Gutter Slope Calculator
Calculate the required gutter slope and drop for proper drainage.
Enter your gutter run length to find the ideal downward pitch.
Gutter slope (also called fall or pitch) is the deliberate downward angle of a gutter run toward the downspout. Correct slope ensures water drains by gravity without pooling (which leads to overflow, sagging, and mosquito breeding) or flowing too fast (which causes noise and erosion at the downspout).
Slope formula: Drop = Run × (Slope per Foot)
Where:
- Drop = total vertical fall across the entire gutter run (inches)
- Run = total horizontal length of the gutter (feet)
- Slope per foot = the fall for every 1 foot of horizontal run
Standard slope specification: 1/4 inch per 10 feet (0.025 inches/foot, or 0.21%)
Or equivalently: 1 inch per 40 feet
Minimum acceptable slope: 1/8 inch per 10 feet (barely drains; prone to debris accumulation) Maximum recommended slope: 1/2 inch per 10 feet (drains fast, visible tilt, may look unsightly)
What each variable means:
- Fall direction — slope always toward the downspout; if the gutter run is very long (40+ feet), two-directional slope (from center toward downspouts at both ends) is common
- Hidden high point — for long runs, mounting the midpoint higher and sloping toward downspouts at both ends hides the visible tilt and maintains proper drainage
- Fascia alignment — gutters attach to the fascia board; slope means the attachment points are at progressively lower heights toward the downspout end
Reference: slope by gutter run length at 1/4" per 10 ft:
- 10 ft run: 0.25" drop
- 20 ft run: 0.50" drop
- 30 ft run: 0.75" drop
- 40 ft run: 1.0" drop
- 50 ft run: 1.25" drop
- 60 ft run: 1.5" drop
Worked example: Gutter run: 45 feet from far end to downspout. Using standard 1/4" per 10 ft slope.
- Total drop = (45 ÷ 10) × 0.25" = 4.5 × 0.25" = 1.125 inches
- Round to 1 and 1/8 inches total drop over the 45-foot run
- Method: Snap a chalk line from the high end to the downspout end, dropping 1.125" over 45 feet; mount brackets along this line
If the gutter level looks too visibly slanted (common on long runs), split into two 22.5-foot runs from a center high point, each dropping 0.563" toward individual downspouts.