1 in
Over a 40 ft run.
High point at the far end, low point at the downspout.
Snap a chalk line from the high point to the low point and hang the gutter to it. Drops are rounded to the nearest 1/16 inch, the finest mark on most tape measures.
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The common rule of thumb is 1/4 inch of fall per 10 feet of gutter, which is 1/40 of an inch per foot. Installers work anywhere from 1/16 inch to 1/2 inch per 10 feet. Less fall looks level from the street but can leave standing water. More fall drains faster but shows as a visible tilt on a long run.
To work it out by hand, divide the run length in feet by 10 and multiply by the slope. A 40 ft run at 1/4 inch per 10 ft drops 40 / 10 x 1/4 = 1 in.
| Run length | 1/8 in per 10 ft | 1/4 in per 10 ft | 1/2 in per 10 ft |
|---|---|---|---|
| 10 ft | 1/8 in | 1/4 in | 1/2 in |
| 20 ft | 1/4 in | 1/2 in | 1 in |
| 30 ft | 3/8 in | 3/4 in | 1 1/2 in |
| 40 ft | 1/2 in | 1 in | 2 in |
| 50 ft | 5/8 in | 1 1/4 in | 2 1/2 in |
| 60 ft | 3/4 in | 1 1/2 in | 3 in |
Drops are rounded to the nearest 1/16 inch. The middle column is the 1/4 inch rule.
A 25 ft garage run, one downspout. 25 / 10 x 1/4 = 5/8 in. Mark the high end, measure 5/8 in down at the downspout end, and snap a line.
A 40 ft back wall, downspout at one corner. 40 / 10 x 1/4 = 1 in. Move the downspout to the middle and each 20 ft half drops 1/2 in.
A 60 ft front run. To one end it needs 1 1/2 in of fall. With a downspout at each end the high point sits in the middle and each 30 ft half drops 3/4 in, so the low ends stay closer to the top of the fascia.
On a long run you can put the downspout in the middle, or one at each end. Either way the run is split in two. Each half is sloped on its own, so each half needs only half the fall. A 40 ft run with a center downspout is two 20 ft runs, each dropping 1/2 in at 1/4 inch per 10 ft, instead of 1 in across the whole length.
With a center downspout the high points are at both ends. With a downspout at each end the high point is in the middle. Pick the layout in the calculator and it splits the run for you.
At 1/4 inch per 10 ft, here is the drop on each half when a run is split by a center downspout (or by a downspout at each end), next to the drop to a single end.
| Total run | One downspout at an end | Split: each half drops |
|---|---|---|
| 40 ft | 1 in | 1/2 in |
| 50 ft | 1 1/4 in | 5/8 in |
| 60 ft | 1 1/2 in | 3/4 in |
| 80 ft | 2 in | 1 in |
| 100 ft | 2 1/2 in | 1 1/4 in |
Hold a level against the fascia at the high end and mark a level line toward the downspout. At the downspout, measure from that line down to the top edge of the gutter. That is the actual drop. Look it up in the table above for your run length.
If the drop is close to zero, or the gutter rises toward the outlet, water will sit in it. A drop well past 1/2 inch per 10 ft usually means a hanger has pulled loose rather than a deliberate pitch.
Re-pitching a single-story run with a few sagging hangers is a fair DIY job with a level and a chalk line. Second-story work is a job for a gutter installer. So is a run where the fascia behind the gutter is soft, because new hangers will not hold in rotted wood.
Crews that clean gutters see bad slope first, as standing water or sludge settled at a low spot. A cleaning visit is a good time to note pitch problems and quote a re-hang as its own line. Write the run length and measured drop on the job so the next visit can check it.
Cleaning gutters rather than hanging them? The gutter cleaning cost calculator prices a job by length and story height, and the gutter cleaning cost guide shows what homeowners typically pay.
The common rule of thumb is 1/4 inch of fall for every 10 feet of gutter. It is a trade convention, not a building code, and installers use anywhere from 1/16 inch to 1/2 inch per 10 feet. At 1/4 inch per 10 feet, a 40 ft run drops 1 in.
About 1/40 of an inch per foot, or 0.025 inches. You cannot mark that on a tape measure, so work it out over the whole run: divide the length by 10 and multiply by 1/4 inch.
3/4 in at 1/4 inch per 10 feet. If water still sits in a flat stretch, go to 1/2 inch per 10 feet, which is 1 1/2 in over 30 feet.
1 in over the full 40 feet with one downspout at the end. With a downspout in the center, each 20 ft half drops 1/2 in.
Yes. Past about 1/2 inch per 10 feet the tilt starts to show from the street, and in a heavy rain the water can run past the outlet instead of dropping into it. On a 50 ft run the working range is 5/16 in to 2 1/2 in.
Some installers hang short runs level and let the depth of the water push it to the outlet. The catch is standing water after every rain, which adds weight to the hangers and rusts steel gutters. A small, even slope is the safer default.
When one run would need so much fall that the low end drops well down the fascia board. A 60 ft run needs 1 1/2 in of fall to one end, or 3/4 in on each half with a downspout in the middle.
Convert first. 1/4 inch per 10 feet is roughly 2 mm per metre. Enter the run in feet (1 metre is 3.28 feet) and the drop comes back in inches, rounded to the nearest 1/16.
