Circular to Rectangular Duct Converter

Convert circular ductwork into a rectangular size while checking airflow, velocity, aspect ratio and pressure loss. Free browser-based calculator from Smart Ventilation Systems Ltd — enter your circular diameter and the depth you have available, and get an equal-area size, an approximate equal-friction size and a recommended manufactured size rounded to a practical increment.

How to convert circular duct to rectangular duct

Start from the circular duct's cross-sectional area, fix the depth you have available, and calculate the width needed to carry the same airflow. Then check equivalent diameter, velocity, aspect ratio and pressure loss before confirming the size — a duct that matches on area alone can still cost you fan pressure.

What is equal-area duct conversion?

Equal-area conversion sizes the rectangular duct so its cross-sectional area matches the circular duct (W = A ÷ H). The average velocity stays approximately the same, but a flat rectangular section has more wetted perimeter per unit of area, so friction is usually higher — particularly at high aspect ratios.

What is equivalent duct diameter?

The equivalent diameter (De) is the diameter of a round duct producing approximately the same pressure loss as the rectangular duct at the same airflow, using the ASHRAE-style relationship De = 1.30 (W·H)^0.625 / (W+H)^0.25. Matching De to the original diameter keeps the rectangular run's friction rate close to the circular duct it replaces.

Why does aspect ratio matter?

Above about 3:1 a duct needs more sheet metal per unit of airflow, loses stiffness (it may need extra reinforcement to meet DW144), generates more friction and can be noisier. Above 6:1 it is usually better to increase the depth, split the airflow into two ducts, or reroute.

Circular to rectangular transition considerations

The transition piece adds a fitting loss that is not part of the straight-duct calculation. Smart Ventilation Systems fabricates square-to-round transitions to order alongside rectangular and spiral ductwork, all to DW144. Request a fabricated transition or call 07710 984 957.

This calculator is intended as a design aid only. Final duct sizes should be verified by a competent HVAC designer against airflow, fan pressure, acoustic requirements, fitting losses and project specifications.