Uniform straight low-pressure run
Project-specific skin weights can support a useful preliminary line-load record before hardware and structure design.
Home & Construction
HVAC Duct Load Calculator
Keep distributed dead load separate from point equipment loads and report demand only; no hanger, anchor, trapeze, structure, or seismic resistance is invented.
SUSPENDED DUCT DEMAND
Separate distributed duct weight from point equipment loads
Sheet, insulation, liner, and distributed accessories form a line load. A planning factor and hanger spacing create span demand; point loads remain separate.
Total planning demand-
Planning line load-
Interior-span distributed load-
Hanger count-
Separate point load-
Distributed service weight-
SUBJECT DECISION ILLUSTRATION
Real-world context followed by an exact current calculation record
The illustration explains the professional setting without pretending to be a data chart. The ledger below preserves the entered values and calculation path.
| Load component | Area / length basis | Weight / factor | Component demand | Line / support result | Boundary |
|---|
How to use
Record suspended duct demand without asserting a support capacity
- Enter the surface area and represented centerline run length.
- Enter product-specific sheet, insulation, and liner weights per area.
- Add distributed accessory weight separately.
- Keep heavy dampers or equipment as point loads.
- Enter a proposed hanger spacing and documented planning factor.
- Pass line, span, and point demands to qualified support and structural design.
Suspended-load fundamentals
Five demand records prevent false capacity claims
- Skin area
- Represented duct surface carrying area-based weights.
- Distributed weight
- Sheet, insulation, liner, and allocated accessory weight over a run.
- Line load
- Distributed weight divided by run length.
- Span demand
- Planning line load multiplied by hanger spacing.
- Point load
- Localized force retained at its actual support region.
Calculation method
Build distributed load by area and preserve localized equipment
Area-based weights multiply the entered duct skin area. Distributed accessories are added before dividing by centerline length. The planning factor applies to this line load. Hanger spacing creates an interior distributed-span screen, while the point load remains separate and is added only to the aggregate planning demand.
Weight boundary
Representative pounds per square foot must match the construction
Gauge, material, reinforcement, joints, stiffeners, liner, insulation, facing, sealant, and accessories determine actual weight. Use a project or manufacturer schedule rather than a generic universal value.
Reaction boundary
Average distributed load is not an actual trapeze reaction
Ends, unequal spacing, fittings, offsets, risers, branches, dampers, access doors, flexible connections, equipment, and support stiffness change reactions. Preserve locations for design.
Pressure and seismic boundary
Dead load is only one demand family
Internal pressure, collapse, vibration, fan thrust, thermal movement, seismic restraint, impact, maintenance loads, and fire/smoke system actions require separate construction and support checks.
Detailed calculation process
Suspended duct equations and default substitution
Wdistributed = A(ws + wins + wliner) + Waccessorywservice = Wdistributed / Lwplanning = wservice × FplanningRspan = wplanning × spacingWtotal,planning = wplanning L + Wpoint
- Sheet weight = 520×1.6 = 832 lb.
- Insulation and liner = 520×(0.35+0.22) = 296.4 lb.
- Add 180 lb distributed accessories for 1,308.4 lb distributed service weight.
- Service line load = 1,308.4/80 = 16.355 lb/ft.
- Planning line load = 16.355×1.15 = 18.808 lb/ft.
- Interior span = 188.083 lb; aggregate planning demand adds the separate 95 lb point load.
The current cards and component ledger provide the live reconciliation.
Result interpretation
Demand communicates what the support system must address
Use line and point-load records to coordinate hangers and supporting structure. Do not label the result safe, allowable, or compliant without defining hardware, anchors, trapeze geometry, structure, load combinations, and the governing standard.
Decision analysis
Move point loads to real support locations
Heavy dampers, access doors, coils, fans, silencers, and equipment connections should not be smeared across the run when local reactions matter. Provide independent support when required by the design.
Evidence and data lineage
Retain construction and support inputs
Keep model/drawing revision, duct section and route, material, gauge, reinforcement, joints, liner and insulation product weights, accessory and point-load schedule, hanger spacing and layout, planning-factor basis, seismic category, anchor and structure references, designer, date, and unrounded demand export.
Limits and exclusions
Automatic exclusions
- No hanger, rod, trapeze, anchor, deck, beam, slab, or structure capacity.
- No pressure, collapse, vibration, thrust, thermal, seismic, fire, impact, or fatigue load.
- No actual end, fitting, branch, riser, equipment, or unequal-span reaction analysis.
- No support-spacing approval, code decision, or construction authorization.
Key terminology
Duct support terms
- Gauge
- Sheet thickness designation.
- Liner
- Material installed on the duct interior.
- Line load
- Distributed force per unit route length.
- Trapeze
- Cross-member support suspended by rods.
- Point load
- Localized load at a defined location.
- Reaction
- Force transferred through a support.
Worked decision cases
Distributed and localized loads demand different records
Damper and silencer cluster
Concentrated components, reinforcement, vibration, and connection forces require localized supports and cannot rely on an average hanger value.
Authoritative basis
References for duct construction and support context
Important note
This page calculates entered dead-load demand only. Qualified mechanical, seismic, and structural design must establish real support spacing, hardware, anchors, structure, load combinations, and compliance.
Frequently asked questions
Why use surface area for weight?
Sheet, liner, and insulation mass generally scale with duct skin area.
Does the point load affect line load?
No. It remains separate so its actual support location can be reviewed.
Is the planning factor a code safety factor?
No. It is an entered demand multiplier and cannot replace governing load combinations.
Why is hanger count one more than spans?
A straight sequence of spans normally has support points at both ends.
Can the page select hanger spacing?
No. Spacing is an input that must be justified by the approved system and design.
Does duct pressure add support load?
Pressure affects construction and can create forces not represented by this dead-load model.