How to Measure Ductwork for a Made-to-Order Quote

Every duct section we make is fabricated to the dimensions you give us — so getting the measurements right first time means the duct fits first time.

The three dimensions that matter

Rectangular duct is described by width × height × length in millimetres. Width and height are the internal airway dimensions; length is measured along the direction of airflow. Sheet gauge, stiffening and flange allowances are handled automatically when you configure a part in our shop.

Measuring existing duct on site

Measure the internal airway at the duct end, check both ends for square, note the connection type (Mez flange, slip joint or raw edge), and never measure over insulation or across flanges.

Lengths, joints and site tolerance

Straight lengths are typically supplied up to 1.5m per section joined with Mez flanges. Allow for flange connections and leave a trimmable section or slip joint to take up site tolerance at the end of a run.

Fittings

Bends need size and angle; reducers need both end sizes and concentric/eccentric orientation; transitions need the rectangular end and spigot diameter; plenum boxes need spigot size, position and face; offsets need offset distance, direction and overall length.

The mistakes that cause remakes

Swapped width and height, measuring over insulation, wrong eccentric reducer orientation, no site tolerance, and assuming imperial duct converts to round metric sizes.

Not sure? Send us the drawings — we price full take-offs every day. Price your ductwork online.