Pipe slope calculator
Find the total fall of a drain line from its length and slope, or work backward from a measured drop. Results in fractions of an inch, percent grade and degrees.
Minimum drain slope by pipe size
| Pipe diameter | Minimum slope | Grade |
|---|---|---|
| 2½" or less | 1/4" per foot | 2.08% |
| 3" to 6" | 1/8" per foot | 1.04% |
| 8" or larger | 1/16" per foot | 0.52% |
Source: International Plumbing Code, Table 704.1. Local codes can differ, so confirm with your jurisdiction before you install.
How the math works
Total drop = slope per foot × run length. A 40-foot run at 1/8" per foot falls 40 × 0.125 = 5 inches. Percent grade is the slope divided by 12 (inches in a foot), times 100.
Plumbers do this math every day on job sites. If you like this kind of problem solving, a plumbing career pays a 2025 median of $63,800 a year, according to the BLS. See how to become a plumber.
Frequently asked questions
What is the standard slope for a drain pipe?
Under the International Plumbing Code (IPC Table 704.1), horizontal drainage pipe 2½ inches or smaller needs at least 1/4 inch of fall per foot, 3 to 6 inch pipe needs at least 1/8 inch per foot, and 8 inch or larger pipe needs at least 1/16 inch per foot. Always check your local code.
How do I calculate pipe slope?
Multiply the slope (inches of fall per foot) by the length of the run in feet. For example, a 20-foot run at 1/4 inch per foot drops 20 × 0.25 = 5 inches. To find the slope from a known drop, divide the drop in inches by the run in feet.
What is 1/4 inch per foot as a percentage?
1/4 inch per foot is a 2.08% grade (0.25 ÷ 12 × 100), or about 1.19 degrees. 1/8 inch per foot is a 1.04% grade.
Can a drain pipe have too much slope?
Yes. Very steep slopes can let water outrun solids and leave waste behind. Many plumbers aim for code minimum or slightly more on long horizontal runs, and use vertical drops where a big change in height is needed.