FieldEngineersKitEngineering Reference & ASME Code Basis
1. Formula & conversion factors
International Plumbing Code (IPC) Table 704.1 — horizontal drain slopes (screening) · ASME B31.3 (2022 Edition) · Process Piping — gravity drain context (screening)
S = ΔH / L · θ = arctan(S)
Ratio = 1 : (1/S) · Pass if S ≥ IPC min(NPS)
IPC Table 704.1–style bands by NPS · ASME B31.3 gravity-drain screening context
Geometric slope from rise (drop) over run length, reported as percent, in/ft or mm/m, and 1:N ratio. IPC Table 704.1–style minima: ≤3 in → 1/4 in/ft; 4–6 in → 1/8 in/ft; ≥8 in → 1/16 in/ft. Screening only — not for pressurized or two-phase lines. Full-pipe Manning velocity is out of scope here.
- ΔH (Rise / drop) — Vertical fall over the run (in or mm).
- L (Run) — Horizontal (or along-pipe) length (ft or m).
- S (Slope) — ΔH/L — also shown as % , in/ft, mm/m, and 1:N.
- NPS (Nominal pipe size) — Selects the IPC Table 704.1–style minimum slope band.
2. Quick reference lookup table
| NPS band | Min slope | As % | As mm/m |
|---|---|---|---|
| ≤ 3 in | 1/4 in/ft | 2.08% | 20.8 |
| 4–6 in | 1/8 in/ft | 1.04% | 10.4 |
| ≥ 8 in | 1/16 in/ft | 0.52% | 5.2 |
Building-drain screening minima. Plant process gravity lines may require steeper slopes — confirm project specs. Example duties (4"·2.5 in/10 ft, 100A·60 mm/3 m) are on the pSEO paths, not code minima.
3. How to use this calculator
- 1
Select NPS
NPS sets the IPC Table 704.1–style minimum slope band for Pass/Fail.
- 2
Enter rise and run
Rise is vertical drop (in or mm). Run is length (ft or m). Toggle metric/imperial in the navbar.
- 3
Read slope % and ratio
Hero shows percent slope with in/ft or mm/m and 1:N ratio, plus IPC Pass/Fail.
- 4
Use drop rows
Result rows scale the same slope to other run lengths for layout checks.
4. Engineering FAQ
About 2.08%. 0.25 in of fall per foot of run is 2.5 ÷ 120 ≈ 0.0208, or roughly a 1:48 ratio.
IPC Table 704.1 typically allows 1/8 in/ft for 4–6 in drains. Steeper slopes such as 1/4 in/ft still comply.
No — by design. This tool focuses on geometric slope and IPC minimum-slope screening. Full-pipe Manning velocity needs diameter and roughness assumptions that overstate precision for most plant drain checks.
Carry NPS into pipe-wall-thickness or flange-dimension-weight for the same line size.