Square / Rectangular Duct Suspension Calculator
Weight model based on SMACNA method (sheet weight × surface area per linear foot). Reference table is noted as "per SMACNA".
Inputs
Results
Reference: Sheet Weights & Method (per SMACNA)
Weight/ft = (2 × (W+H) / 12) × (sheet weight for selected gauge). Sheet weights shown are typical galvanized steel values consistent with SMACNA tables.
2.656 lb/ft²
2.156 lb/ft²
1.656 lb/ft²
1.406 lb/ft²
1.156 lb/ft²
0.906 lb/ft²
0.781 lb/ft²
This calculator estimates bare duct weight only (no liner, insulation, flange, or accessory loads). Always verify against project specifications.
How to Calculate Rectangular Duct Weights
This tool simplifies the math required by SMACNA standards to determine the hanging weight of square or rectangular galvanized steel ducts.
Enter Dimensions: Input the duct Width and Height in inches. The calculator automatically computes the perimeter and surface area per linear foot.
Select Gauge: Choose the sheet metal gauge (standard 26ga to 16ga). The tool applies the standard SMACNA density for galvanized steel.
Set the Span: Enter the distance between your supports (typically 8ft or 10ft).
Verify Capacity: Select your cable system rating.
Note: Because rectangular ducts require a trapeze setup (one hanger on each side), the calculator automatically doubles the cable rating to determine the Effective Safe Working Load (SWL) per support location.
The Cable Advantage for Rectangular Duct
Suspending rectangular ductwork traditionally involves cutting two threaded rods, measuring a strut channel, and assembling washers and nuts at four connection points. Cable suspension replaces this entire assembly with lightweight wire and locking mechanisms.
Fast Leveling: Rectangular ducts must be perfectly level to avoid twisting. Cable locks allow for instant, independent adjustment on the left and right sides without using tools.
Reduced Vibration: Flexible cable systems often transmit less vibration noise to the building structure compared to rigid threaded rods.
Improved Airflow: Cable suspension is less bulky than strut channels, reducing airflow obstruction in crowded plenum spaces.
Frequently Asked Questions (Rectangular Duct)
Why does the calculator double the cable rating?
Rectangular ducts are almost always suspended by a “Trapeze” method, meaning there are two hangers at every support location (one on the left, one on the right). If you use a 100 lb rated cable on both sides, your total capacity at that support location is 200 lbs. The calculator accounts for this automatically.
Does this calculate weight for lined or insulated duct?
No. This calculator estimates the weight of bare galvanized steel only. If your duct has internal acoustic liner or external wrap, you must add that weight manually or include a safety margin in your fittings allowance.
What formula is used for the weight?
We use the standard Surface Area method: Weight = (Perimeter / 12) × Sheet Weight × Length. The sheet weights used (e.g., 0.906 lbs/sq ft for 26ga) are taken directly from standard SMACNA reference tables.
Can I use this for Stainless Steel or Aluminum?
No. This tool is calibrated for Galvanized Steel density. Aluminum is roughly 1/3 the weight of steel, while Stainless Steel is slightly heavier. Using this tool for Aluminum would result in a very conservative (safe) estimate, but using it for heavy-gauge Stainless Steel might underestimate the load.
Legal Disclaimer & Usage Notice
The results provided by this calculator are for informational and estimation purposes only. While the weight models are based on standard SMACNA industry data, actual duct weights may vary depending on the specific sheet metal manufacturer, seam type, reinforcement, insulation, and liners used.
Important:
Verify Capacities: Always reference the specific cable manufacturer’s data sheets for Safe Working Loads (SWL) and safety factors.
Professional Review: These calculations do not constitute professional engineering advice. A licensed structural engineer or qualified project manager must review and sign off on all load calculations and suspension methods prior to installation.
Liability: www.cablesuspension.com assumes no liability for errors, omissions, or damages resulting from the use of this tool. It is the user’s responsibility to ensure compliance with all local building codes and project specifications.