Girth is the number the shop works in
Rectangular duct is specified by width and height, but nothing in the shop cares about those separately. What matters is the distance round the outside, which is twice the width plus twice the height, and that single figure sets the blank width, the transverse joint length, the reinforcement member length and the weight per foot. A 24 by 12 duct and an 18 by 18 duct have the same girth of 72 inches and cost the same in metal, even though one carries noticeably more air than the other.
That equivalence is why duct sizing and duct fabrication pull in different directions. For a given cross-sectional area, the squarer shape has the smaller girth, so it uses less metal and has less surface to lose heat through. The flatter shape fits above a ceiling. Which one wins is a job decision, and the metal cost of losing it is exactly the girth difference times the length of the run.
| Duct | Free area (sq in) | Girth (in) | Sheet per foot of run (sq ft) |
|---|---|---|---|
| 18 x 18 | 324 | 72 | 6.00 |
| 24 x 12 | 288 | 72 | 6.00 |
| 36 x 8 | 288 | 88 | 7.33 |
| 48 x 6 | 288 | 108 | 9.00 |
The last row is the one to look at. Same air, half again the metal, and that is before the extra reinforcement a wide flat side needs.
Blanks, pieces and where the seams go
A section of duct is rarely one piece of sheet. Two L-shaped halves is the common arrangement, giving two longitudinal seams on opposite corners; four flat sides gives four; a single wrapper with one seam is possible on small duct where the girth fits inside the sheet width. The choice is driven by the sheet you can buy and by what the brake and the lock former will take, and it changes the blank width by a factor of two or four.
Each longitudinal seam consumes blank width, and that consumption belongs to the lock you actually run. A Pittsburgh lock takes a pocket on one edge and a plain flange on the other; a grooved seam takes a fold on both. Numbers get quoted for these and the numbers differ between shops, between machines and between gauges. The only figure worth using is one measured off a sample formed on your own equipment, which is why this page takes it as an input rather than supplying one.
The ends of a section work the same way. Some transverse joints are formed out of the duct itself and eat blank length; others are separate components that add nothing to the blank but plenty to the parts list. Set the end allowance to zero for the second kind.
Sheets, not square feet
Total blank area divided by sheet area gives a number that is always too small, because blanks do not tile a sheet. The calculator lays them out as a straight grid in whichever orientation fits more, which is the honest lower bound for a piece that has to come out in one flat rectangle. On a 48 by 120 sheet with a blank 37 by 62, you get one per sheet and three fifths of the metal goes to the drop, and the fix is to change the section length so two blanks fit rather than to argue with the arithmetic.
That is the practical value of running the take-off before cutting. Section length is often the only free variable, and moving it from 60 inches to 58 can be the difference between one blank and two per sheet. Rectangular nesting in general is the sheet nesting calculator.
What the specification decides and this page does not
Gauge, reinforcement spacing, reinforcement member size, joint class, sealing class and pressure class are all set by the construction standard the job was specified to, and by whatever authority adopted it. They vary with duct size, with pressure, and with the standard in force where the work is. Nothing here selects any of them; the gauge box, the spacing box and the member weight box are inputs, and the answers are only as good as what goes in them.
The gauge selector deserves a specific warning. Gauge is not one standard. The manufacturers standard gauge used for carbon and galvanized sheet, the stainless steel gauge series and the Brown and Sharpe series used for aluminum give three different thicknesses for the same number. The selector here covers galvanized and carbon steel only, and anything else needs a caliper reading in the override field. Full conversions both ways are on the sheet metal gauge chart.
Weight, and why it comes up late
Metal weight per square foot is thickness times density times 144. For 24 gauge galvanized, taking the coating into account, that is about 1.13 pounds, so a 40 foot run of 24 by 12 needs around 257 square feet of blank and carries close to 290 pounds of metal before a single hanger. That figure decides how the sections get up the stairs and what the hangers are carrying, and it is worth having before the truck arrives rather than after. Weight of other stock shapes is the metal weight calculator.
Questions people ask
How do I calculate sheet metal for rectangular duct?
Start with the girth, which is twice the width plus twice the height. That is the width of flat sheet one running foot of duct needs, before seams. Divide the girth by the number of pieces you make a section from, add the blank consumed by each longitudinal seam, and that is your blank width. The blank length is the section length plus whatever each end takes for the transverse joint. Multiply blank width by blank length by the number of blanks and you have the flat area; convert to sheets by laying the blanks out as a grid rather than dividing areas.
What gauge should duct be?
That comes from the construction standard the job is specified to, and it depends on duct size, pressure class and reinforcement spacing rather than on any single rule. Larger duct and higher pressure move to heavier metal, and heavier reinforcement at closer spacing can allow lighter metal at the same size. This page takes the gauge as an input because the answer belongs to your specification and to the authority that adopted it, not to a calculator.
Why is my sheet count higher than the square footage suggests?
Because blanks are rectangles that have to come out whole. Dividing total blank area by sheet area assumes the offcuts can be reassembled, and they cannot. If a blank is 37 inches wide, only one fits across a 48 inch sheet and eleven inches goes to the drop on every sheet. Section length is usually the variable you can move: shortening a section by a couple of inches so two blanks fit where one did changes the yield more than any other decision in the take-off.
How much does a lock seam take off the blank?
Enough that guessing costs you sheets, and not a number anyone else can give you accurately. It depends on the seam type, on the gauge, and on how your particular lock former is set. The reliable method takes about ten minutes: cut a strip of the gauge you are running, form the seam on your machine, and measure how much narrower the strip finished than it started. That figure goes in the input box and stays right until the machine is adjusted.
Does this cover elbows and fittings?
No. It takes off straight rectangular duct only, and fittings are where the metal and the labour actually concentrate on most jobs. A radius elbow, a square throat with turning vanes, an offset and a transition each have their own development, and the sheet they need bears little relation to their length. Round to round transitions can be laid out with the cone and cylinder flat pattern calculator; the rest are drawn rather than calculated.