Duct Weight Calculator
A takeoff tool for sheet metal quantities: enter the duct size, run length, gauge and material, and it returns weight per metre and for the whole run, plus the insulation and lagging surface area that usually gets priced alongside it. Galvanised steel, aluminium and stainless densities are built in, with a seam and reinforcement allowance you can set.
Sheet metal for 24.0 in × 12.0 in
337.4 kg
11.07 kg/m · 744 lb total · 24 ga (from pressure class)
| Gauge and nominal thickness | 24 ga · 0.701 mm · 0.0276 in |
| Perimeter | 72.0 in |
| Weight per metre | 11.07 kg/m · 7.44 lb/ft |
| Weight for the run | 337.4 kg · 744 lb |
| Surface area per metre | 1.829 m²/m · 6.000 ft²/ft |
| Surface area for the run (insulation) | 55.7 m² · 600 ft² |
| Cross-sectional area | 0.1858 m² |
| Allowance applied | 10% on bare sheet |
Straight duct only. Fittings develop more metal per unit length than straight duct and are not included — count them separately. Gauge selected automatically is indicative; confirm against the SMACNA construction tables for the reinforcement class and joint type actually specified.
Why the perimeter, not the area, decides the price
Duct is sized on area, because area carries the air. Duct is bought by weight and insulated by surface, and both of those follow the perimeter. The two diverge fast, and that divergence is where estimates go wrong.
Take two ducts with almost identical free area. A 500 × 480 section encloses 0.24 m² with a perimeter of 1.96 m. A 1200 × 200 section encloses the same 0.24 m² with a perimeter of 2.80 m. Same air, same velocity, same friction rate to within a little — but 43 per cent more sheet metal, 43 per cent more insulation, more hangers because it is heavier, and more stiffener because a 1200 mm flat side drums. On a 200 metre run that difference is a real number on a real quotation, and it was decided the moment somebody chose the aspect ratio.
How to use the duct weight calculator
- Enter the duct size and the run length. Internal dimensions and the developed length of the run — the length the metal actually follows, not the plan-view distance.
- Choose the material. Galvanised steel at 7850 kg per cubic metre is the default. Aluminium at 2710 weighs about a third as much for the same gauge, stainless at 8000 slightly more than steel.
- Let the gauge follow the pressure class, or set it by hand. Automatic selection uses the pressure class and the governing dimension. If the specification names a gauge, switch to manual and use it.
- Set the seam and reinforcement allowance. A bare perimeter calculation misses seams, joints, stiffeners and angle. Ten per cent is a reasonable default for normal work; heavily reinforced high-pressure duct needs more.
- Read weight and surface area together. Weight prices the metal and sizes the supports. Surface area prices the insulation, the lagging and the paint, and it is usually the bigger number on a quotation.
Formulas and a worked example
- Weight per metre: P × t × ρ × (1 + allowance)
- Perimeter, round: P = πD
- Perimeter, rectangular: P = 2(a + b)
- Perimeter, flat oval: P = πa + 2(W − a)
- Surface area per metre: equal to the perimeter — the figure insulation is priced on
Worked example
A 600 mm round duct in 24 gauge galvanised steel. Perimeter = π × 0.6 = 1.885 m. Nominal thickness for 24 gauge is 0.701 mm, and galvanised steel is 7850 kg/m³, so bare sheet is 1.885 × 0.000701 × 7850 = 10.37 kg per metre. Add a 10 per cent seam and reinforcement allowance and the takeoff figure is 11.41 kg per metre. Over a 30 metre run that is 342 kg of metal and 56.5 m² of surface to insulate.
Gauge thickness and material density
| Gauge | mm | in | kg/m² (steel) |
|---|---|---|---|
| 26 ga | 0.551 | 0.0217 | 4.33 |
| 24 ga | 0.701 | 0.0276 | 5.50 |
| 22 ga | 0.853 | 0.0336 | 6.70 |
| 20 ga | 1.006 | 0.0396 | 7.90 |
| 18 ga | 1.311 | 0.0516 | 10.29 |
| 16 ga | 1.613 | 0.0635 | 12.66 |
Nominal thicknesses for galvanised sheet. Densities used: galvanised steel 7850, stainless 8000, aluminium 2710 kg/m³. Gauge selection by pressure class is set out on the SMACNA duct gauge chart.
Frequently asked questions
How do you calculate the weight of ductwork?
Multiply the duct perimeter by the sheet thickness to get the volume of metal per unit length, then by the density of the material. For a 600 mm round duct in 24 gauge galvanised steel: perimeter 1.885 m, thickness 0.701 mm, density 7850 kg/m³, giving 10.4 kg per metre of bare sheet. Add an allowance for seams, joints and reinforcement — 10 per cent brings it to 11.4 kg per metre.
Why is the surface area the same as the perimeter?
Because surface area per metre of duct is simply the perimeter multiplied by one metre. It is worth stating explicitly because that same figure is what insulation, lagging, cladding and paint are all priced on, and on an insulated duct the insulation line usually exceeds the sheet metal line on the quotation.
What allowance should I add for seams and joints?
Ten per cent is the common planning figure and is the default here. It covers longitudinal seams, transverse joints and light stiffening. Heavily reinforced duct — high pressure class, long unsupported spans, large flat sides — carries considerably more angle and stiffener, and 15 to 20 per cent is closer for that work. For a firm price, take the reinforcement schedule off the SMACNA tables and count it.
Does a flatter duct weigh more?
Yes, materially. A 1200 × 200 duct and a 500 × 480 duct enclose almost the same area, but their perimeters are 2800 mm and 1960 mm — the flat one needs about 43 per cent more metal per metre, plus more insulation, more hangers and more reinforcement because the wide flat sides need stiffening. Aspect ratio is a cost decision long before it is an airflow decision.
Should I use nominal or actual gauge thickness?
This calculator uses the nominal thicknesses published for galvanised sheet — 0.551 mm for 26 gauge up to 1.613 mm for 16 gauge. Mill tolerance means delivered sheet varies either side of nominal, and galvanising adds a small amount of coating weight on top. For estimating, nominal is the right basis; for a contractual weight, work from the mill certificate.
Does this include fittings?
No — it covers straight duct only. Elbows, tees, transitions, take-offs and end caps all consume more metal per unit length than straight duct because of their pattern development and waste. On a congested plant room run the fittings can add 20 to 40 per cent to the sheet metal, so count them separately rather than scaling this figure up.
Last updated: 26 July 2026