Duct Aspect Ratio: Width, Height and Design

Duct aspect ratio calculation

Measure the internal width and height in the same units. Divide the larger dimension by the smaller one and express the result as a ratio to one. A 600 mm by 200 mm duct has an aspect ratio of 3:1. A 400 mm by 300 mm duct has an aspect ratio of about 1.33:1.

The order of width and height does not change the numerical aspect ratio. It does change the installation drawing, so keep both dimensions and their orientation. Internal dimensions determine air area; outside dimensions and supports determine the space the assembly needs.

Same area, different shape

Both example ducts have an internal area of 0.12 m². At 0.6 m³/s, the average air velocity is Q/A = 5 m/s in either section. Their perimeters differ: the 600 by 200 mm duct has 1.6 m of perimeter, while the 400 by 300 mm duct has 1.4 m.

That difference matters because air interacts with the duct walls and because the fabrication uses a different amount of surface for the same length. Equal area and equal average velocity therefore do not establish equal resistance or cost. Fittings, surface condition and the detailed construction remain part of the comparison.

Our duct velocity guide explains the airflow-to-area calculation. Keep aspect ratio as a geometry check, not a complete sizing method.

Hydraulic diameter and equivalent round duct

For a rectangular section with sides a and b, hydraulic diameter is D_h = 2ab/(a+b). Use consistent units. The 600 by 200 mm example gives 300 mm; the 400 by 300 mm example gives about 343 mm.

Hydraulic diameter is a geometric quantity used in appropriate flow calculations. It is not automatically the diameter of a round duct that will have identical friction at the same volume flow. Equivalent-diameter methods have their own assumptions and calculation basis.

When comparing a round alternative, use the relevant design method and fitting data for the whole route. Our round versus rectangular duct guide covers the practical choice.

Rectangular duct aspect ratio and available space

A shallow ceiling space may encourage a wide, flat duct. Check whether the width then conflicts with structure, other services or maintenance access. Bends and transitions need room too; the straight section is only one part of the envelope.

Review wall support, stiffness, fabrication and joining with the supplier. A wide face can behave differently from a compact section, and the specified pressure conditions matter. Avoid treating a familiar ratio limit from another material or application as a universal rule for chemical ductwork.

For chemical extraction, describe corrosion exposure, temperature and cleaning requirements alongside the geometry. These conditions can determine which construction is practical.

What to send with a duct layout

Provide airflow, internal dimensions, available outside space, route length, bends and transitions. Mark the direction of each rectangular side through changes in direction. Include the selected fan duty and pressure allowance so the route can be checked as a system.

We fabricate and install PVC chemical ductwork, including circular sizes from 150 to 400 mm diameter. Send your proposed arrangement so we can discuss a suitable route and construction. The ductwork design guide brings the dimensional, chemical and installation questions together.