100% money-backBook a walkthrough
Construction glossary · MEP

What is ductin construction?

A duct in construction is the sheet metal (or occasionally fiberglass or fabric) passage that moves supply, return, and exhaust air between HVAC equipment and the spaces it serves. In commercial estimating, ductwork is taken off in pounds of sheet metal or square feet of duct surface, computed from the duct dimensions, the metal gauge, and the run length. It is the core of the sheet metal contractor's scope and usually the biggest line in the mechanical bid after the equipment itself.

Book a walkthrough Get a Free Takeoff

Free 30-minute walkthrough on your own drawings. No card.

Updated August 2026 · Reviewed by the Ruh construction team

Measured by lb of sheet metalGauge by duct size & pressureShapes rect, round, oval

Rectangular duct section

Sheet metal wallAir pathSheet metal wall24 ga24x12 in
The definition

Duct, explained.

Duct is the distribution network of the HVAC system, and in commercial work it lives on the mechanical (M series) sheets as single line or double line layouts with sizes noted as width x depth in inches, such as 24x12. The sheet metal contractor owns the scope, usually as a subcontractor to the mechanical contractor or directly to the GC, fabricating rectangular duct in a shop from galvanized steel and buying round and spiral duct from a supplier. Rectangular duct dominates tight ceiling plenums, round and spiral run cheaper per pound and leak less, and flex duct handles the last few feet to a diffuser. New estimators get burned in predictable spots: taking off linear feet without converting to pounds, ignoring fittings (elbows, transitions, taps) that can carry a third or more of the labor, missing that drawing dimensions are inside clear dimensions so lined duct takes more metal, and skipping the gauge changes that come with pressure class. A duct takeoff that is just lf of straight runs is not a duct takeoff.

How it is measured

Sheet metal duct is measured in pounds or in square feet of duct surface. The estimator takes the duct size off the mechanical plan, computes the perimeter, multiplies by the run length to get sf of surface, then multiplies by the weight of the specified gauge (per the galvanized sheet gauge table and SMACNA construction standards) to get pounds. Fittings are counted and converted to equivalent pounds or equivalent lf because they cost far more labor per pound than straight duct. Round and spiral duct is often taken off in lf by diameter instead. Cost drivers are gauge, pressure class, fitting density, liner, and height of install. The sizes live on the duct layout; the gauges live in the spec and the SMACNA schedule, not on the plan.

Worked example

A worked duct example.

Worked example

Take 100 lf of 24x12 rectangular duct at 26 gauge. Step 1, perimeter: 2 x (24 in + 12 in) = 72 in, which is 6 ft. Step 2, surface: 6 sf of metal per lf of run, so 100 lf x 6 sf = 600 sf. Step 3, weight: 26 gauge galvanized sheet weighs 0.906 lb per sf, so 600 sf x 0.906 = 543.6 lbs of straight duct. Step 4, allowance: seams, joints, reinforcement, and hangers commonly add around 25 percent, so 543.6 x 1.25 = 679.5, carried as 680 lbs. Step 5, price: at an illustrative fabricated and installed rate of $12 per lb, 680 lbs x $12 = $8,160 for the run. Note what gauge does to the math: the same run at 22 gauge (1.406 lb per sf) would weigh 600 x 1.406 x 1.25 = 1,054.5 lbs before a single fitting is added.

How Ruh handles it

How Ruh handles duct.

100%

Try Ruh on a real bid. 100% money-back guarantee if you are not satisfied.*

*Scoped delivery, terms apply. Read the guarantee terms

Start with a walkthrough

Duct: frequently asked questions

How is ductwork measured in a construction estimate?+

Rectangular sheet metal duct is converted to pounds: perimeter times run length gives square feet of surface, and the gauge weight in lbs per sf turns surface into weight, following SMACNA-based fabrication practice. Some estimators carry sf of duct surface instead and price per sf. Round and spiral duct is commonly taken off in lf by diameter, and flex duct in lf as part of the diffuser runout assembly.

What is the difference between rectangular, round, and spiral duct?+

Rectangular duct is shop fabricated from flat sheet and fits tight ceiling spaces, but it uses more metal per cfm of air and costs more per pound to make. Round duct is stronger per pound, leaks less, and handles pressure better; spiral duct is machine wound round duct that is often left exposed as a finished look. Many systems mix them: rectangular trunks in the plenum, spiral mains in open ceilings, round runouts to diffusers.

Do duct sizes on drawings include the liner?+

Duct sizes on mechanical plans are clear inside dimensions, the free area the air actually sees. When a duct is internally lined, the sheet metal must be fabricated larger, two times the liner thickness in each direction, so a 24x12 duct lined with 1 in liner is really 26x14 of metal. Takeoffs that compute weight from the plan dimensions undercount the steel on every lined run, which is a common and quiet estimating miss.

Still measuring by hand? Your next takeoff can run while you review this one.

See it on your plans

Related terms

Duct insulationVAV boxDuctwork gaugeRTU unit (HVAC)AHU (air handling unit)DiffuserDamperVRF system

Keep going: read the full guide or explore AI construction takeoff and estimating software.

The MEP estimating cheat sheet
Every unit, waste factor, and conversion on one page. Free PDF.
Previous term
Diffuser
Next term
Duct insulation

Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.

100% money-back guarantee

Try Ruh on a bid you are pricing now.

Put your next takeoff on Ruh.