📐 Ductwork Offset Calculator
Calculate piece lengths, angles & equivalent run to route ducts around obstacles
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Offset Results
Offset Angle Reference Table
| Angle | Sin | Cos | Run for 6" Rise | Run for 12" Rise | Piece Length (6") | Piece Length (12") |
|---|---|---|---|---|---|---|
| 30° | 0.500 | 0.866 | 10.39" | 20.78" | 12.00" | 24.00" |
| 45° | 0.707 | 0.707 | 6.00" | 12.00" | 8.49" | 16.97" |
| 60° | 0.866 | 0.500 | 3.46" | 6.93" | 6.93" | 13.86" |
Piece Length by Angle & Rise — Spec Grid
| Rise | 30° Piece | 30° Run | 45° Piece | 45° Run | 60° Piece | 60° Run |
|---|---|---|---|---|---|---|
| 3" | 6.00" | 5.20" | 4.24" | 3.00" | 3.46" | 1.73" |
| 4" | 8.00" | 6.93" | 5.66" | 4.00" | 4.62" | 2.31" |
| 6" | 12.00" | 10.39" | 8.49" | 6.00" | 6.93" | 3.46" |
| 8" | 16.00" | 13.86" | 11.31" | 8.00" | 9.24" | 4.62" |
| 10" | 20.00" | 17.32" | 14.14" | 10.00" | 11.55" | 5.77" |
| 12" | 24.00" | 20.78" | 16.97" | 12.00" | 13.86" | 6.93" |
| 14" | 28.00" | 24.25" | 19.80" | 14.00" | 16.17" | 8.08" |
| 16" | 32.00" | 27.71" | 22.63" | 16.00" | 18.48" | 9.24" |
Common Obstacle Reference
| Obstacle Type | Typical Rise Needed | Recommended Angle | Notes |
|---|---|---|---|
| Floor Joist (2×10) | 4–6" | 45° | Clear bottom flange |
| Floor Joist (2×12) | 6–8" | 45° | Add 1" clearance |
| LVL / Beam | 8–12" | 30°–45° | Check full depth |
| Water Pipe (1") | 3–4" | 60° | Minimal run needed |
| Water Pipe (2–4") | 5–7" | 45°–60° | Allow clearance each side |
| Attic Knee Wall | 6–8" | 45° | Use insulated duct |
| Basement Beam | 8–10" | 30° | Gentle angle preferred |
| Structural Header | 10–14" | 30° | May need 3-piece offset |
Planning Tips
Ductwork offset are when section of ductwork must move sideways or vertically for circle something on its way In new buildings, ductwork occasionally are offset some inches in closets or other places because of electrical cables or other obstacles. That happens very commonly in HVAC installations.
Exist particular products for spiral pipe and accessories used in HVAC systems. Round metal ductwork use offset connectors, for instance 4-inch round elbow tube, for help the pipe circle obstacles. Some of them are adjustable from 0 until 90 degrees for alter the direction of the air in round branches.
Duct Offsets: How to Measure and Make Them
They are very useful when you need rigid tube for heating, venting or air conditioning.
For rectangular ductwork, count offsets are a bit more mathematical. The main formula bases on the width of the ductwork divided by its length. For instance, if ductwork are 20 inches broad and 5 feet long, and it requires 2-inch offset, you count: 20 multiplied by 2 are 40, and divided by 5, that gives 4 inches extracted every side.
Do the offset are broad or high depend on the case. It seems difficult initially, but you learn it by means of prcatice and skill.
Here other example. If the offset are 3 inches and the width is 4 inches on bit of 60 inches, that results in 0.2 inches extracted every aspect. Simply said, the formula is made up of mitering the ductwork for create the necessary corner.
For offset from three bits, the calculation becomes a bit more complex: you share the result by means of 2.
Know to count offsets directly on the job site are very helpful. Like this you can adapt to surprises without need re-order material from the store. The best part is when you do that twice on the same bit for create simultaneously offset and rise.
Exist even app called Easy Offset, that automatically counts the distance for mitering and have various useful utilities.
Rigid ductwork-elbows from several sections can be rotated less than 90 degrees. Pair of elbows rotated to equal corner create exactly the involved offset. If is enough space, little offset it is possible to do very easily.
In AC systems, the ceiling tubes to the unit no always are in direct line; they are offset, included of the connection at the AC-device. For simplify the draft, you can use offset-box, what are well recall during the planning of the way.
