Circular and rectangular duct sizing

• Design airflow
• Target air velocity
• Available rectangular duct height
• Duct length, insulation thickness and fittings allowance

Start by defining the part of the system covered by the calculation. Enter project values rather than leaving example values unchanged. Keep all flow rates, temperatures, dimensions and pressures consistent with the units shown beside each field. If a value comes from existing equipment, use the manufacturer data sheet or a measured value and keep a record of its source.

Run the calculation once with the expected operating conditions, then repeat it with a relevant low-load or high-load condition when this may affect the selection. A single result does not describe every operating point. Compare the output with the neighbouring standard size when the calculator offers one, because a theoretical diameter or capacity is not automatically an available product size.

• Required free area
• Suggested circular diameter
• Equivalent rectangular dimensions
• Indicative insulation surface

The main result is a preliminary engineering value, not a final product selection. Read it together with the secondary checks displayed by the calculator. Velocity, pressure loss, available pressure, temperature range and safety margins can be as important as the first diameter or capacity shown. A result near a limit should lead to a review of the assumptions rather than an automatic selection of the next size.

Keep the input values and the result with the project notes. This makes it possible to understand why a size was selected and to update the calculation when equipment, routes or design conditions change. The French and English interfaces use the same calculation engine, so equivalent inputs must produce the same numerical result.

• Select an available standard duct size
• Check pressure loss and noise
• Include fittings and transitions
• Confirm insulation and fire-performance requirements

This tool supports preliminary sizing only. It does not replace a complete design, a coordinated drawing, a specification, commissioning data or a calculation approved for construction. The user remains responsible for the project inputs and for checking the result against applicable standards, local requirements, safety rules and manufacturer documentation.

Real systems may include additional pressure losses, control devices, diversity, minimum equipment flow rates, water treatment requirements, acoustic constraints, installation tolerances and maintenance needs. These effects are not all represented by a quick calculator. Before installation, confirm the selected equipment and dimensions with the complete system calculation and the actual products intended for the project.