Area of Rectangular Duct Calculator
Enter the side dimensions, length and quantity of a rectangular duct to get the total sheet-metal surface area, connector strip (shina) length and corner piece count, with an optional seam allowance.
Ventilation Duct Surface and Component Calculator
Area of Rectangular Duct Formula
Where:
- \(A\), \(B\) — side dimensions of the duct cross-section (mm)
- \(L\) — length of one duct section (mm)
- \(N\) — number of duct sections
- \(A_s\) — total surface area of the duct walls (m²)
- \(P_s\) — total length of the connector strip, or shina, for the flanges (m)
- \(N_c\) — number of flange corner pieces (pcs)
- \(SC = 0.02\) m if seams are considered, otherwise \(SC = 0\)
- \(S = 0.12\) m if seams are considered, otherwise \(S = 0\)
All dimensions are entered in millimetres and converted to metres inside the formula. The first expression is the perimeter of the duct cross-section (plus the seam allowance) multiplied by the length; the second gives the total strip length needed for the flange frames on both duct ends; the third counts eight corner pieces per duct section — four for each end flange.
Area of Rectangular Duct – Calculation Example
Example: a straight rectangular duct with Side A = 500 mm, Side B = 300 mm, Length = 1000 mm, one duct section, and seams considered.
\[ A_s = 2\left(0.5 + 0.3 + 0.02\right) \cdot 1 \cdot 1 = 1.64 \ \text{m}^2 \]
\[ P_s = 2\left(\left(0.5 + 0.3\right) \cdot 2 - 0.12\right) \cdot 1 = 2.96 \ \text{m} \]
\[ N_c = 8 \cdot 1 = 8 \ \text{pcs} \]
Result: surface area of the duct walls — 1.64 m²; connector strip (shina) — 2.96 m (shown as 2.96 m² in the result field); corner pieces — 8 pcs.
How to Use the Calculator
- Enter Side A — the first dimension of the duct cross-section, in millimetres (e.g. 500).
- Enter Side B — the second dimension of the duct cross-section, in millimetres (e.g. 300).
- Enter Length — the length of one straight duct section, in millimetres (e.g. 1000).
- Enter Number — how many identical duct sections you need to fabricate.
- Leave Consider seams checked if the ducts will be made with folded seams (Pittsburgh lock or similar). This adds a 20 mm allowance per side to the surface area and subtracts 120 mm from the shina length to account for the corner cuts. Uncheck it for a pure geometric calculation.
- Read the results: Area products — total sheet-metal surface area; Tire — total shina (flange strip) length; Corner — number of flange corner pieces.
What the Results Mean
The Area products value is the total area of the four duct walls. This is the number you need when ordering galvanized sheet metal, and it is also the basis for estimating insulation, sealant and paint. Note that it is the surface area of the finished duct, not the flat blank size — fabricators still add their own folding and joint allowances when cutting the sheet.
The Tire result (from "shina", the term for the flange strip used in duct fabrication) is the total length of the connector profile that frames both ends of the duct. This flange strip is mounted along the perimeter of each duct end so that sections can be bolted together with gaskets. The formula takes the cross-section perimeter, subtracts the space taken up by the corner pieces, and multiplies by two for the two ends of the duct.
The Corner result is simply eight corner pieces per duct section — four corners on each of the two end flanges. These stamped or extruded corners hold the flange strip together and provide the bolt holes for connecting duct sections.
Together, these three figures cover the main bill of materials for a straight rectangular duct: sheet metal for the walls, strip for the flanges, and corners for the frame. For a complete ventilation project you will also need fittings (bends, tees, reducers), which this straight-duct calculation does not include. To size the ductwork itself, first determine the required airflow with the Ventilation Airflow Requirement calculator, then check fan capacity with the Ventilation Power calculator.
Assumptions and Practical Tips
- The calculator handles straight duct sections only. Bends, tees, reducers and other fittings must be estimated separately.
- The seam allowance is fixed: 0.02 m (20 mm) added to the half-perimeter for the surface area, and 0.12 m (120 mm) deducted from the shina length. If your fabrication technology uses different allowances, uncheck Consider seams and adjust the results manually.
- The Tire result is a linear quantity (metres of strip), even though the calculator displays it with an m² suffix — treat the number as a length when ordering the flange strip.
- Sheet thickness and material grade are not accounted for; add a waste margin of 5–10% to the surface area when ordering metal.
- All inputs must be in millimetres. The results are converted to metres automatically.
Once the ducts are sized and fabricated, you can complete the system layout with the Ventilation Diffuser Count and Ventilation Grid Count calculators to distribute the air correctly in each room.
Frequently Asked Questions
How do I calculate the surface area of a rectangular duct by hand?
Convert all dimensions to metres first. The area of the four walls is the cross-section perimeter times the length: \( A = 2(A + B) \times L \). For a 500 × 300 mm duct that is 1 m long: \( 2 \times (0.5 + 0.3) \times 1 = 1.6 \, \text{m}^2 \). Multiply by the number of identical sections, and add a seam allowance if the duct is folded from sheet metal. This calculator automates the same steps, including the seam and flange extras.
Why does the calculator show the shina (flange strip) result in m²?
This is a display quirk, not a real area. The Tire result is a linear length in metres — the total length of flange strip needed to frame both ends of every duct section. Ignore the m² suffix and order that many metres of connector profile. For example, 2.96 means 2.96 metres of strip, not 2.96 square metres of material.
What is the seam allowance and should I include it?
Include seams if the duct will be fabricated with folded joints such as a Pittsburgh lock or snap lock. The calculator then adds 20 mm per side to the sheet-metal area for the fold material and subtracts 120 mm from the strip length to account for corner-piece spacing. If you order pre-insulated ducts or use welded joints with no folds, uncheck the box for a pure geometric figure. Your shop may use different allowances — adjust manually if so.
Is the calculated area the same as the flat sheet-metal blank size?
No. The result is the surface area of the finished duct walls, which is correct for estimating insulation, sealant and paint coverage. The flat blank you cut from a sheet is larger because of extra material for seams, folds and stiffening beads — fabricators add those allowances themselves. For ordering sheet metal, take the calculator's figure and add a waste margin of roughly 5–10%, plus any fabrication allowances your workshop requires.
Can I use this calculator for duct fittings like bends, tees and reducers?
No — it only computes straight duct sections. Fittings have curved or transition surfaces that need different formulas. Estimate bends, tees, reducers, flexible connections and diffuser plenums separately. As a rough rule of thumb, fittings add 10–20% to the total sheet-metal tonnage of a ventilation project, but exact figures depend on the fitting schedule. The straight-duct numbers from this calculator are still the backbone of your material list.
What inputs and units does the calculator expect?
All dimensions — Side A, Side B and Length — must be entered in millimetres, even though the results come out in square metres and metres. The calculator divides by 1000 internally. If you only have dimensions in inches, multiply by 25.4 to get millimetres first. Number is how many identical duct sections you need; everything (area, strip, corners) scales linearly with it.
How much sheet metal, strip and corners do I need for a whole duct run?
Calculate one straight section with this tool and multiply by the count of identical sections, then repeat for each different size in the run. Per section you get: wall area, strip length (perimeter of both ends minus corner spacing) and 8 corner pieces — four per end flange. Add a 5–10% waste margin on the sheet metal, a spare package of corners for damage during installation, and gasket material for every flange joint.
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