Ductwork weight calculator: sheet metal pounds by run

How duct is taken off

Sheet metal is bought, fabricated, and bid by the pound, so a duct takeoff is a length to weight conversion. You work the mechanical sheets run by run: read the size tag off each segment (a rectangular duct is called out width by depth, and the width is the dimension you see on plan), pick up the length, and record the gauge the specification and the pressure class require at that size. Every size change and every gauge change starts a new line. A 36 by 18 trunk and a 14 by 10 branch are not comparable per foot, so they never share a line.

Then the shop ticket confirms the unit. Straight duct is cut from coil or flat sheet, fittings are laid out as developed patterns, and both get weighed in pounds before they get priced. Sheet metal labour is commonly carried in hours per pound rather than hours per foot as well, so the pound sits on both sides of the priced line. A linear foot of duct is not a quantity of anything until you know the perimeter and the gauge.

The formulas

Perimeter (in) = 2 x (width + depth) Square feet of sheet per linear foot = perimeter / 12 Sheet metal area (sf) = square feet per linear foot x length in feet Weight (lb) = area x pounds per square foot for the gauge Pounds per linear foot = square feet per linear foot x pounds per square foot

The perimeter over 12 step is the whole trick. It converts a cross section measured in inches into the square feet of flat sheet needed to wrap one foot of run. A 24 by 12 duct has a 72 in perimeter, so it takes 6 sf of sheet per foot of duct, and at 24 gauge that is 6.94 lb for every foot you install.

Galvanized sheet weights by gauge

Duct is galvanized steel and the weights are published, not estimated. The values below are the galvanized column of the Manufacturers' Standard Gauge for Sheet Steel, which is the gauge system SMACNA duct construction is written against:

• 26 gauge: 0.0217 in nominal, 0.906 lb per sq ft
• 24 gauge: 0.0276 in nominal, 1.156 lb per sq ft
• 22 gauge: 0.0336 in nominal, 1.406 lb per sq ft
• 20 gauge: 0.0396 in nominal, 1.656 lb per sq ft
• 18 gauge: 0.0516 in nominal, 2.156 lb per sq ft

Those printed figures are rounded. The galvanized table is set in ounces per square foot, and each galvanized gauge is the US Standard Gauge weight plus 2.5 oz per sq ft for the zinc coating, so the exact values are 14.5, 18.5, 22.5, 26.5 and 34.5 oz per sq ft, which is 0.90625, 1.15625, 1.40625, 1.65625 and 2.15625 lb. The calculator carries the exact values. As a sanity check on any gauge chart, the standard is based on 41.82 lb per square foot per inch of thickness, so 0.0336 in x 41.82 lands on 1.405.

Here is the gotcha, and it is on a lot of charts. Two gauge tables exist. Read weights off a US Standard Gauge column and you are 2.5 oz per sq ft light on every square foot of duct, which is about 17 percent at 26 gauge and about 7 percent at 18 gauge. If a chart shows 26 gauge near 0.75 lb per sq ft instead of 0.906, it is the uncoated table and it is the wrong one for galvanized duct.

Gauge is not a preference, which is why you cannot price duct by the foot

Rectangular duct gauge comes out of the SMACNA HVAC Duct Construction Standards, Metal and Flexible. Two rules from the rectangular duct chapter change how a takeoff is structured. First, on gauge selection: "The greater duct dimension determines the duct gage for all four sides. This applies to reinforced and unreinforced ducts." A 36 by 6 duct is gauged on the 36, not on an average and not on the small side. Second, the gauge schedules are one table per static pressure class, published for 1/2, 1, 2, 3, 4, 6 and 10 in wg, with duct dimension down the side and reinforcement spacing across the top (10 ft down to 2 ft), and each cell giving both a minimum gauge and a minimum reinforcement grade. Read the pressure class off the drawings, because the standard requires the contract documents to identify it, and use the edition your specification references.

The pricing consequence is direct. Going up in size raises the perimeter and can force a heavier gauge at the same time, so pounds per foot climb on two axes at once. In the calculator defaults, the 14 by 10 branch at 26 gauge is 3.63 lb per lf and the 36 by 18 trunk at 22 gauge is 12.66 lb per lf. Same job, same system, three and a half times the metal per foot. A single dollars per linear foot rate applied across that mix is wrong twice, high on the branches and low on the trunk, and the errors do not cancel because the mix is different on every job.

What straight duct weight misses

The number this calculator returns is straight duct, and straight duct is the easy part of a sheet metal bid. Four things sit outside it.

Fittings. Elbows, transitions, offsets, takeoffs, tees, and end caps carry more metal and far more labour per foot of run than straight duct. A 90 degree elbow is a developed pattern with a throat, a heel, and two cheeks, it uses more sheet than the length it occupies, and it takes shop time to lay out, cut, form, and seam. The accurate way to handle fittings is to count them off the plans and convert each one to its developed sheet area, which is what a fabrication shop does off the ticket. A percentage is the shortcut for a bid that has to go out today.

There is no published figure for that percentage. On a fitting-light system, a warehouse main with long straight runs, the allowance sits low, in the 10 to 20 percent range. On a fitting-dense system, a tenant fit-out with a takeoff every few feet, tight transitions, and offsets around structure, the allowance can approach or exceed the weight of the straight duct itself. The 25 percent in the calculator field is a placeholder that keeps the arithmetic visible, not a recommendation. Set your own number the only way that holds up: pull the fabrication pounds off finished jobs, compare them against the straight run pounds from the same takeoff, and keep that ratio by job type.

Hangers and reinforcement. Hanger rod, strap, trapeze angle, and intermediate reinforcement members are counted and priced separately, at the spacing the standard requires for the duct size and pressure class.

Sealant and gasket. Priced against joint and seam length and the seal class in the specification, not against the pounds.

Insulation. External wrap and internal liner are measured in square feet of duct surface, which is the area number this calculator gives you, not the weight number. That is a separate line at a separate unit price, and on many jobs a separate subcontractor.

Scrap is the fifth item and it belongs to purchasing rather than the bid quantity. Blanks are nested out of coil or sheet stock and the drops do not come back, so metal purchased runs above metal installed. That percentage is a property of your nesting and your stock widths, so take it from your own material records rather than a rule of thumb.

Worked example

Take the calculator defaults, four runs off a small system.

• 36 x 18 trunk, 32 ft, 22 gauge: perimeter 2 x (36 + 18) = 108 in, so 108 / 12 = 9 sf per lf, 9 x 32 = 288 sf, and 288 x 1.40625 = 405 lb
• 24 x 12 main, 48 ft, 24 gauge: perimeter 72 in, 6 sf per lf, 6 x 48 = 288 sf, and 288 x 1.15625 = 333 lb
• 18 x 12 branch, 64 ft, 26 gauge: perimeter 60 in, 5 sf per lf, 5 x 64 = 320 sf, and 320 x 0.90625 = 290 lb
• 14 x 10 branch, 96 ft, 26 gauge: perimeter 48 in, 4 sf per lf, 4 x 96 = 384 sf, and 384 x 0.90625 = 348 lb

That is 240 linear feet of duct, 1,280 sf of galvanized sheet, and 405 + 333 + 290 + 348 = 1,376 lb of straight duct. At the placeholder 25 percent fitting allowance, add 344 lb, for 1,720 lb. The 240 lf, the 1,280 sf, and the 1,376 lb are measurement. The 344 lb is judgment, and it belongs on its own visible line in the estimate where a reviewer can argue with it.

Now the two numbers that make the point. Straight duct averages 1,376 / 240 = 5.73 lb per lf across this system. The individual runs are 3.63 lb per lf on the 14 by 10 and 12.66 lb per lf on the 36 by 18. Price this system at any single rate per foot and the mix decides whether the job makes money.

From pounds to a priced line

Pounds are the quantity half of a sheet metal bid. The priced line applies a material rate per pound, a shop fabrication rate, and a field install rate, then adds the items that never price per pound at all: hangers, sealant, insulation, dampers, grilles and diffusers by the each, access doors, and test and balance. For how those pieces come together into a submitted number see the guide on how to bid HVAC as a subcontractor. The wider mechanical workflow, including where the pound-based quantities land in a bid, is covered on the HVAC estimating software page.

That is the step Ruh handles. It reads the drawing set, measures the scope run by run off the mechanical sheets, and prices those measured quantities against the contractor's own price book, with every line traceable back to the sheet it came from. Your estimator reviews the run list, the gauges, the allowances, and the rates before anything goes into the bid.