Understanding your result
The headline is the drop over each slope, with the slope rate and the low end’s position. The stats give the slope length, the high end and the low end. The table lists the chalk-line marks at a quarter, half and three quarters of each slope.
The thing to use is the marks: snap the line, set the hangers to it, and the gutter drains. The low end row is the warning: if it is far below the drip edge, split the run or use a shallower slope. Downspout positions come from the downspout calculator; the hangers that hold the line from the hangers calculator.
How we calculate this
There is nothing more to it than a straight line, which is the point: the hard part of gutter slope is not the arithmetic but transferring a 2½-inch drop over 40 feet onto a fascia board that may itself be out of level. Measuring down from the drip edge rather than up from the ground avoids that error, because the drip edge is what the water leaves from. The run lengths come from the gutter length calculator.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Slope | 1/16 in per ft | Gutter makers’ installation guides (1/16 to 1/8 in per ft) |
| High end | ½ in below the drip edge | Installer practice |
| Run | 40 ft | One eave of the site’s example house |
| Split runs | over about 40 ft | Installer practice: downspouts at both ends |
Assumptions last reviewed October 7, 2026.
The calculator assumes the drip edge is straight and level; on a sagging fascia, measure from a string line. It does not size the gutter (the downspout calculator does). The downspout placement guide covers where the low points belong.
Two worked examples
A 40-foot run to one downspout
40 feet, 1/16 inch per foot, downspout at one end, high end ½ inch below the drip edge.
- Drop: 40 × 1/16 = 2.5 in
- Low end: 0.5 + 2.5 = 3 in below the drip edge
- Marks: 1.13 in at 10 ft, 1.75 in at 20 ft, 2.38 in at 30 ft
Three inches at the low end is past the comfortable limit: a second downspout at the other end would halve it.
A 50-foot run with downspouts at both ends
50 feet, 1/8 inch per foot, high point in the middle.
- Each slope: 25 ft; drop: 25 × 1/8 = 3.13 in
- Low ends: 3.63 in below the drip edge
- Marks on each half: 1.28 in at 6.3 ft, 2.06 in at 12.5 ft
At 1/8 inch per foot even 25 feet is a lot of drop; 1/16 would bring each end to about 2.1 inches.
Where to find your inputs
Run length. From the gutter length calculator, end cap to end cap or corner.
Downspout position. From the house: where water can be discharged away from the foundation.
High end. Hold a level against the drip edge and measure; ½ to 1 inch below.
Common mistakes
- Sloping by eye. A 1/16 inch per foot slope is invisible. Use a chalk line.
- Measuring from the ground. Ground and fascia are rarely parallel. Measure from the drip edge.
- One slope on a long run. Split it and add a downspout.
- Too much slope. The low end drops below the roof edge.
- Dips between hangers. Set every hanger to the line.
- Outlets above the gutter bottom. Cut outlets flush.
Questions people ask
- How much slope should a gutter have?
- About 1/16 inch per foot is the usual minimum and 1/8 inch per foot drains faster; many installers use 1/4 inch per 10 feet, which is 1/40 inch per foot, on seamless gutter. A 40-foot run at 1/16 inch per foot drops 2½ inches. More than about 1/4 inch per foot looks crooked and lets water overshoot at the low end.
- Should a long gutter slope one way or both ways?
- Runs over about 40 feet should slope from a high point in the middle to a downspout at each end, which halves the drop and the distance water travels. A 50-foot run at 1/8 inch per foot drops 6¼ inches one way, or 3⅛ inches each way from the middle.
- How far below the drip edge should a gutter be?
- The back of the gutter sits just below the drip edge at its high end, about ½ to 1 inch, so water from the shingles drops in. At the low end it is lower by the drop. If the low end ends up more than about 2½ inches below the drip edge, water can overshoot or run behind; shorten the slope.
- How do I mark the gutter slope on the fascia?
- Mark the high point below the drip edge, mark the low point lower by the calculated drop, snap a chalk line between them, and set every hanger to the line. The calculator gives intermediate marks so you can check the line on a long fascia that is not straight.
- Why does my gutter hold water?
- Either it has no slope, it dips between hangers, or the outlet is higher than the bottom of the gutter. Re-hang it to a chalk line at the planned slope and add hangers where it sags.
- Does a gutter need slope at all?
- Some designers hang seamless gutters dead level with enough downspouts, since level gutters carry their full rated capacity. Most installers slope them slightly so water clears and debris does not settle; a small slope costs nothing in capacity.