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The standard allowance most suppliers and estimating guides assume for ordinary work.
Rectangular takes a width and a height; round takes a diameter.
Choose the section the duct schedule gives. The area is the section's perimeter times the length either way: twice the width plus the height for a rectangle, π times the diameter for a round duct.
The duct's cross-sectional width (the wider side, by convention).
Measured as the outside sheet metal dimension. Combined with height, this sets the duct's perimeter, which drives the surface area per unit length.
The duct's cross-sectional height (the narrower side, by convention).
For surface area the two sides count equally, so transposing them changes nothing here — which is exactly why the convention is worth keeping. Duct is scheduled width first and height second, and a duct entered the other way round gives the right insulation quantity for a duct that will not fit the ceiling void it was drawn into. This page will not catch that; the schedule is where it has to be right.
The length of the straight duct run being fabricated.
Fittings, transitions, and elbows are not straight runs — take those off separately by piece, not as a length here.
An extra percentage of material added to cover seams, laps, and cutting waste.
Commonly around 10% for shop-fabricated duct with efficient nesting; field fabrication with more cutting waste may warrant a higher allowance. This is an estimating convention, not a code requirement.
Sheet metal surface area needed
167.9 sq ft
- Base surface area (before waste)
- 152.63 sq ft
They open the calculator with your figures already in it
Duct Sheet Metal Surface Area Calculator (Rectangular and Round): 168 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Surface area = 2 × (Width + Height) × Length (perimeter × length), the standard method for straight rectangular duct fabrication material takeoff — fittings/transitions are taken off separately by piece. A waste/scrap allowance (commonly ~10% for shop-fabricated duct, more for field fabrication) is a standard estimating convention, not a code requirement.
- Round duct: the circumference of a circle is π × its diameter, so a straight round duct's surface is π × diameter × length, the same perimeter-times-length method as the rectangular case
Inputs used
- Duct Shape
- Rectangular
- Duct Width
- 16 in
- Duct Height
- 11.75 in
- Duct Diameter
- 6 in
- Straight Run Length
- 33 ft
- Waste/Scrap Allowance (%)
- 10
Intermediate steps
- Base surface area (before waste)
- 152.63 sq ft
What this calculation does not cover
- Returns area, not a metal order. Sheet gauge comes from the duct's larger dimension and its pressure class in the SMACNA construction tables, and it is the gauge that sets weight, price per square foot, and whether the sides oil-can at pressure. The same 15 m² is two very different purchases at 26 gauge and at 20 gauge.
- Excludes everything that holds the duct together and up. Transverse joint hardware (drive cleats, S-slips, TDC/TDF flange), stiffening angles on the larger sizes, and hanger strap and rod all scale with the number of joints and the hanger spacing, not with surface area — a long run made up in short sections carries far more of this than the area suggests.
- The dimensions entered are the metal size. An internally lined duct has to be made larger by twice the liner thickness on each axis to keep the free area the airflow was sized for, so entering the airflow dimensions on a lined duct buys metal for a duct that is undersized the moment the liner goes in.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-11 · v1.0.1
Regulatory standards & verification citations2
- Surface area = 2 × (Width + Height) × Length (perimeter × length), the standard method for straight rectangular duct fabrication material takeoff — fittings/transitions are taken off separately by piece. A waste/scrap allowance (commonly ~10% for shop-fabricated duct, more for field fabrication) is a standard estimating convention, not a code requirement.
- Round duct: the circumference of a circle is π × its diameter, so a straight round duct's surface is π × diameter × length, the same perimeter-times-length method as the rectangular case
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