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CFM per ton: the chart, and the duct size that carries it

The rule is 400 CFM per ton of cooling, so a 3 ton system moves 1,200 CFM. The Reference tab has the table, and putting 1,200 into the ductulator gives a 14.6 in exact duct, a 16 in round.

2 min read · Updated 2026-10-09

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What does the 400 CFM per ton table say?

Find it under Reference > Airflow per ton of cooling. DuctCalc lists seven common sizes with the kW equivalent and the metric airflow.

400 CFM per ton, as listed in the app
TonskWCFML/s
1.55.3600283
27.0800378
2.58.81,000472
310.61,200566
3.512.31,400661
414.11,600755
517.62,000944

The footer adds the exceptions: dry climates run up to 450 CFM per ton, and humid climates or heat pumps in heating often drop to 350. The table is fixed at 400, so for 450 or 350 you multiply yourself and type the CFM.

From tons to a duct size

  1. Look up the airflow: 3 tons is 1,200 CFM.
  2. Go to the Ductulator tab, Size a duct, and type 1200 in Airflow.
  3. With Friction rate at 0.1, Required round duct reads 14.6 in at 1,028 fpm.
  4. The stock line says 16 in at 859 fpm and 0.064 in.wg/100ft.

Look at the badge here. The exact 14.6 in would be above the 1,000 fpm maximum for a residential supply main, but the app rates the stock size, and 16 in at 859 fpm is Within recommended range. A 14 in duct is the one that bites: 1,123 fpm and 0.124. An 18 in is quiet at 679 fpm and 0.036, and costs more metal.

If the trunk will be rectangular, the Rectangular equivalents card starts with 14 x 14 and 16 x 12, and goes on through 20 x 10 and 26 x 8 to 36 x 6 for the same 1,200 CFM. The last one gets the 6:1 chip.

What this is not

It is a trunk size for the equipment airflow. Splitting the air to rooms needs the room loads, and each branch is then a separate calculation with its own CFM. Run Size a duct once per branch and Name this duct run in History to keep them straight.

Questions and answers

How many CFM is a 3 ton AC?
1,200 CFM at the 400 CFM per ton rule, which is 566 L/s. Dry climates use up to 450 per ton and humid ones or heat pumps in heating often 350, so the equipment's own data wins.
Can the app calculate tonnage from a house?
No. There is no load calculation in DuctCalc. It starts from an airflow you already have.
What duct size for 2 tons?
2 tons is 800 CFM. At 0.1 in.wg/100ft in galvanized steel the exact duct is 12.6 in and the stock size is 14 in, running at 748 fpm. The first rectangular pair is 12 x 12, then 14 x 10.

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