Total fall (drop) of a horizontal drain over common runs: fall = run in feet × inches of fall per foot. 1/4" per foot is a 2.08% grade, 1/8" per foot 1.04% and 1/16" per foot 0.52%.
Minimum slope for horizontal drainage piping, IPC Table 704.1 (2021): 1/4" per foot for 2½" and smaller, 1/8" per foot for 3" to 6", 1/16" per foot for 8" and larger. The IRC (Table P3005.3) uses the same values for houses; the UPC (708.1) calls for 1/4" per foot and allows 1/8" for 4" and larger only where the AHJ approves. Local codes and amendments may differ, so treat this chart as a reference, not an approval.
Fall for runs of 5 to 100 feet
Example: 40 ft of 2" drain at 1/4" per foot falls 40 × 0.25 = 10".
Total fall in inches, by run and slope| Run | 1/16" per ft | 1/8" per ft | 1/4" per ft |
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| 5 ft | 5/16" | 5/8" | 1 1/4" |
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| 10 ft | 5/8" | 1 1/4" | 2 1/2" |
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| 15 ft | 15/16" | 1 7/8" | 3 3/4" |
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| 20 ft | 1 1/4" | 2 1/2" | 5" |
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| 25 ft | 1 9/16" | 3 1/8" | 6 1/4" |
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| 30 ft | 1 7/8" | 3 3/4" | 7 1/2" |
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| 35 ft | 2 3/16" | 4 3/8" | 8 3/4" |
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| 40 ft | 2 1/2" | 5" | 10" |
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| 45 ft | 2 13/16" | 5 5/8" | 11 1/4" |
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| 50 ft | 3 1/8" | 6 1/4" | 12 1/2" |
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| 60 ft | 3 3/4" | 7 1/2" | 15" |
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| 70 ft | 4 3/8" | 8 3/4" | 17 1/2" |
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| 80 ft | 5" | 10" | 20" |
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| 90 ft | 5 5/8" | 11 1/4" | 22 1/2" |
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| 100 ft | 6 1/4" | 12 1/2" | 25" |
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Slope in percent and degrees
Percent grade = inches of fall per foot ÷ 12 × 100, and the angle is arctan(inches per foot ÷ 12).
Slope, grade and the pipe sizes it is the IPC Table 704.1 minimum for| Slope | Grade | Angle | IPC minimum for |
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| 1/16" per ft | 0.52% | 0.3° | 8" and larger |
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| 1/8" per ft | 1.04% | 0.6° | 3" to 6" |
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| 1/4" per ft | 2.08% | 1.19° | 2½" and smaller |
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Fall is rounded to the nearest 1/16", percent and degrees to two decimals.
Estimates only. Check measurements, product labels and local building codes before you buy or build.