DuctForge — HVAC duct takeoff calculator

Step one

Fitting

Fitting

Step two

Straight duct dimensions

A plain rectangular run.

Also called Plain duct, Run.

Straight duct dimensions
mm
mm
mm

Quantity and allowance

%

Gauge

System, floor or area. Lines with the same name are totalled together.

Anything that helps you recognise this line later.

6.7200 m² · 36.96 kg

What this is

A duct takeoff you can check by hand

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 as a PDF quantity sheet you set up yourself.

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.

It works with no signal at all. Open it once on a connection and your phone keeps a copy of the workspace, the standards page and the guides — so use Add to Home screen and the icon opens a working calculator in a plant room, a basement or a lift shaft. Everything it needs is already on the device; there was never a server doing the arithmetic.

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.

Where your takeoffs are saved

In this browser’s local storage, on this device, and nowhere else. There is no server holding them, which is why they survive a refresh but do not follow you from your phone to your desktop, and why clearing your browser data clears them.

Every job you start is kept, so a few days of use leaves several in the list — that is the takeoffs you made, not a cache. Switch between them in the bar at the top, delete one with the bin, or clear the lot from Material and allowances. If a job matters, use Save: it writes a project file you can back up, move between machines, or hand to somebody else.

Two standards, kept apart

Commercial billing
Nominal mean perimeter × centreline length — BOQ / IS 655 / DW 144 practice. What a consultant, client or quantity surveyor accepts on a claim.
Shop fabrication
The true unfolded blank, with slant hypotenuses, heel arc expansion and gore development. What a sheet metal shop actually cuts.

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

Ten fittings and eight flat pieces

  • Straight duct

    A plain rectangular run.

    A = 2(W + H) × L

  • Transition

    Changes one rectangular size to another.

    A = (W₁ + H₁ + W₂ + H₂) × L

  • Elbow

    Radiused bend through an angle.

    A = 2(W + H) × [θπ/180 × (R + W/2)]

  • Offset

    Steps the run sideways, same section throughout.

    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

  • Square to round

    Rectangular one end, round the other.

    A = [2(W + H) + πD]/2 × L

  • Rectangular plate

    One flat piece, width by height — no length.

    A = W × H

  • Round plate

    A flat disc — one diameter.

    A = πD²/4

  • Ring

    A flat disc with a round hole in the middle.

    A = π(D² − d²)/4

  • Plate with round hole

    A rectangular plate with a round hole in the middle.

    A = W × H − πd²/4

  • Rectangular frame

    A rectangular plate with a rectangular opening in the middle.

    A = W × H − w × h

  • Flat-oval plate

    Two half circles joined by straight sides — a flat-oval duct's end.

    A = (W − H) × H + πH²/4

  • Triangular plate

    A flat triangle — base and height.

    A = B × H / 2

  • Trapezoid plate

    Two parallel edges and the height between them.

    A = (T + B)/2 × H

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

Estimators, surveyors and sheet metal shops

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 transition and an offset 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

What it does not do

  • Gauge is by size only. Real SMACNA selection also depends on pressure class and reinforcement spacing, and round duct has its own lighter table that this app does not carry. Override the gauge on any line where your specification differs.
  • Sheet counts are a nesting estimate. Gross area over one sheet, rounded up, per gauge. It cannot know how your shop nests.
  • Transitions are concentric. Eccentric reducers and cones are not modelled, and the Y-piece excludes its crotch plate.
  • It reads no drawings. Dimensions are typed. There is no DWG, DXF or PDF import, because inferring duct sizes from a drawing produces confident wrong quantities.
Every formula, gauge band and constant →How to use it, step by step →