How to Measure for Custom Ductwork: What to Include in Your Quote Request

This guide walks through every measurement and specification a fabricator needs from you, so your custom ductwork quote request is complete from the start.

Why the Right Measurements Matter More Than You Think

Incorrect measurements are the single most common cause of custom ductwork fabrication errors. Ducts that arrive undersized choke airflow. Oversized sections drop velocity below the threshold needed to push conditioned air into distant rooms. Either way, the system underperforms — and the fix costs significantly more than getting the spec right the first time.

According to NIST and Lawrence Berkeley National Laboratory research, residential duct systems commonly lose 25–40% of their effective cooling and heating capacity through a combination of leakage, undersizing, and uninsulated runs. Precision at the measurement stage is the first line of defence against those losses.

The ACCA Manual D standard — the recognised framework for residential duct system design — exists precisely because improvised sizing consistently produces poor outcomes. When you measure correctly and communicate clearly, the fabricator can build to that standard with confidence.

Measuring Duct Dimensions: Round, Rectangular, and Oval

Round ducts: Measure the internal diameter straight across the circular cross-section. A 10" round duct has an internal diameter of 10". Use callipers or a flexible tape — the outside diameter includes the metal wall thickness, which is not the spec the fabricator needs.

Rectangular ducts: Measure both the width (W) and height (H) of the internal opening. Record both dimensions in order — for example, "14 × 8" — since the orientation affects how the duct fits into a ceiling cavity or wall chase. Never assume a square section; many rectangular ducts have a 3:1 or 4:1 aspect ratio, and the fabricator needs to know which dimension runs vertical.

Oval ducts: These are often used where clearance is tight. You need the major axis (the widest point) and the minor axis (the narrowest), plus the cross-sectional area, which should match the equivalent round duct it replaces. An online conversion tool can confirm equivalency — provide both sets of numbers in your quote request rather than just one.

Total Length and Run Configuration

Fabricators price and fabricate by linear run length, so your measurements here directly determine cost and lead time. For each duct run, record:

• Total length of straight sections, measured in inches or feet
• Number of bends or elbows, and the angle of each (45°, 90°, or other)
• Number of transitions — points where the duct changes size or shape
• Branch takeoff locations — measured from the plenum or start of the main trunk

The mistake most people make is measuring only the straight runs and forgetting that every elbow adds equivalent friction length. A 90° elbow in a 12" round duct can add 15–30 feet of equivalent resistance to the system. That matters for system performance and for the fabricator's cut list.

If your run passes through a tight floor joist bay, note the clearance dimensions there too. A 12 × 6 rectangular duct cannot navigate a 10"-deep joist cavity without a transition — the fabricator needs to design around that constraint, but only if you tell them it exists.

Material Specifications: Don't Leave This Blank

Custom ductwork can be fabricated from several materials, and the choice affects cost, lead time, weight, and performance. The three most common are:

Galvanised steel is the industry standard for most residential and commercial supply and return systems. It offers excellent strength, corrosion resistance from its zinc coating, and long service life. Most fabricators default to this if no material is specified, but it is still worth confirming.

Aluminium is lighter and naturally corrosion-resistant, making it a practical choice where weight is a concern or where moisture exposure is ongoing. It is common in crawlspace installations and coastal climates.

Stainless steel is specified for kitchen exhaust, specialty ventilation, and high-humidity commercial environments where galvanised steel would corrode. It carries a premium in both material cost and fabrication time.

You also need to specify gauge — the sheet metal thickness. Residential light-commercial ductwork typically runs 26-gauge for branch runs and 24-gauge for main trunks. SMACNA's HVAC Duct Construction Standards define the minimum gauge by pressure class; for most residential applications, these defaults hold, but a commercial or high-pressure application may require 22-gauge or heavier.

Fittings, Transitions, and Accessories: The Part Everyone Forgets

Straight runs are easy. It is the fittings — elbows, transitions, offsets, plenums, caps, and collars — that cause the most back-and-forth between customers and fabricators. Every fitting needs its own spec.

For each fitting, include:

• Type (elbow, reducer, offset, boot, wye, tee, cap)
• Inlet and outlet dimensions for transitions and reducers
• Turning radius for elbows — a long-radius sweep has far lower friction loss than a short-radius bend, and costs more to fabricate
• Connection type at each end — slip joint, flanged, S-cleat, drive cleat, or TDC/TDF (Transverse Duct Connection) flange

Dampers, volume control registers, and access doors should also be listed if they are part of the fabricated assembly rather than field-supplied. Include their placement relative to the nearest fitting or branch takeoff.

Plenums deserve particular attention. A supply plenum or return plenum that is slightly undersized chokes the entire system before a single branch run even begins. Specify plenum dimensions (length × width × height), the number and size of collar openings, and the connection method to the air handler.

Insulation Requirements and Install Environment

Where your ductwork runs determines whether it needs insulation — and what type. Ducts in unconditioned spaces (attics, vented crawlspaces, garages) lose conditioned air to the surrounding environment through both leakage and conduction. Specifying the correct insulation in your quote request means the fabricator can pre-wrap or pre-line sections before delivery, saving significant field time.

For most unconditioned attic runs, R-8 insulation is now the expected standard for compliance with current energy codes. If your jurisdiction follows the IECC 2021 or later, confirm the required R-value with your local building department before ordering — building on an older R-6 spec may fail inspection.

Tell the fabricator whether ducts will be in a conditioned or unconditioned space, the ambient temperature range of that space if it is extreme (attic temperatures in hot climates routinely exceed 130–140°F in summer), and whether a vapour barrier is required. All of this affects material selection and wrap specification.

If you are ordering custom sheet metal ductwork for a commercial kitchen, clean-room, or other specialty environment, the environmental spec becomes the most important part of the quote request. Do not assume a standard residential spec applies.

Quantities, Timeline, and Project Context

A quote that includes only dimensions is still incomplete. Fabricators need:

Quantities. How many of each straight section, fitting, and accessory. Batch fabrication is typically faster and more cost-efficient than single-piece orders — if you are fitting out a full floor of a building, order all runs together rather than in multiple small batches.

Delivery timeline. In 2026, extended steel tariffs and ongoing labour shortages across the HVAC fabrication sector are pushing lead times on custom and welded ductwork. Providing your required delivery date upfront allows the shop to flag any constraints before fabrication begins rather than after.

Project type. New construction, renovation, or replacement? Each has different priorities. A replacement run needs to match existing connection types exactly. New construction may allow the fabricator more flexibility on fitting geometry, which can reduce cost.

Drawing or sketch. Even a hand-drawn layout with annotated dimensions is worth including. Shops using CAM fabrication software can catch dimensional conflicts in your drawing before the metal is cut — but only if they have a drawing to check against.

The Complete Custom Ductwork Quote Checklist

Before you submit, confirm your request includes:

• Internal dimensions of all straight runs (diameter for round; W × H for rectangular)
• Total length of each run, measured in linear feet or inches
• Number and type of all fittings, with inlet/outlet dimensions
• Elbow turning radius for each bend
• Connection type at each end of every section
• Material (galvanised steel / aluminium / stainless steel) and gauge
• Insulation specification (R-value, vapour barrier, pre-wrap or field-applied)
• Quantity of each item
• Required delivery date
• Project type (new construction / renovation / replacement)
• Any access constraints — ceiling height, joist bay dimensions, crawlspace clearances

A complete quote request does not slow the process down. It speeds it up. Fabricators who receive full specifications can turn custom work faster than those who need two or three rounds of clarification before cutting begins.

Conclusion

Getting custom ductwork fabricated accurately starts well before the shop floor — it starts with the information you provide. Precise internal dimensions, material specifications, fitting details, insulation requirements, and project context are not optional extras; they are the spec package the fabricator works from. Leave any element vague and the result is either a delay for clarification or, worse, components that need modification on-site.

If you are ready to put that information to work, submit your custom ductwork quote request and a member of the team will review your specifications and come back with an accurate, no-guesswork quote.