Duct Size by Tonnage Calculator
Trunk and return duct size from AC tonnage.
Enter your values
Enter the system size in tons.
Enter the trunk velocity in FPM.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Trunk and return duct size from AC tonnage. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | System size, Trunk velocity |
| Main output | System airflow |
| Formula | CFM = tons x 400, then size duct at design velocity |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- System size(tons)
- Enter the system size in tons.
- Trunk velocity(FPM)
- Enter the trunk velocity in FPM.
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.
What do I need to enter into the Duct Size by Tonnage Calculator?
Just 2 values: system size, trunk velocity. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Duct Size by Tonnage Calculator work out the 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. It applies the formula CFM = tons x 400, then size duct at design velocity and shows the working so you can check each step by hand.
Is the Duct Size by Tonnage Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.