. Local codes may ask for more downspouts, so check them before you choose the positions.
Mistakes to avoid
- Eyeballing the slope. Mark the drop on the fascia and snap a line instead.
- Using the whole roof edge for every downspout. Each downspout drains only the run that feeds it.
Common questions
How do I calculate gutter fall for a roof gutter?
Enter the run from the high end to the downspout and the slope from your gutter system's instructions. The total drop shows how far the downspout end sits below the high end.
What slope should gutters have?
Use the figure in your gutter system's installation instructions. Installers commonly work within 0.125 to 0.25 inch per 10 ft, and the calculator starts at 0.125 inch per 10 ft, the low end of that range.
Is gutter pitch the same as roof pitch?
No. Roof pitch describes the slope of the roof surface, while gutter pitch, also called gutter slope, is the small drop along the gutter toward the downspout. For the roof itself, use the roof pitch calculator.
How far can a gutter run go to a downspout?
The result note flags any run longer than the maker's limit of 40 ft of gutter per downspout. Past that length, add downspouts, and check local codes for the count you need.
What are the common gutter sizes?
Most home K-style gutters are 5 inches or 6 inches wide. The 5 inches size is the most common, and the 6 inches size is often used on larger or steeper roofs that shed more water. Width is a separate choice from slope, so once you know it, use this calculator to set the drop.