A GC estimating a 40-unit apartment building in Chattanooga opens five HVAC bids and sees a 20% spread between low and high. Nobody is price-gouging. The mechanical subs read the same drawings and priced different scopes.
That pattern is common, and the spread is usually scope, not price. Most ductwork estimating content comes from two places: service vendors selling outsourced takeoffs and software vendors selling fabrication databases. This article takes the GC's chair instead. You'll see how duct is quantified and priced, then how to pressure-test a mechanical number before you carry it into your bid.
Why Duct Is the Line Item Where Mechanical Bids Drift Apart
Most GCs sub out HVAC, but you still own the budget. When two duct numbers sit 20% apart, the gap rarely comes from the price of galvanized sheet metal. It comes from insulation, fittings, dampers, access doors and testing, which one sub carried and another assumed you would.
Duct is hard to compare because it's a bundle. A single lump sum can hide a dozen decisions about what's in and what's out. To compare two bids, you need your own picture of the quantities underneath them.
Self-Perform vs. Sub-Out: Who Actually Needs a Duct Takeoff
A sheet metal contractor needs a detailed takeoff to fabricate and price: gauge by gauge, fitting by fitting. A GC who subs the work needs quantities for two other jobs: checking the budget and leveling bids.
The pricing mechanics below serve both readers. The plan-reading, leveling and cross-trade sections are written for the GC.
The Budget-Stage Problem: Pricing Duct Before the Mechanical Sub Is Hired
At 50% drawings, you still need a defensible HVAC number for an owner or lender. The usual shortcut is a dollars-per-square-foot figure for the whole mechanical system.
That figure ignores how the building is laid out. A compact 40-unit building with short runs and a dense plan prices very differently from a spread-out office floor with long trunk lines. Even a rough quantity pass, with duct length, sizes and fitting counts, gives you something to defend that a per-square-foot guess can't.
How Ductwork Estimating Works: Pounds, Linear Feet, Fittings and Labor
Sheet metal contractors price duct by the weight of metal plus labor hours, not by simple linear feet. A foot of 8" round pipe and a foot of 36x20 rectangular trunk share a unit of measure and almost nothing else. Fittings carry much of the labor and weight, which is why a pure length count misleads.
Rectangular, Round, Spiral and Flex: Different Materials, Different Math
Rectangular duct is measured by perimeter times length to get square feet, then converted to weight by gauge. Gauge follows the pressure class and the longest side. For reference, 24-gauge galvanized weighs roughly 1.16 lb per square foot, and 22-gauge about 1.41, per this galvanized sheet metal weight chart.
Spiral and round duct are priced by diameter and length, and they install faster per foot. Flex duct is typically a per-foot material and labor price with a connection cost at each end. Spiral and rectangular runs aren't interchangeable in a unit price because the material, fabrication and install labor all differ. Mixing them into one $/LF number is where budgets go wrong.
Why Fittings Drive the Number
Elbows, transitions, tees and takeoffs are fabricated one at a time, which makes them labor-heavy. Take the same 200 LF of 24x14 main trunk in 24-gauge, which comes to about 1,464 lb of straight duct.
These rates are illustrative, not published figures. Assume straight duct installed at $8 per lb and fittings at $16 per lb.
- Light fitting count: about 220 lb of fittings. Total: $11,712 + $3,520 = $15,232, or about $76 per LF.
- Heavy fitting count: about 512 lb of fittings from added elbows, transitions and tees. Total: $11,712 + $8,192 = $19,904, or about $100 per LF.
Same length, same size, a 31% difference. A per-LF figure hides that completely.
Insulation, Liner, Hangers and Accessories
External wrap, internal liner and hangers add real cost, and subs handle them inconsistently. So do volume dampers, access doors, fire and smoke dampers, and the diffusers, grilles and registers at the ends of the run.
Some subs carry all of it in the duct price. Others list insulation separately, and some leave fire and smoke dampers to the GC or the life-safety scope. Treat each one as a question to ask, not an assumption.
From Pounds to Dollars: Labor Hours and Unit Costs
The conversion has two parts. Material cost is weight times a metal price plus waste. Labor is quantity times a production rate, in hours per hundred pounds or per fitting, times your crew's loaded rate. Overhead and markup go on top.
Production rates vary by region, union status, access conditions and the shop's own history. Don't borrow a published rate and treat it as yours. Use rates from your own past jobs or from the sub's numbers when you can get them.
Reading the Mechanical Plan Set for Ductwork Estimating
Duct information is spread across the M-series. Know where each piece lives and you can build a quick quantity check without a full HVAC takeoff. For the rest of the set, see how to read MEP drawings.
Plans, Schedules, Details and the Spec Section
Duct layout sheets show routing, sizes and fittings, usually at 1/8" or 1/4" scale. Air device schedules give diffuser and grille counts with CFM. Equipment schedules show RTUs, air handlers and VAV boxes. Riser diagrams show vertical runs, and detail sheets show hangers, seismic bracing and connections.
The specification is where scope gets set. In MasterFormat, Division 23 holds the requirements: 23 31 00 for ducts and casings, 23 07 13 for duct insulation and 23 05 93 for testing, adjusting and balancing. Look there for the SMACNA pressure class, sealing requirements, liner and insulation, and who tests what. The specs often add cost the plans never show.
Where Takeoff Errors Hide
Scale errors are the classic one: a sheet printed at 11x17 instead of 24x36 turns every length wrong. Vertical risers often don't appear in plan views, so shafts get undercounted. Revisions between addenda change sizes and routing, and the takeoff doesn't get updated. Duct also collides with beams, joists and ceiling heights that the mechanical sheets don't show.
The super who's been in the field for 20 years can often spot a routing problem on a trunk line in 30 seconds. He'll see 24" of duct running under a 20" beam in a corridor with a 9' ceiling. The problem is that nobody sends him the plans before bid day, so the conflict surfaces as a change order after award. A 15-minute review with your super or a mechanical foreman before you finalize a number is cheap insurance.
Leveling Mechanical Sub Bids: A Ductwork Estimating Checklist for GCs
The point of leveling is to make sure a low bid isn't low because of what it leaves out. One GC we talked to, on a $14M senior-living project, told us his HVAC sub was $60K under the next bidder. After award, he learned the sub had excluded test and balance and the penetration patching, which he ended up carrying at roughly $38K. The low bid wasn't a bargain. It was a smaller scope.
The Inclusions and Exclusions That Move the Number
These are the items that most often swing a mechanical bid. Build a column for each in your leveling sheet and require a yes, no or by-others from every sub.
- Duct insulation and internal liner
- Fire, smoke and combination dampers
- Access doors and volume dampers
- Seismic bracing and vibration isolation
- Controls and BAS integration
- Test and balance
- Roof curbs, rigging and crane
- Permits and inspection fees
- Patching and firestopping of penetrations
Sanity-Checking a Duct Price Against Your Own Takeoff
Run a rough quantity pass from the plans: total duct pounds or square feet, fitting count, and device count. Apply your own unit costs, then compare each sub's number to yours.
If a bid comes in far under your check, ask what it's carrying. If it's well over, ask whether the extra is insulation, liner or heavier pressure class. Frame it as a scope question: "Your duct line is about 25% below our check. Can you confirm insulation, fire dampers and test and balance are included?" That gets a useful answer. "Are you sure about your price?" doesn't.
Handling Sub Questions and Addenda Without Losing a Day
Mechanical subs ask a lot of questions on duct: pressure class, insulation thickness, who supplies the dampers. Each answer has to reach every bidder, and each addendum can change someone's number.
Struvia drafts answers to subcontractor questions on a bid package, and its bid leveling normalizes scope across sub bids and flags gaps and exclusions, so a missing damper line or a test and balance exclusion shows up in the comparison. You still review the answers and make the scope calls. The tool reduces the tracking work. It doesn't replace judgment.
HVAC Estimating Software vs. Spreadsheets for Duct Takeoff
Most teams take off duct in one of three ways. Some measure manually in Bluebeam Revu (tiers at $260, $330 and $440). Others use a dedicated takeoff tool such as PlanSwift ($1,595 one-time license or about $1,749 a year) or STACK ($2,999 a year for a single seat). The third group uses mechanical estimating software built for sheet metal shops. For more on one of these, see our STACK review and our roundup of free estimating software.
What Mechanical Estimating Software Does Well, and What a GC Doesn't Need
Mechanical estimating software is built around fabrication databases, fitting libraries and labor units. For a sheet metal shop that's the whole point: you need to price every elbow and drive the shop floor from the estimate.
A GC who subs HVAC usually doesn't need that depth. Budget checks and bid leveling require quantities and a price book, not a fabrication database. Paying for HVAC estimating software that you use for 5% of its features is a common mismatch.
Takeoff-Only Tools vs. Takeoff-to-Estimate
Per-seat takeoff tools such as Togal ($299 per user per month, billed yearly) stop at quantities, which you export to Excel and price yourself. Struvia carries the takeoff into a priced estimate in your own Excel template, and bills on usage in credits instead of seats.
Struvia isn't a sheet metal fabrication or shop-floor system. It's built for the GC's side of the job.
A Practical Workflow: Takeoff, Price Book, Sub Bids
The workflow looks like this:
- Upload the mechanical plan PDF.
- Generate AI-assisted quantities.
- Price from your company price book.
- Compare the result against sub bids.
Reviewing the output stays your job. Check scale, boundaries and duct classes before the numbers go anywhere.
Ductwork Doesn't Live Alone: Coordinating With Electrical, Plumbing and Demo Scopes
Gaps and double-counting happen at the seams between trades. Mechanical, electrical and plumbing each assume the others will pick up the connection. If you want to see how the electrical side is typically estimated, our guide to Accubid alternatives covers it.
Electrical and Plumbing Touchpoints
Put a name next to each of these before bids go out:
- Power and disconnects for RTUs, air handlers and exhaust fans
- Condensate drains from coils and units
- Gas piping to rooftop units and furnaces
- Controls and thermostat wiring
Electrical estimating software and electrical takeoff software handle counts and conduit runs for their own trade, and plumbing estimating software does the same for piping. Those quantities don't talk to the mechanical bid, so you have to check the overlap by hand. The usual result is a gap, or the same disconnect priced twice.
Renovation Work: Demo, Reuse and Tie-Ins
Remodels add existing duct demolition, capping, reuse and tie-ins to the new system. Subs often say "demo by others" or "reuse existing as-is," and neither is a price.
Coordinate your demo line with the mechanical bid, whether it comes from demolition estimating software or a manual line in your template. Walk the space and decide what's coming out, what's staying and what gets capped. Then confirm in writing which scope each party carries.
Build a Duct Price Book From Your Own Jobs
Every awarded mechanical bid is a data point. Turn them into unit costs such as dollars per pound, per LF and per CFM, and your budget checks improve with each job.
What to Capture From Each Awarded Mechanical Bid
Record the scope inclusions and exclusions, building type, square footage and tonnage. Add the duct quantity if you know it, the final subcontract value and any change orders. The change orders matter most because they show what the original number really missed.
Keep the data in a consistent format so you can compare across jobs. Don't treat it as precise. It's a reference range for your own market and your own subs.
Frequently Asked Questions
How do you estimate ductwork cost?
Measure duct by type and size, then convert to sheet metal weight. Add fittings, insulation, accessories and labor hours. Apply overhead and markup to get the price.
Is ductwork priced per linear foot or per pound?
Sheet metal contractors typically price by weight plus labor. GCs often use per-LF or per-square-foot figures for budgets. Per-LF figures hide the fitting count, so two runs of the same length can differ by 30% or more.
How accurate is a rough HVAC budget from square footage?
It's useful at concept stage only, and it varies widely by building type and system. Move to a quantity-based check as soon as mechanical drawings exist. A per-square-foot number can't see routing, fittings or insulation.
What should be in an HVAC subcontractor bid?
A complete bid covers equipment, duct and insulation, dampers, controls, test and balance, permits and rigging. It should also state its exclusions clearly. If a bid doesn't list them, ask.
Can AI do a duct takeoff from a PDF plan set?
AI-assisted takeoff can speed quantity capture from uploaded PDFs. The estimator still has to verify scale, boundaries and classes. Struvia works this way: it generates quantities from your plan PDF and you review them before pricing.
What is the difference between HVAC estimating software and general takeoff software?
HVAC estimating software typically embeds duct fabrication databases and labor units. General takeoff and estimating tools measure and price from your own data. The first fits a sheet metal shop, and the second fits a GC doing budget checks and bid leveling.
The Bottom Line on Ductwork Estimating
A duct number is only as reliable as its scope and quantities. Know what's included, run your own rough takeoff, and ask the sub about any gap before award. That's what keeps a 20% spread from turning into a change order.
If you want to test this on a live job, upload a mechanical plan set free, run the takeoff and compare your mechanical bids against it. New accounts start with 600 free credits.
*Reviewed by Weston Burnett, Co-Founder and CTO of Struvia.*
