Reference
Everything the calculator does, written out — including what it simplifies. The tables below are generated from the same code that computes your takeoff, so they cannot drift out of step with it.
Open the calculatorIn one line
Surface area comes from the fitting’s own dimensions. Gross area is that plus the allowance you set. Weight is gross area times the sheet density printed beside it. Multiply the two numbers on screen and you get the third — every time.
The two standards
A quantity surveyor and a sheet metal shop measure the same fitting differently, and both are right. DuctForge keeps them apart and states which one produced every figure.
Commercial billing
Nominal measurement to BOQ / IS 655 / DW 144 practice: the perimeter of the duct multiplied by the length along its centreline. This is what MEP consultants, clients and quantity surveyors accept on an invoice claim.
Shop fabrication
The flat sheet a fabricator actually cuts, including slant hypotenuses on transitions, heel arc expansion on bends and wrapper triangulation. It is a cutting quantity, not a billing one.
They do not always differ, and where they agree it is worth knowing why. A straight duct has no slant and no arc, so both give the same area. An elbow does too — its 2·cheek + heel + throat development simplifies exactly to mean perimeter × centreline arc, which is Pappus’s theorem rather than a coincidence. An offset goes the other way: its shop blank is smaller than its billed area, because the side cheeks are parallelograms and shearing a parallelogram adds no area.
Formulas
W and H are the duct's width and height, L its length, R the inside (throat) radius, θ the included angle, O an offset's lateral step and F a collar's flange lip. On the flat pieces, D and d are an outside and a hole diameter, w and h a frame's opening, and B and T a base and a top edge.
| Fitting | Commercial billing | Shop fabrication |
|---|---|---|
| Straight ductA plain rectangular run. | A = 2(W + H) × L | A = 2(W + H) × L |
| TransitionChanges one rectangular size to another. | A = (W₁ + H₁ + W₂ + H₂) × L | A = (W₁ + W₂)·√(L² + ((H₁−H₂)/2)²) + (H₁ + H₂)·√(L² + ((W₁−W₂)/2)²) |
| ElbowRadiused bend through an angle. | A = 2(W + H) × [θπ/180 × (R + W/2)] | A = 2·A_cheek + A_heel + A_throat, A_cheek = θπ/360·[(R+W)² − R²], A_heel = θπ/180·(R+W)·H, A_throat = θπ/180·R·H |
| OffsetSteps the run sideways, same section throughout. | A = 2(W + H) × √(L² + O²) | A = 2(L × H) + 2(W × √(L² + O²)) |
| CollarBranch takeoff with a flange lip. | A = 2(W + H) × (L + F) | A = 2(W + H)·L + 2(W + H)·F + 4F² |
| Y-pieceTrouser splitting one duct into two. | A = A_B1 + A_B2, A_Bn = (W₁/2 + H + Wₙ + H) × θπ/180·(R + Wₙ/2) | A = Σ branches [ 2·A_cheek + A_heel + A_throat ], each branch taken on its own width Wₙ |
| Round ductA plain round or spiral run. | A = πD × L | A = πD × L |
| Round elbowGored bend in round duct. | A = πD × [θπ/180 × R] | A = πD × [θπ/180 × R] |
| Round reducerConcentric cone between two diameters. | A = π(D₁ + D₂)/2 × L | A = π(D₁ + D₂)/2 × √(L² + ((D₁−D₂)/2)²) |
| Square to roundRectangular one end, round the other. | A = [2(W + H) + πD]/2 × L | A = W·√(L² + ((H−D)/2)²) + H·√(L² + ((W−D)/2)²) + 4 × ½∫₀^{π/2} r·√(L² + (r − (W/2)cos φ − (H/2)sin φ)²) dφ, r = D/2 |
| Rectangular plateOne flat piece, width by height — no length. | A = W × H | A = W × H |
| Round plateA flat disc — one diameter. | A = πD²/4 | A = πD²/4 |
| RingA flat disc with a round hole in the middle. | A = π(D² − d²)/4 | A = π(D² − d²)/4 |
| Plate with round holeA rectangular plate with a round hole in the middle. | A = W × H − πd²/4 | A = W × H − πd²/4 |
| Rectangular frameA rectangular plate with a rectangular opening in the middle. | A = W × H − w × h | A = W × H − w × h |
| Flat-oval plateTwo half circles joined by straight sides — a flat-oval duct's end. | A = (W − H) × H + πH²/4 | A = (W − H) × H + πH²/4 |
| Triangular plateA flat triangle — base and height. | A = B × H / 2 | A = B × H / 2 |
| Trapezoid plateTwo parallel edges and the height between them. | A = (T + B)/2 × H | A = (T + B)/2 × H |
Straight ductA plain rectangular run.
TransitionChanges one rectangular size to another.
ElbowRadiused bend through an angle.
OffsetSteps the run sideways, same section throughout.
CollarBranch takeoff with a flange lip.
Y-pieceTrouser splitting one duct into two.
Round ductA plain round or spiral run.
Round elbowGored bend in round duct.
Round reducerConcentric cone between two diameters.
Square to roundRectangular one end, round the other.
Rectangular plateOne flat piece, width by height — no length.
Round plateA flat disc — one diameter.
RingA flat disc with a round hole in the middle.
Plate with round holeA rectangular plate with a round hole in the middle.
Rectangular frameA rectangular plate with a rectangular opening in the middle.
Flat-oval plateTwo half circles joined by straight sides — a flat-oval duct's end.
Triangular plateA flat triangle — base and height.
Trapezoid plateTwo parallel edges and the height between them.
Gauge
The common size-only shortcut from the SMACNA duct construction standards. The metric and imperial bands are two published tables rather than conversions of each other — 12 inches is 304.8 mm — so a job is graded on the table matching its own units.
| Gauge | Thickness | Largest dimension (metric) | Largest dimension (imperial) | kg/m² | lb/ft² |
|---|---|---|---|---|---|
| 26 ga | 0.55 mm | up to 300 mm | up to 12″ | 4.32 | 0.885 |
| 24 ga | 0.70 mm | 301 – 750 mm | 13″ – 30″ | 5.50 | 1.126 |
| 22 ga | 0.85 mm | 751 – 1000 mm | 31″ – 40″ | 6.67 | 1.366 |
| 20 ga | 1.00 mm | 1001 – 1500 mm | 41″ – 60″ | 7.85 | 1.608 |
| 18 ga | 1.20 mm | 1501 – 2100 mm | 61″ – 84″ | 9.42 | 1.929 |
| 16 ga | 1.60 mm | over 2100 mm | over 84″ | 12.56 | 2.572 |
26 ga
24 ga
22 ga
20 ga
18 ga
16 ga
This is a simplification, and it matters. Real gauge selection also depends on the duct’s pressure class and on reinforcement spacing. Treat the table as a starting point, check it against the project specification, and override the gauge by hand on any line where the spec differs — the calculator carries the override through to the weight, the sheet count and the export.
Densities are not transcribed, they are derived: thickness × 7850 kg/m³, which reproduces the published table exactly. That is bare metal — it excludes any coating, stiffeners, flange steel, gaskets and fixings.
Round duct carries an extra caveat. Round duct is graded on the rectangular table here, because that is the table this app has. SMACNA publishes a separate and generally lighter one for round and spiral duct — a cylinder is stiffer than a flat panel — so this over-specifies. Override the gauge on any line where your specification differs.
A gauge is a thickness, so the band table above survives a change of material untouched — only the density moves, and with it the weight.
| Material | Density | Note |
|---|---|---|
| Galvanised steel | 7850 kg/m³ | Bare steel. The zinc coating adds roughly 1% and is not counted. |
| Stainless steel | 8000 kg/m³ | Austenitic (304 / 316). Ferritic grades run nearer 7700 kg/m³. |
| Aluminium | 2700 kg/m³ | The gauge table is a steel standard. The THICKNESS is what carries over — aluminium duct is normally specified a gauge or two heavier for the same duty, so check it against your specification. |
Galvanised steel
Stainless steel
Aluminium
Also counted
Three quantities an estimator has to price alongside the sheet. Every one of them is DERIVED from the geometry you already typed — none is a new measurement, and each is off until you switch it on.
Insulation
The same billing formula re-run with every cross-section dimension grown by twice the thickness. Lagging a 600 × 400 duct with 25 mm measures it as 650 × 450.
The centreline never moves. A rectangular elbow’s R is its throatradius, so it shrinks by the thickness while the width grows by twice it — get that backwards and insulating a bend silently lengthens it. A round elbow’s R is already a centreline radius, so it is left alone.
Flanges
Every piece is counted with a flange at each end, and a straight run is as many pieces as the supplied length divides into — a 6 m run of 1.2 m duct is five pieces, so ten ends.
Two flanges meet at every joint and both are material you buy, which is why this counts ends rather than joints. Corner pieces are four per rectangular end; a round flange has none.
Hangers
One support for every piece, plus one more for each further full spacing of centreline run.
A rule of thumb rather than a structural calculation. Real hanger spacing depends on duct size, weight and the building — check it against the specification.
Rates are the same idea in reverse: your figure per kg or per m², applied to the quantities above. This app has no prices of its own and never will.
Allowances
The allowance is your decision, not a measurement. These are the bands as the trade quotes them; any line can carry its own figure, and every export states which was used.
| Allowance | Where it applies |
|---|---|
| 0% — net BOQ | Pure theoretical area, for direct subcontractor invoicing. |
| 8% — slip & drive | Factory-run straight duct in light gauge, minimal seam scrap. |
| 12% — industry standard | SMACNA / TDF / TDC transverse flange roll-forming: a 35 mm flange lip per side plus the longitudinal Pittsburgh seam. |
| 15% — heavy fittings | Complex multi-branch fittings and angle-iron companion flanges. |
| 20% — high wastage | Heavy gauge sheet, awkward nesting, high offcut loss. |
0% — net BOQ
8% — slip & drive
12% — industry standard
15% — heavy fittings
20% — high wastage
Sheets
Gross area divided by one commercial sheet — 2.88 m² (1200 × 2400 mm) or 32 ft² (4 × 8 ft) — rounded up, and counted separately for each gauge.