Duct Size by Tonnage Calculator
Trunk and return duct size from AC tonnage.
Enter your values
ⓘ Live results
Results
- System airflow
- 1,200 CFM
- Trunk round diameter
- 16.6 in
- Return duct diameter (600 FPM)
- 19.1 in
- Registers at 120 CFM each
- 10
Quick answer
A common rule of thumb is 400 CFM per ton of cooling, which is then used to size the main trunk and return duct.
CFM = tons x 400, then size duct at design velocity
How to use the Duct Size by Tonnage Calculator
- 1Enter the equipment's rated tonnage.
- 2Use 400 CFM per ton as the standard residential design airflow.
- 3Size the trunk and return duct from that airflow at their respective design velocities.
Worked example
Using the values the calculator loads with:
Inputs
- System size3 tons
- Trunk velocity800 FPM
Results
- System airflow1,200 CFM
- Trunk round diameter16.6 in
- Return duct diameter (600 FPM)19.1 in
- Registers at 120 CFM each10
Frequently asked questions
Is 400 CFM per ton always right?
It's a solid rule of thumb; humid climates sometimes target 350 CFM per ton for better dehumidification.
Does this replace a full Manual D design?
No, it's a quick sizing check — a full room-by-room and friction-rate design is still the accurate method.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.