The standard for household drains is ¼ inch of fall per foot of run (a 1:48 slope, about 2%). But there is a twist most people miss: bigger pipes want less slope, not more. Pipes 3 inches and up drop to ⅛ inch per foot (1:96, about 1%).
Slope by pipe size
| Pipe size | Minimum slope | As a ratio |
|---|---|---|
| 1¼ – 2½ in (32–65 mm) | ¼ in per foot | 1:48 (2.1%) |
| 3 – 6 in (75–150 mm) | ⅛ in per foot | 1:96 (1.0%) |
| 8 in and larger (200 mm+) | &frac1;16 in per foot | 1:192 (0.5%) |
Why not just make it steeper?
Drainage relies on the water carrying the solids along with it. Get the slope right and liquid and solids move together. Too little slope and the water sits, solids settle, and the pipe clogs. Too much slope and the water races ahead and drains away from the solids, leaving them stranded to build up and block the line — the classic mistake on a DIY sewer run. There is a right band, and steeper is not safer.
Drain Slope Calculator
Enter the run length and pipe size — it gives the fall, gradient, percent and inches-per-foot.
Setting the fall on site
Fall is just slope × run. For a 20 ft branch at ¼ inch per foot, the outlet end sits 5 inches lower than the start (20 × ¼). Set it with a level and a marked block, or a laser, and check it before you backfill — a dip or a back-fall in a buried line is a call-back waiting to happen.
Working in metric?
Slopes are usually given as a ratio: 1:40 to 1:60 for waste pipes (40–50 mm), 1:80 or so for a 110 mm foul drain — the same idea that big pipes want a gentler grade. The calculator shows the fall in mm, the gradient as 1:X, and the percent, whichever your local code uses.
Check your local code
These are the common minimums, but plumbing codes vary and some jurisdictions set their own figures and maximums. Treat this as the starting point and confirm against the code that applies to your job.