Gutter Slope Calculator
Get the fall right: how many inches to drop a gutter over its run so it drains instead of pooling — at the standard quarter inch per ten feet.
Calculator
Over 40 ft, drop the gutter 1.00 in toward the downspout (1/4 in per 10 ft).
A gutter must tilt, just slightly, toward its outlet or it becomes a long thin pond that breeds mosquitoes, sags under standing water and overflows in the next storm. The trade-standard pitch is a quarter inch of drop for every 10 feet of run — barely perceptible to the eye, but enough to keep water moving. This tool turns a run length into the total drop you set between the high end and the downspout so the gutter hangs with a purposeful, consistent fall.
On a short run you simply drop the whole length toward one downspout. On a long run — beyond roughly 40 ft — it is common to pitch from a high point in the middle down to a downspout at each end. That halves the distance any water travels and halves the drop needed at either outlet, which keeps the gutter looking level from the ground while still draining properly. The result box shows both the single-fall drop and the from-center figure for exactly this reason.
Formula
Linear drop at a quarter inch per 10 ft:
drop_in = ( run_ft ÷ 10 ) × 0.25\n\nlong runs (> 40 ft): pitch from a high center,\n drop each half ≈ ( half_run ÷ 10 ) × 0.25
The 0.25 is the quarter-inch standard; some installers use a slightly steeper fall on very long or debris-prone runs, but a quarter inch per 10 ft is the widely cited baseline.
Worked example
A 40-ft run draining to one downspout:
drop_in = ( 40 ÷ 10 ) × 0.25 = 1.0 in
Set the high end one inch higher than the outlet over the 40 ft. If instead you pitched a 60-ft run from the center to a downspout at each end, each 30-ft half drops (30 ÷ 10) × 0.25 = 0.75 in, so no point sits more than three quarters of an inch below the peak.
Setting the pitch on the wall
To lay out the fall, snap a level chalk line along the fascia, then measure down from it — the computed drop at the outlet end — and strike a second line that is your gutter's top edge. Hang the back of the gutter to that sloping line and the pitch takes care of itself. Space hangers every couple of feet so the run does not sag between them and undo the slope; a belly in the middle traps water no matter how carefully you set the ends. Too little pitch pools; too much looks visibly crooked and can let water rush past a downspout in a downpour — the quarter-inch-per-10-ft target threads that needle. This is a planning estimate; confirm against the gutter and hanger maker's guidance.
Reference table
| Gutter run | Total drop (single fall) | Drop per half (from center) |
|---|---|---|
| 10 ft | 0.25 in | 0.13 in |
| 20 ft | 0.50 in | 0.25 in |
| 30 ft | 0.75 in | 0.38 in |
| 40 ft | 1.00 in | 0.50 in |
| 50 ft | 1.25 in | 0.63 in |
| 60 ft | 1.50 in | 0.75 in |
Runs longer than about 40 ft are usually pitched from a high point in the middle toward a downspout at each end, halving the drop needed at either outlet.