HVAC Duct Takeoff:
Linear Feet, Fittings, and Sheet Metal

Separate duct by type

Never mix duct types on a single line item. Rectangular, oval, round, and spiral duct each carry different gauge requirements, fabrication labor rates, and material prices per linear foot. Treating them as a single pool leads to systematic errors that compound across a large mechanical set (Regal Estimating / Wendes, 2026).

Within each type, break out every unique cross-section as its own line. A 12x8 and a 14x10 rectangular duct are not the same commodity — sheet metal weight scales with perimeter, so even a few inches of difference changes pounds-per-foot and material cost. Separate sizes also make the mechanical schedule cross-check clean.

Flexible duct deserves its own section entirely. It is counted separately from rigid sheet metal, priced by the foot with a different labor unit, and is subject to installation length limits under SMACNA guidelines that affect how much you can legitimately quote. Mixing flex into a rigid duct line will understate labor and overstate material on the flex portion simultaneously.

• Rectangular, oval, round, and spiral duct — separate line items by type and by size
• Sheet metal weight scales with perimeter (width + height) × 2 × length
• Flexible duct counted separately from rigid; subject to SMACNA length limits

Measure linear feet and sanity-check

Once duct types are separated, measure main trunk, branch runs, and individual takeoffs as distinct categories rather than one total run length. Main trunk runs carry the highest static pressure and often require heavier gauge and reinforcement, so conflating them with branch runs distorts both material cost and labor hours. Takeoffs — the short stubs that connect a trunk to a branch — are sometimes missed entirely on plan view.

Add vertical risers in mechanical chases that do not appear in the floor plan. On multi-story construction, these can represent a significant fraction of total linear footage and they are easy to overlook because they show only in elevation or section drawings. A disciplined estimator works through the plan view, then checks every mechanical room and shaft detail before closing the quantity sheet.

Use benchmark ranges to sanity-check your totals before pricing. An 1,800 sq ft home with roughly 10 rooms and a basement typically requires 300–500 linear feet of duct; a 1,000–2,500 sq ft residential project runs approximately 150 LF (Modernize / Chase Estimating, 2023). Commercial projects scale differently by occupancy type, but having a rough LF-per-square-foot expectation for your typical project type will flag gross misses before they become bid errors.

• Measure main trunk, branch runs, and takeoffs as separate quantities
• Add vertical risers from elevation and section views — not just plan view
• Sanity-check: ~300–500 LF for an 1,800 sq ft residential project with 10 rooms

Count fittings and transitions

Fittings are where ductwork takeoffs most often go short. Every elbow, tee, transition, offset, and end cap is a discrete fabricated piece with its own material weight and installation labor — often two to three times the labor per piece compared to a straight foot of duct. A systematic fitting count is not optional on a mechanical bid.

Work through the drawing methodically: elbows first, then tees and wyes at every branch connection, then transitions where duct size changes, then offsets and end caps. Tally each category by duct size — a 24-inch elbow and an 8-inch elbow are not the same fabrication cost.

Air devices — diffusers, registers, grilles, and volume dampers — must be verified against the mechanical schedule, not just counted from the plan. The schedule is the controlling document. Hangers and supports are typically priced as an allowance: code spacing for sheet metal duct is every 8–10 feet, so total linear footage gives a reliable minimum count.

• Elbows, tees, transitions, offsets, end caps — tally each by duct size
• Verify diffusers, registers, grilles, and dampers against the mechanical schedule
• Hangers and supports: code spacing typically every 8–10 ft for sheet metal

Convert to sheet metal weight

Once linear feet and fittings are tallied, the next step is converting duct surface area into a sheet metal quantity. The formula is straightforward: perimeter times length gives surface area in square feet, and gauge determines the weight per square foot. For a 12x8 duct, perimeter is 40 inches (3.33 ft), so 100 linear feet yields 333 sq ft of surface area. Multiply by the pounds-per-square-foot for the applicable gauge to get your material tonnage.

Gauge selection follows SMACNA pressure class requirements. Low-pressure supply systems typically use 26-gauge for smaller sizes, stepping up to 24- or 22-gauge for larger rectangular duct and higher static pressure classes. Return air at lower pressures can run lighter. Heavier gauges and external reinforcement apply when ducts exceed the span limits in the SMACNA HVAC Duct Construction Standards, which is the reference document your fab shop will use regardless.

Insulation and liner takeoff follows the same surface area math. Where drawings specify internal liner, apply the liner square footage directly. Add a material waste factor — typically 5–10% for sheet metal and 10–15% for insulation — before pricing.

• Sheet metal quantity = duct perimeter × length (surface area) at SMACNA-specified gauge
• Heavier gauges and reinforcement for larger ducts and higher static pressure classes
• Insulation/liner taken off at the same surface area; add 5–15% waste factor

Estimate labor hours

Labor is typically the largest cost line on a ductwork bid, and the productivity benchmark that most estimators use for planning is approximately 25 linear feet of main duct per man-day on a clean installation. By that standard, 200 linear feet of main trunk equals roughly 8 man-days, or 64 hours (Chase Estimating, 2023). Branch runs and flex duct typically install faster per foot; fitting-heavy sections install slower, sometimes as low as 10–15 LF per man-day depending on access and fitting density.

Cross-check duct sizing against equipment CFM before finalizing labor estimates. The rule of thumb is approximately 1 CFM of airflow per 1 to 1.25 square feet of conditioned floor area (ServiceTitan, 2024). If your duct quantities produce a total CFM capacity that is far off from the equipment schedule, either the takeoff has an error or the design has a scope gap — either way it is worth surfacing before bid day rather than during construction.

Adjust for access conditions. Exposed warehouse ductwork installs faster than ductwork in a congested mechanical room or a finished ceiling cavity. A difficulty factor of 1.0 to 1.5 on base labor is a reasonable range — calibrate it from your crew's own history on similar project types.

• Benchmark: ~25 LF per man-day for main duct; 200 LF ≈ 8 man-days (64 hours)
• Fitting-heavy runs reduce productivity to 10–15 LF per man-day
• CFM cross-check: ~1 CFM per 1–1.25 sq ft of floor area (ServiceTitan, 2024)
• Apply access/difficulty multiplier (1.0–1.5) based on project conditions

Questions estimators actually ask