Ductwork takeoff and HVAC quantity takeoff

Reading mechanical drawings, in order

1. Legend and abbreviations: symbols, duct types, notes. Confirm which dimension is width.
2. Equipment schedules: AHUs, RTUs, fans, VAV boxes, diffusers with sizes and CFM.
3. Plans: duct routing with sizes, diffuser locations, equipment.
4. Sections, risers and details: heights, drops, offsets, connections, supports.
5. Specifications (Division 23): duct construction standard, pressure classes by system, sealing, materials, insulation and liner, testing.

Read the duct construction spec first

Most specs reference the SMACNA HVAC Duct Construction Standards, now ANSI/SMACNA 006-2020 (4th edition). Three things drive cost:

• Pressure class by system: ½, 1, 2, 3, 4, 6 or 10 in. w.g. Higher classes mean heavier gauge, closer reinforcement and tighter sealing.
• Seal class: SMACNA ties Class A to 4 in. w.g. and above, Class B to 3 in., and Class C to 2 in., and recommends Class C at lower pressures. Energy codes and specs often require more, so read Division 23.
• Leak testing: the 2024 IECC requires ducts designed for 3 in. w.g. and above to be tested on at least 25% of duct area to leakage class 4.0 or better. Specs may require more, including low-pressure systems.

Duct takeoff by size

Field	Example
System	Supply, return, exhaust, outside air
Size	24×12 rectangular, 12 in round, 30×12 flat oval
Length	LF from plan, plus risers and drops from sections
Pressure class	Per spec, by system
Material and gauge	Galvanized, stainless, aluminum; gauge from SMACNA tables
Insulation / liner	SF by thickness and facing

Keep systems separate. It helps pricing, review and project tracking.

Worked example: weight of one run (illustrative)

60 LF of 24×12 supply duct, 2 in. w.g. class, assumed 24 gauge from the SMACNA tables for that size and reinforcement:

• Perimeter = 2 × (2.0 + 1.0) = 6.0 ft
• Surface area = 6.0 × 60 = 360 SF
• Weight = 360 × 1.156 lb/SF = 416 lb straight duct
• Add seams, reinforcement and waste from your own history. A blanket percentage is only a rule of thumb; heavy fittings content on short runs can add far more.
• External wrap, if required: 360 SF plus overlap, by thickness.

A branch takeoff (illustrative)

A supply branch serving four offices: 16×10 duct for 60 ft, reducing to 12×8 for 40 ft, with four 8 in round runouts of about 10 ft each, volume dampers and flex connections to diffusers.

Item	Quantity	Unit
16×10 duct	60	LF
12×8 duct	40	LF
Reducer 16×10 to 12×8	1	EA
8 in round taps with volume dampers	4	EA
8 in round duct	40	LF
Flex connections (length per spec limit)	4	EA
Diffusers	4	EA

Many specs limit flex duct length at each diffuser. Check the limit before taking off long flex runs.

Fittings

Count fittings by type and size: elbows (with or without turning vanes), transitions, takeoffs, tees, offsets and end caps. For rectangular sheet metal, shops price either by weight, with fittings weighed individually or as an allowance from history, or by piece. Pick one method and use it consistently so quotes compare.

Devices and accessories

• Diffusers, registers and grilles by type, size and neck size.
• Dampers: volume, fire, smoke, combination fire/smoke, with actuators and access doors where required.
• VAV and terminal units.
• Access doors, flexible connectors, turning vanes, sound attenuators.
• Hangers and supports, per the spec and seismic requirements where they apply.

Fire and smoke dampers carry wiring, access and inspection requirements; confirm who wires actuators.

Insulation and liner

Measure external wrap by outside surface area (perimeter × length) and keep it separate by thickness and facing. Note which systems need it, typically supply and outside air in unconditioned spaces, per the energy code and spec. Lined duct is usually sized up so the inside clear dimension matches the drawing; check which dimension the drawings show.

Checks before pricing

• [ ] Sizes taken from plans, not assumed from adjacent runs.
• [ ] Vertical risers and drops included from sections and riser diagrams.
• [ ] Pressure class assigned per system.
• [ ] Dampers, access doors, turning vanes and flexible connectors counted.
• [ ] Insulation and liner taken off separately.
• [ ] Exhaust, return and outside air systems all included.
• [ ] Leak testing quantity and pressure from the spec and code.

Revit models

When a model is available, duct lengths and fittings can be scheduled from it. Check that it's complete for your scope, that fittings are modeled, and that sizes match the issued drawings.

Where RenderDraw fits

RenderDraw reads mechanical plans with schedules and specs, proposes equipment counts and duct runs with the source attached, and puts uncertain items in front of your estimator first. Your gauges, labor and pricing apply to reviewed rows only.