Guide

How to take off a duct job

Eight steps, about five minutes to read, and you will not need it twice. The one part worth reading properly is the first: which measurement standard you are working to decides every number that follows.

Nothing here is stored anywhere but your own device, so you can experiment with a job freely — there is nothing to break and nothing to undo.

Open the calculatorEvery formula and constant

The two standards

First, the one decision that matters

The same duct is a different quantity depending on who is asking. Pick the wrong one and every figure below it is wrong in a way that looks completely reasonable.

Billing

Nominal measurement — mean perimeter multiplied by centreline length, the BOQ / IS 655 / DW 144 practice. Use this when you are claiming against a client, a consultant or a quantity surveyor. It is what they will check your invoice against.

Shop

The true unfolded blank a fabricator cuts, including slant heights on transitions, heel arc expansion on bends and the gore development on round elbows. Use this when you are buying sheet or setting a shop to work. It is not a billing quantity and should never be claimed as one.

Step by step

The workflow

  1. Set the standard, the units and the material

    The billing/shop switch and the metric/imperial switch are both in the bar at the top, and both apply to the whole takeoff. Switch units whenever you like — the geometry is stored in millimetres, so nothing is lost or rounded in the process.

    Sheet material lives in the Material and allowances panel below the schedule. Galvanised steel is the default; stainless and aluminium change the weight and nothing else, because a gauge is a thickness.

    What you'll see
    MetricImperial
    BillingShop
    Material and allowancesGI
    1. Units. Everything on screen switches; your dimensions are converted, not retyped.
    2. The measurement standard. This is the decision above — get it right before anything else.
    3. Material, in the panel below the schedule. Changes the weight, never the area.

  2. Pick the fitting

    Six rectangular fittings, four round ones and eight flat pieces. If your run is plain duct, use Straight; if it changes size, Transition; if it turns, Elbow; if it steps sideways, Offset; if it branches, Collar or Y-piece. Coming off an air-handling unit into spiral, use Square to round. A plate, a blank or an end cap has no length at all — pick it from Flat pieces by its shape.

    The picker keeps whichever zone you were working in, so you can move through a dozen fittings in AHU-1 without retyping it.

    What you'll see

    Rectangular

    Straight ductTransitionElbowOffsetCollarY-piece

    Round and spiral

    Round ductRound elbowRound reducerSquare to round

    Flat pieces

    Rectangular plateRound plateRingPlate with round holeRectangular frameFlat-oval plateTriangular plateTrapezoid plate
    1. Rectangular: straight duct, transition, elbow, offset, collar and Y-piece.
    2. Round and spiral: plain duct, a gored elbow, a concentric cone, and a square-to-round transition.
    3. Flat pieces: a rectangular plate, a round plate, a ring, a plate with a round hole, a rectangular frame, a flat-oval plate, a triangle and a trapezoid. Each is one face, never counted for flanges or hangers.

  3. Type the dimensions

    Every field is labelled with the symbol it uses in the formula, so the working shown under the result reads back against the boxes you filled.

    Two of them catch people out. On a rectangular elbow, R is the inside (throat) radius, not the centreline. On a round elbow it is the centreline radius, which is the convention for round duct — usually 1.5 times the diameter.

    What you'll see
    W Width600mm
    R Inside radius300mm

    A = 2(W + H) × L

    2 × (600 + 400) × 3000 = 6 000 000 mm² = 6.000 m²

    1. The symbol beside each label is the one used in the formula.
    2. R on a rectangular elbow is the INSIDE radius. On a round elbow it is the centreline.
    3. The working, with your numbers in it. Check any line against the calculator on your desk.

  4. Check the drawing

    Blueprint shows the fitting dimensioned, and every dimension in its formula appears on it. Flat pattern shows the blanks a shop would cut: solid lines are cuts, dashed lines are folds. Isometric shows the object, which is the fastest way to notice you picked an offset when you meant a transition.

    Seam laps and flange material are deliberately not drawn into the flat pattern — the waste allowance covers them numerically, and drawing them too would count them twice.

    What you'll see
    BlueprintFlat patternIsometric
    Example: a 90° elbow, dimensioned600R 30090°600400section
    1. Three views. Your choice sticks — changing a dimension will not switch it back.
    2. Every dimension in the formula is on the drawing, and nothing is on the drawing that is not in the formula.

  5. Set the quantity and the waste allowance

    Pieces multiplies the line. The allowance is your decision: 0% for a net BOQ claim, 8% for factory-run straight duct, 12% for standard flanged work, 15–20% for complex fittings and heavy gauge.

    Gauge is chosen for you from the largest dimension, but you can override it on any line — and you should, wherever the specification says something different.

    What you'll see
    Pieces4
    0%8%12%15%20%
    From the table24 ga22 ga
    1. Pieces. One line can be any number of identical fittings.
    2. The allowance, as a preset or typed. 12% is the standard flanged figure.
    3. Gauge. Left to the table, or set by hand where your specification differs.

  6. Add it to the takeoff

    Nothing enters the schedule until you press Add. Until then the fitting is a draft: change it, watch the numbers move, and only commit when it is right.

    Every line can be edited, duplicated or removed afterwards. Duplicating is the quick way to take off a run of similar fittings at different lengths.

    What you'll see
    + Add to takeoff

    Elbow ×4

    W 600 · H 400 · R 300 · θ 90°

    EditCopyRemove
    1. The one orange button on the screen. Nothing is scheduled until it is pressed.
    2. Each line can be edited, duplicated or removed. Duplicate, then change the length.

  7. Zones, insulation, flanges and hangers

    A zone is whatever you bill by — AHU-1, Level 3, Kitchen. Give lines the same zone name and they total together.

    In Material and allowances you can also have it count insulation on the outer face, flange ends from the length your duct is supplied in, and hangers at your spacing. All three are off until you switch them on, because a quantity nobody asked for is a quantity nobody checked.

    What you'll see
    ZoneAHU-1

    Applies to every line

    Material and allowances

    GI · 25 mm insulation · flanges · hangers

    1. The zone. Type it once — it carries to the next fitting you pick.
    2. Insulation, flanges and hangers, all off until you set them.

  8. Dampers, terminals and everything else

    VCDs, fire dampers, grilles, diffusers, access doors, flexible duct — and anything of your own, like labour or transport — go under Extras, a list of their own. Pick the type and the item, give it a size, a quantity and a unit, and a rate if you want it priced.

    Extras are counted, not calculated. They print on the PDF as their own table, grouped by type, and have their own block in both CSVs — but they are never added to the duct's area or weight, and the sheet says so.

  9. Read the totals and get it out

    Totals give you net area, waste, gross area and weight, then material by gauge with an estimated sheet count for each. Put in your rate per kg or per m² and it prices the job at your own figures.

    Everything that leaves the app is under Export. The PDF is the quantity sheet you issue: switch off any part you don't want — the basis notes, a column, the credit line — add your own company, client and notes, pick the paper, and check the live preview before Save as PDF. It starts with the standard, the units, the allowance and every assumption printed on it. CSV exports every input alongside every result, so the file still explains itself in a year. Save writes a project file you can reopen or hand to someone else.

    What you'll see

    Net area

    24.000 m²

    Waste

    2.880 m²

    Gross area

    26.880 m²

    Weight

    147.84 kg

    24 ga0.70 mm18.240 m²6 sheets
    22 ga0.85 mm8.640 m²3 sheets
    SaveExport
    1. The bottom line. Gross is net plus your allowance; weight is gross times the density shown.
    2. By gauge, because 22 ga cannot be cut from a 24 ga sheet. Sheet counts are an estimate.
    3. Save keeps the job as a file. Export gives you the PDF sheet — set up and previewed — and the CSVs.

Limits

Worth knowing

  • Gauge is chosen by size alone

    Real SMACNA selection also depends on pressure class and reinforcement spacing. Treat the table as a starting point and check it against the specification.

  • Round duct is graded on the rectangular table

    SMACNA publishes a separate and generally lighter table for round and spiral duct, which this app does not carry — so round comes out over-specified. Override the gauge where your spec differs.

  • Sheet counts are an estimate

    Gross area divided by one sheet, rounded up, per gauge. It cannot know how your shop nests, and offcut reuse moves it in both directions.

  • Everything lives on this device

    Clearing your browser data clears your takeoffs. If a job matters, Save it to a file — that file is the copy you can move between machines.

Open the calculator