Rectangular offset calculator
Use when you know the offset amount, duct/layout width, and available length. This gives the clean O.W.L. layout result without choosing an angle.
O.W.L. field formula
O.W.L. = Offset × Width ÷ Length
O = Offset. How far the duct needs to move.
W = Width. The duct width or layout width you are using for the pattern.
L = Length. The available offset length / run.
O.W.L. layout result = Offset × Width ÷ Length
Square/rectangular slope check: angle = atan(offset × width ÷ length)
Common fitting travel: travel = offset ÷ sin(angle)
Round/spiral rolling offset: true offset = √(rise² + roll²)
Pipe-style / rolling offset calculator
Use for round or spiral when the offset moves in one direction or rolls both sideways and vertically. Enter fitting takeoffs only if your foreman/shop gives them to you.
Round/spiral fitting angle multipliers
| Angle | Travel multiplier true offset ÷ sin(angle) | Run multiplier true offset ÷ tan(angle) |
|---|---|---|
| 15° | 3.864 | 3.732 |
| 22.5° | 2.613 | 2.414 |
| 30° | 2.000 | 1.732 |
| 45° | 1.414 | 1.000 |
| 60° | 1.155 | 0.577 |
How to use this in the field
Measure the real offset and the real available run. Confirm the duct tag, elevation, airflow direction, insulation thickness, service access, and hanger/support before cutting or ordering.
For round/spiral, fitting takeoff values are not universal. Enter the takeoff only when your shop, fitting, manufacturer, or foreman gives it to you.
This calculator is a layout helper. It does not approve cutting, drilling, rerouting, changing fittings, modifying equipment, changing fire/smoke/rated assemblies, or replacing drawings/specs/manufacturer instructions/foreman direction.