Equal-friction duct sizing
Use this free duct sizing calculator to compare round and rectangular duct dimensions before preparing an ACMV layout. Equal friction selects a diameter that achieves the entered straight-duct pressure gradient at the specified airflow. It is useful for an initial branch sizing exercise and for checking an existing duct size. Calculate without an account, then review the velocity and physical space before choosing a practical fabrication size.
Formula and inputs
The existing duct-math engine uses Darcy–Weisbach pressure loss with a Colebrook friction factor, Reynolds number and standard-air assumptions. It assumes galvanized steel roughness of 0.0003 ft and air density of 0.075 lb/ft³. Enter airflow and either friction rate, velocity or round diameter, selecting the appropriate units. Rectangular equivalents use the Huebscher relationship: De = 1.30(ab)^0.625 / (a + b)^0.25.
Worked example: 500 L/s
Assume airflow of 500 L/s and a friction rate of 0.8 Pa/m for standard air in galvanized duct. The engine returns a round diameter of 356.01 mm and velocity of 5.02 m/s. Setting rectangular increments to 50 mm gives approximate options of 200 × 550 mm, 250 × 450 mm and 300 × 350 mm. These are rounded alternatives rather than identical hydraulic performances.
Using a rectangular alternative
For the same calculated equivalent diameter, fixing one side at an assumed 300 mm gives an exact other side of approximately 354.14 mm. Rounding to 300 × 350 mm changes the equivalent diameter slightly; check the resulting friction and velocity rather than claiming an exact match. Also check ceiling clearance, insulation thickness, access, aspect ratio, available fittings and coordination with the structural and electrical services along the route.
Limits of the straight-duct result
Equal friction does not calculate a complete fan duty. The numerical result excludes the cumulative losses through elbows, branches, dampers, filters, coils, terminals and other components. Use actual lengths and fitting loss data when preparing the pressure budget. High velocity can create noise and balancing difficulties even when the friction target is satisfied. Different duct materials, leakage, air temperatures and site conditions can change the appropriate design assumptions.
Singapore project review
Treat this output as a draft for review, not a stamped design or an approval. For a Singapore submission, check the applicable SS553 requirements and the project specification with the responsible designer. Verify the input schedule, record the assumptions, and reconcile the calculation with drawings and manufacturer data. This tool does not identify every applicable requirement, certify compliance, or replace professional judgement about installation, operation, safety, and maintenance. Retain the calculation date and revision so later changes can be traced to their original assumptions.