Work out the fall of a drain or sewer pipe. Enter the run length and a slope such as 1/4 in per foot to get the total drop, or enter the drop and length to find the slope, and check it against minimum drain, waste and vent (DWV) slopes for the pipe size.
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Solve for
Drain, waste and vent (DWV) horizontal drain size.
Invert height at the upper end, in the drop unit. Gives the height at the lower end.
Enter your values to see the result.
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How to use the Pipe Slope Calculator
Choose what to solve for: total drop, slope or run length.
Enter the known values: the run length and the slope, or the measured drop.
Pick the pipe size to check it against the minimum code slope.
Read the drop in decimals and fractions, plus the slope in %, mm/m and degrees.
Use the layout table to mark hangers or a string line along the run.
Pipe slope formulas
Total drop = run length (ft) × slope (in per ft)
Slope = total drop (in) ÷ run length (ft)
Run length = total drop ÷ slope
Percent grade = slope (in/ft) ÷ 12 × 100; mm per m = grade × 10; angle = atan(grade ÷ 100)
Example: a 3 in drain running 28 ft at 1/8 in per foot drops 28 × 0.125 = 3.5 in. At 1/4 in per foot it would drop 7 in, which may not fit within a floor or crawlspace.
Minimum slope for horizontal drainage pipe
Pipe size
Minimum slope (IPC)
Grade
Drop per 10 ft
2½ in or smaller
1/4 in per ft
2.08%
2½ in
3 in to 6 in
1/8 in per ft
1.04%
1¼ in
8 in or larger
1/16 in per ft
0.52%
⅝ in
Common slope conversions
in per ft
Percent
mm per m
Degrees
Ratio
1/16
0.52%
5.2
0.30°
1 in 192
1/8
1.04%
10.4
0.60°
1 in 96
3/16
1.56%
15.6
0.90°
1 in 64
1/4
2.08%
20.8
1.19°
1 in 48
1/2
4.17%
41.7
2.39°
1 in 24
Tip: set the high end first, then measure down from a level string line at each hanger using the layout marks. Check local code before installing, since some jurisdictions use the Uniform Plumbing Code or their own amendments.
Frequently asked questions
What is the minimum slope for a drain pipe?
Under the International Plumbing Code, horizontal drains 2½ in and smaller need at least 1/4 in per foot (2.08%), 3 to 6 in drains need at least 1/8 in per foot (1.04%), and drains 8 in and larger need at least 1/16 in per foot. Many plumbers use 1/4 in per foot wherever space allows.
How much drop is 1/4 inch per foot over 10 feet?
Multiply the run by the slope: 10 ft × 1/4 in = 2.5 in of drop. At 1/8 in per foot the same 10 ft run drops 1.25 in, and over 20 ft it drops 2.5 in.
How do I convert 1/4 inch per foot to a percent?
Divide the drop per foot by 12 inches and multiply by 100: 0.25 ÷ 12 × 100 = 2.08%. 1/8 in per foot is 1.04% and 1/2 in per foot is 4.17%.
Can a drain pipe have too much slope?
Codes set minimums, not a firm maximum for most horizontal drains, and a pipe sloped 45° or more counts as vertical. Very steep horizontal runs can let water outrun solids, so long runs are usually kept between 1/8 and 1/2 in per foot, using 45° fittings for larger changes in height.
How do I measure the slope of an existing pipe?
Measure the height of the pipe (or its bottom, the invert) at two points and the horizontal distance between them. Slope = difference in height ÷ distance. A 4 ft level with a shim also works: a 1 in shim under one end of a 4 ft level that reads level means 1/4 in per foot.
Does a sewer line need the same slope as indoor drains?
The building sewer follows the same pipe-size minimums, so a 4 in sewer usually needs at least 1/8 in per foot and 1/4 in per foot is common. Long sewer runs to a septic tank or main are often designed close to the minimum to keep the outlet from getting too deep.
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