Duct Size Calculator
Estimate round and rectangular duct sizes from airflow, duct use, and target velocity.
Results
This is a quick duct sizing estimate, not a full Manual D duct design. Final duct design should also check friction rate, total effective length, fittings, and static pressure.
Rectangular options
Balanced
14 x 9 in
126 sq in at 686 FPM
Compact
12 x 12 in
144 sq in at 600 FPM
Wide
24 x 8 in
192 sq in at 450 FPM
How to use this calculator
- 01Enter the design airflow.
- 02Choose supply or return duct use.
- 03Use the preset velocity or enter a custom target velocity.
- 04Read the round duct size, rectangular options, area, and actual velocity.
If you still need to estimate room airflow first, use the CFM Calculator. For cooling equipment size, use the AC Tonnage Calculator.
Understanding the math
The calculator divides airflow by target velocity to estimate duct area. It then converts that area into a round diameter and practical rectangular options.
duct area = airflow / target velocity round diameter = sqrt((4 x area) / pi) actual velocity = airflow / actual duct area
This is a quick duct sizing estimate, not a full Manual D duct design.
Duct sizing quick reference
These examples use the supply preset velocity. Return ducts often need more area because they usually run at lower velocity.
| Airflow | Round duct | Rectangular example |
|---|---|---|
| 100 CFM | 6 in | 6 x 4 in |
| 200 CFM | 8 in | 9 x 6 in |
| 400 CFM | 12 in | 12 x 8 in |
| 600 CFM | 14 in | 14 x 9 in |
| 800 CFM | 16 in | 18 x 12 in |
| 1,000 CFM | 18 in | 18 x 12 in |
| 1,200 CFM | 18 in | 24 x 16 in |
Frequently asked questions
How do you calculate duct size?
Divide airflow by target velocity to get duct area. Then convert that area to a round duct diameter or rectangular width and height.
How many CFM can a 12x12 duct handle?
A 12x12 duct has 1 sq ft of area. At 500 FPM it carries about 500 CFM, and at 700 FPM it carries about 700 CFM. Higher velocity can add noise and pressure loss.
What is the rule of thumb for HVAC duct sizing?
Use CFM and target velocity, not square footage alone. Common rough targets are about 700 FPM for supply ducts and about 500 FPM for return ducts.
What is the difference between supply and return duct sizing?
Return ducts often use lower velocity, so they usually need more duct area for the same airflow. Lower return velocity can help reduce noise and restriction.
Can I use this calculator with metric measurements?
Yes. Use the Imperial / Metric toggle. Inputs, results, and reference values follow the active unit system where applicable.