How to Measure for Custom Ductwork

Ordering custom ductwork saves time on the jobsite, but only if the measurements are right. A mismeasured fitting means refabrication, shipping delays, and a crew standing around waiting. This guide covers how to measure for custom rectangular and round duct fittings so the pieces arrive ready to install.

What You Need

• Tape measure (25 ft minimum)
• Notepad or phone for recording dimensions
• Angle finder or protractor for elbows and offsets
• Straightedge for checking if existing duct is square

Take every measurement twice. Write it down immediately. The number one cause of refabrication orders is transposed width and height dimensions.

Measuring Rectangular Duct

Rectangular duct has two cross-section dimensions: width and height. The convention in the HVAC industry is:

• Width = the first dimension, measured horizontally (the wider face when installed in a joist bay)
• Height = the second dimension, measured vertically (the narrower face in a joist bay)

When you order a 12 x 8 fitting, you are ordering 12 inches wide by 8 inches tall. If the fitting arrives and you meant 8 x 12, it will not fit the cavity. Always measure width first, height second, and label them clearly.

Measuring Length

For straight duct sections, measure the distance between connection points. Then account for how the pieces connect:

With slip connections, the male end slides about 1.5 inches into the female end. If you need 48 inches of duct between two fittings using slip connections, order 48 inches and the overlap is built into the connection. The critical measurement is the centerline distance between where each fitting ends.

Measuring for Elbows

An elbow changes direction. You need three measurements:

1. Cross-section (width x height)
2. Angle (90 degrees is standard, but 45, 60, and custom angles are common)
3. Throat radius (the inside curve — tighter radius saves space but increases pressure drop)

For replacement elbows, measure the existing elbow at the widest point of the outer curve and the tightest point of the inner curve. The difference gives you the throat dimension. If you are designing a new run, a throat radius equal to the duct width gives the best balance of compact size and low pressure drop.

Measuring for Transitions and Reducers

A transition connects two different duct sizes. A reducer steps down the trunk as airflow decreases. For both, you need:

• Inlet dimensions (width x height of the larger end)
• Outlet dimensions (width x height of the smaller end)
• Length (distance from inlet to outlet along the centerline)

Keep the taper angle under 15 degrees per side to minimize turbulence. If the size change is large, a longer transition performs better aerodynamically. A 14 x 10 to 10 x 8 transition should be at least 8 inches long.

Measuring for Tees and Wyes

A tee splits airflow into two directions. A wye does the same at an angle. Measurements needed:

• Main trunk dimensions (width x height)
• Branch dimensions (width x height — often smaller than the trunk)
• Branch length (how far the branch extends before the first connection)
• Branch angle (90 degrees for tees, typically 30 or 45 degrees for wyes)

Measuring Round Duct

For round duct, measure the outside diameter with a tape wrapped around the circumference, then divide by pi (3.14) to get the diameter. Or measure across the opening if accessible.

Common mistake: measuring the inside diameter when the connection is slip-fit. The male end OD must match the female end ID. When ordering, specify the nominal diameter — the fabricator accounts for the slip allowance.

Measuring for Square-to-Round Fittings

A square-to-round transition connects rectangular trunk to round branch runs. Measure:

• Rectangular end (width x height)
• Round end (diameter)
• Length (centerline distance)

Common Measuring Mistakes

• Swapping width and height. A 12 x 8 piece does not fit an 8 x 12 opening. Label every measurement.
• Measuring inside vs outside. For cross-section, measure the outside of the duct. The fabricator subtracts the material thickness.
• Forgetting the connection type. Slip, drive, TDC, and flanged connections all have different engagement lengths and requirements. Specify which end gets which connection.
• Not accounting for offsets. If a duct run needs to jog around an obstacle, measure the offset distance (horizontal shift) and the available run length. An offset fitting is cleaner and faster than two elbows with a short straight piece.
• Rounding to standard sizes. The whole point of custom ductwork is that you can order the exact size. If you measure 11.5 x 7.25, order 11.5 x 7.25.

Measurement Checklist

Before placing an order, confirm you have recorded for each fitting:

1. Fitting type (straight, elbow, tee, transition, etc.)
2. All cross-section dimensions (width x height, or diameter)
3. Length or angle as applicable
4. Connection type on each end (inlet and outlet)
5. Material (galvanized, aluminum, or stainless steel)
6. Gauge (26 ga is standard; 24 or 22 ga for larger sizes or higher pressure)

At PMX Ductwork, you enter these dimensions directly into the configurator and get instant pricing. Every fitting is fabricated to your exact specifications in galvanized, aluminum, or stainless steel, from 26 gauge down to 20 gauge.