HVAC takeoff
Enter a fitting’s dimensions and get its surface area, GI sheet weight, SMACNA gauge and a BOM schedule you can export. Six fittings — straight duct, reducer, elbow, dropper, collar and Y-piece — measured to either the commercial billing standard (mean perimeter × centreline length) or the true shop flat pattern. Metric or imperial.
Every formula, gauge band and constant it uses →
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What this is
DuctForge turns a fitting’s dimensions into the numbers a ductwork job is actually priced on: surface area in m² or ft², galvanised sheet weight in kg or lb, the SMACNA gauge that size calls for, and a bill of materials you can export as CSV or print as a quantity sheet.
It runs entirely in your browser. There is no account, nothing is uploaded, and a takeoff you start today is still on this device tomorrow. That also means it keeps working on a site with one bar of signal.
Every figure on screen shows its own arithmetic — the formula with your numbers substituted into it — so you can check any line against the calculator on your desk. That is the whole design: a quantity nobody can audit is a quantity nobody should invoice.
Two standards, kept apart
The same duct is a different quantity under each. The standard you chose travels with every result, every exported row and every printed sheet.
Coverage
Straight duct
A plain rectangular run.
A = 2(W + H) × L
Reducer
Transition between two rectangular sizes.
A = (W₁ + H₁ + W₂ + H₂) × L
Elbow
Radiused bend through an angle.
A = 2(W + H) × [θπ/180 × (R + W/2)]
Dropper
Offset or swan neck.
A = 2(W + H) × √(L² + O²)
Collar
Branch takeoff with a flange lip.
A = 2(W + H) × (L + F)
Y-piece
Trouser splitting one duct into two.
A = A_B1 + A_B2, A_Bn = (W₁/2 + H + Wₙ + H) × θπ/180·(R + Wₙ/2)
Round duct
A plain round or spiral run.
A = πD × L
Round elbow
Gored bend in round duct.
A = πD × [θπ/180 × R]
Round reducer
Concentric cone between two diameters.
A = π(D₁ + D₂)/2 × L
Alongside the sheet it will count insulation area on the outer face, flange ends and corner pieces from the length your duct is supplied in, and hangers at your spacing. Group lines by zone — AHU, floor, area — and apply your own rate per kg or per m² to turn a quantity into a value.
Who it’s for
If you price ductwork you are doing this arithmetic already, usually in a spreadsheet where the formula is invisible, the measurement standard is implicit, and the gauge is a lookup somebody did once. This does the same job with the working shown and both standards named.
The drawings are there for the same reason. A reducer and a dropper are very different objects that look nearly identical as a row of numbers — seeing the fitting is how you catch that you picked the wrong one before the quantity reaches a tender.
Limits