Quick answer: pipe slope = total drop ÷ run length. In field units: slope (in/ft) = drop in inches ÷ run in feet. For percent grade, divide the in/ft figure by 12 and multiply by 100 — so ¼″/ft = 0.25 ÷ 12 × 100 ≈ 2.08%.
The three ways slope is written
The same physical grade shows up in three notations, and every trade seems to prefer a different one:
- Inches per foot (in/ft) — North American plumbing standard: "¼ inch per foot".
- Percent grade (%) — civil/site work: "a 2% slope". Percent = rise ÷ run × 100 in the same unit.
- Ratio / gradient (1:X) — metric and Canadian code notation: "1:50" means 1 unit of fall per 50 of run. See drainage fall & gradient in metric.
Conversion table
| Slope (in/ft) | Percent grade | mm per metre | Angle |
|---|---|---|---|
| 1/16″/ft | 0.52% | 5.2 mm/m | 0.30° |
| ⅛″/ft | 1.04% | 10.4 mm/m | 0.60° |
| 3/16″/ft | 1.56% | 15.6 mm/m | 0.89° |
| ¼″/ft | 2.08% | 20.8 mm/m | 1.19° |
| ⅜″/ft | 3.13% | 31.3 mm/m | 1.79° |
| ½″/ft | 4.17% | 41.7 mm/m | 2.39° |
| ¾″/ft | 6.25% | 62.5 mm/m | 3.58° |
| 1″/ft | 8.33% | 83.3 mm/m | 4.76° |
These are exactly the presets built into Pipe Slope Calculator — the app shows in/ft, percent and metric together on every result.
Worked example 1: find the slope you have
You're checking an existing 3″ branch. The run is 16 feet and a laser level shows the far end is 2.5 inches lower.
- Slope = 2.5″ ÷ 16 ft = 0.156 in/ft (just under 3/16″/ft)
- Percent = 0.156 ÷ 12 × 100 ≈ 1.3%
- Verdict: clears the IPC's ⅛″/ft (0.125 in/ft) minimum for 3″ pipe — but falls short of the UPC's ¼″/ft. Same pipe, different code, different answer.
Worked example 2: find the drop you need
New 2″ laundry drain, 22-foot run, code requires ¼″/ft:
- Drop = 22 ft × 0.25 in/ft = 5.5 inches of fall across the run
- If you only have 3″ of height available, the slope would be 3 ÷ 22 = 0.136 in/ft — below minimum. Time to re-route, not to fudge.
Skip the arithmetic: do it in the app
Pipe Slope Calculator runs both directions of this math with code checking on top:
- Choose Find Slope (length + drop → slope) or Find Drop (length + slope → drop).
- Enter your numbers — sliders, presets or the keyboard.
- Read all three notations at once: total drop, slope in in/ft (or mm/m), and percent grade, plus a code-compliance verdict for your pipe size under IPC, UPC or NPC.
- Tap Copy Results to paste the full calculation anywhere.
Related questions
What is the formula for pipe slope?
Slope = drop ÷ length. In field units: in/ft = drop (inches) ÷ run (feet). Percent = in/ft ÷ 12 × 100.
What is a 2% slope in inches per foot?
2% = 0.24 in/ft; the field equivalent is ¼″/ft (2.08%). Over 10 ft, 2% drops about 2.4″.
How do I calculate slope in metric?
Fall (mm) ÷ run (m) = mm/m. 1:50 = 20 mm/m = 2%; 1:100 = 10 mm/m = 1%. See the metric gradient guide.
The formula, automated and code-checked
Length in, answer out — drop, slope and grade with IPC/UPC/NPC compliance. 100% offline, $2.99 once.
