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Plumbing Math the Exam Expects — Offsets, Fall, Head and Pressure, Volume and Weight of Water, Fixture-Unit Sums
Closed-book exams cannot ask you to look up a table, so they ask you to *use* a few constants. These are the same under any code.
2018 UPC and 2018 IPC · Reviewed 2026-09-20
1. Offsets (the 45-degree rule and its cousins)
An offset moves a run of pipe sideways. Offset = the sideways distance; travel = the length of the diagonal pipe between the two fittings; run = the distance covered along the original direction.
| Fitting angle | Travel = offset × | Run = offset × |
|---|---|---|
| 45° | 1.414 | 1.000 |
| 60° | 1.155 | 0.577 |
| 22-1/2° | 2.613 | 2.414 |
| 11-1/4° | 5.126 | 5.027 |
Example: a 12-in offset with 45° fittings → travel = 12 × 1.414 = 16.97 in (about 17 in); the run is also 12 in. Subtract the fitting allowances (center-to-face) to get the cut length. A rolling offset uses the true offset = √(offset² + rise²) before multiplying.
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