Quick answer: ¼ inch per foot means the pipe drops 0.25″ for every foot of run — a 2.08% grade, about 20.8 mm/m, or an angle of roughly 1.19°. Over 20 feet that's 5 inches of total drop.
¼″/ft is the plumbing slope — the IPC minimum for small drains, the UPC standard for everything, and the number every apprentice hears first. Here's the full arithmetic so you never have to redo it on drywall.
Drop chart: ¼ inch per foot
| Run length | Total drop | Run length | Total drop |
|---|---|---|---|
| 1 ft | 0.25″ | 15 ft | 3.75″ |
| 2 ft | 0.5″ | 20 ft | 5″ |
| 3 ft | 0.75″ | 25 ft | 6.25″ |
| 4 ft | 1″ | 30 ft | 7.5″ |
| 5 ft | 1.25″ | 40 ft | 10″ |
| 6 ft | 1.5″ | 50 ft | 12.5″ |
| 8 ft | 2″ | 75 ft | 18.75″ |
| 10 ft | 2.5″ | 100 ft | 25″ |
| 12 ft | 3″ | — | — |
Rule of thumb hiding in the chart: 1 inch of drop every 4 feet, or about a foot of drop every 50 feet.
¼″/ft in every notation
| Notation | Value |
|---|---|
| Inches per foot | 0.25 in/ft |
| Percent grade | 2.08% (codes round to "2 percent") |
| Metric fall | ≈20.8 mm/m |
| Ratio / gradient | ≈1:48 (the NPC's 1:50 ≈ ¼″/ft) |
| Angle | ≈1.19° |
Where ¼″/ft is required
- IPC 2024: minimum for horizontal drains 2½″ and smaller (3″+ may go flatter — see the full minimum-slope chart).
- UPC 2024: minimum for all sizes, with a possible AHJ-approved exception at 4″+.
- NPC 2020 (Canada): 1:50 — effectively the same grade — for pipe up to 3″.
Marking it out with a level
On a standard 24″ torpedo level, ¼″/ft means the downhill end reads ½″ low over the level's length. Many levels have a marked 2% vial for exactly this. No level handy? Your phone works too — see measuring pipe slope with an iPhone.
Get any drop instantly in the app
- Choose Find Drop mode.
- Tap the ¼″/ft preset (it's one of eight built-in presets from 1/16″ to 1″ per foot).
- Enter your exact run length — 17½ ft doesn't appear on any chart, but the app doesn't care.
- Read the drop, grade and mm/m equivalents, with a code check for your pipe size layered on top.
Related questions
How much drop is ¼″/ft over 10 feet?
2.5″. Run (ft) × 0.25 = drop in inches.
What percent grade is ¼″/ft?
≈2.08% (0.25 ÷ 12 × 100). Metric: ≈20.8 mm/m. Angle: ≈1.19°.
Which pipes require ¼″/ft?
IPC: 2½″ and smaller. UPC: all sizes (4″+ AHJ exception possible). NPC: up to 3″ (as 1:50).
Charts are for round numbers. Jobs aren't.
Exact drop for your exact run, in one tap — offline, code-checked, $2.99 once.
