Drain pipe slope calculator: how much slope does a drain pipe need?
5 in of fall
1/8 in per ft (1.04%), the 3 in pipe minimum · for a 40 ft run
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
5 in of fall
1/8 in per ft (1.04%), the 3 in pipe minimum
Source: International Plumbing Code 2021, Section 704.1 and Tables 704.1 and 710.1(1), read October 27, 2026. Read the source
A 3 in drain pipe must fall at least 1/8 in per foot by IPC 2021 Table 704.1, so a 40 ft run drops 5 in; pipe of 2 1/2 in or less takes 1/4 in per foot and 8 in or larger 1/16.
A worked example
A 4 in building drain carrying 190 drainage fixture units over a run of 37 ft 6 in needs 9 3/8 in of fall. Table 704.1 asks 1/8 in per ft of a 4 in pipe, but Table 710.1(1) lets it carry only 180 DFU at that slope; at 1/4 in per ft it carries 216. So the run falls 37.5 × 1/4 = 9.375 in, marked as 9 3/8 in.
What is the minimum slope per foot for a drain pipe?
By IPC 2021 Table 704.1: 1/4 in per ft for pipe of 2 1/2 in or less, 1/8 in for 3 to 6 in, and 1/16 in for 8 in and larger. Over 10 ft that is 2 1/2 in, 1 1/4 in and 5/8 in of fall.
What is 1/4 inch per foot in percent?
About 2 percent, as the code itself writes it in 704.1: 0.25 ÷ 12 is 2.08%. 1/8 in per ft is 1.04% and 1/16 in per ft 0.52%.
Does more slope let a drain carry more?
Yes, on a building drain: Table 710.1(1) lets a 4 in drain carry 180 fixture units at 1/8 in per ft, 216 at 1/4 and 250 at 1/2. A 3 in drain goes from 36 to 42 to 50.
What else it is called
Also searched as drain pipe slope per foot, sewer pipe slope calculator or drain fall calculator. Plumbers say fall or pitch; the code says slope, in inches per foot.
The full IPC tables
| Pipe | Minimum slope | Fall per 10 ft | DFU at 1/16 in/ft | at 1/8 | at 1/4 | at 1/2 |
|---|---|---|---|---|---|---|
| 1 1/4 in | 1/4 in per ft | 2 1/2 in | — | — | 1 | 1 |
| 1 1/2 in | 1/4 in per ft | 2 1/2 in | — | — | 3 | 3 |
| 2 in | 1/4 in per ft | 2 1/2 in | — | — | 21 | 26 |
| 2 1/2 in | 1/4 in per ft | 2 1/2 in | — | — | 24 | 31 |
| 3 in | 1/8 in per ft | 1 1/4 in | — | 36 | 42 | 50 |
| 4 in | 1/8 in per ft | 1 1/4 in | — | 180 | 216 | 250 |
| 5 in | 1/8 in per ft | 1 1/4 in | — | 390 | 480 | 575 |
| 6 in | 1/8 in per ft | 1 1/4 in | — | 700 | 840 | 1,000 |
| 8 in | 1/16 in per ft | 5/8 in | 1,400 | 1,600 | 1,920 | 2,300 |
| 10 in | 1/16 in per ft | 5/8 in | 2,500 | 2,900 | 3,500 | 4,200 |
| 12 in | 1/16 in per ft | 5/8 in | 3,900 | 4,600 | 5,600 | 6,700 |
| 15 in | 1/16 in per ft | 5/8 in | 7,000 | 8,300 | 10,000 | 12,000 |
Minimum slope: IPC 2021 Table 704.1, for all horizontal drainage pipe; 1/4 in per ft upstream of a grease interceptor. Drainage fixture units: Table 710.1(1), the most a building drain or building sewer of that size carries at each slope; — means the size is not listed at that slope. A building drain serving a water closet is at least 3 in.
Where the numbers come from
- Slope: International Plumbing Code 2021, Section 704.1 and Table 704.1, as adopted by Colorado, whose amendments to chapter 7 do not touch sections 704 or 710. Horizontal drainage pipe of 2 1/2 in or less at no less than 1/4 in per ft, 3 to 6 in at 1/8, 8 in or larger at 1/16; upstream of a grease interceptor, 1/4 in per ft (2-percent slope). Read October 27, 2026. IPC 2021 chapter 7 on UpCodes
- Load: IPC 2021 Table 710.1(1), building drains and sewers, the most drainage fixture units by diameter and slope, e.g. 3 in carries 36 DFU at 1/8 in per ft, 42 at 1/4 and 50 at 1/2; 4 in carries 180, 216 and 250. Note a: a building drain serving a water closet is at least 3 in. Read October 27, 2026. IPC 2021 Table 710.1(1) on UpCodes
- The rest is arithmetic: fall in = run ft × slope in per ft, rounded up to the 1/16 in; percent grade = in per ft ÷ 12 × 100.
The slopes are the IPC minimums. The Uniform Plumbing Code, used instead of the IPC in some states, is another code and is not included. The fixture unit check is for building drains and sewers; horizontal fixture branches use Table 710.1(2), with no slope column.
By Javier Miralles · Last updated . How this site is made.
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