Rectangular Metal duct sizing chart
Rectangular return duct sizing chart by airflow at 0.08 inch water-column friction rate, with a transparent screening method.
What this duct sizing chart answers
This table adapts a lower friction target to rectangular metal where ceiling depth limits the available height.
This page is maintained by the HVAC Calculator Pro Editorial Team. We publish the inputs, calculation basis and limitations so a technician can reproduce the screening result and decide what must be checked in the field. It is a reference aid, not a substitute for the governing code, a licensed designer or the equipment manufacturer's instructions.
Airflow to duct size
screening reference| Airflow CFM | Suggested size | Velocity fpm | Estimated friction in. w.c. / 100 ft |
|---|---|---|---|
| 100 | 7 × 4 | use equivalent area | — |
| 150 | 8 × 4 | use equivalent area | — |
| 200 | 9 × 5 | use equivalent area | — |
| 250 | 10 × 6 | use equivalent area | — |
| 300 | 10 × 6 | use equivalent area | — |
| 400 | 12 × 8 | use equivalent area | — |
| 500 | 13 × 9 | use equivalent area | — |
| 600 | 14 × 10 | use equivalent area | — |
| 800 | 15 × 11 | use equivalent area | — |
| 1000 | 17 × 13 | use equivalent area | — |
| 1200 | 18 × 14 | use equivalent area | — |
| 1600 | 21 × 17 | use equivalent area | — |
| 2000 | 23 × 19 | use equivalent area | — |
Field interpretation
Keep the aspect ratio practical and avoid narrow slots that add surface friction and fabrication complexity.
Definitions: Airflow is the design volume in CFM; cross-sectional area links CFM to velocity; velocity is the average air speed in fpm; friction rate is the pressure-loss target per 100 ft; equivalent length converts fittings and transitions into comparable straight-run loss.
Worked example: Select a candidate cross-section from the 500 CFM row, then validate area and equivalent length in a Manual D worksheet.
The generated row uses area and target velocity to create a candidate and reports a Darcy-Weisbach friction estimate for round ducts. Rectangular and flex corrections, fittings and the blower pressure budget still require a Manual D worksheet.
How to use this rectangular metal sizing chart
- Start with the room or trunk airflow from a load calculation.
- Choose a candidate size and record the assumed friction target and velocity.
- Add straight-run length, equivalent fitting length, filter, grille and terminal losses before approving the design.
Worked example: A chart row is a candidate dimension, not a guarantee that the installed system will deliver its design airflow.
Record the result with the project address, equipment model, code edition and assumptions. That audit trail is more useful than copying a single number without its conditions.
Continue with the right tool
Use boundary: This chart does not include fitting equivalent lengths, duct heat gain/loss, acoustic limits, altitude correction or a room-by-room Manual J airflow. Verify the longest run with a ductulator or Manual D worksheet.
Common questions
Is this a final Manual D duct size?
No. It is a screening candidate based on airflow, area and target velocity. Final sizing must include friction, fittings, terminals and the blower pressure budget.
Why can two ducts with the same CFM need different sizes?
Shape, roughness, flex compression, fitting losses and the available static pressure change the acceptable size.