Duct Size Calculator (CFM)
Enter your system's tons and get the total airflow it moves, the round trunk size that carries it, the branch size per register and how many supply registers to expect.
Duct size
C09Scenario A
Main trunk diameter (round)
12 outlets on 6" branches. A simplified Manual D; it doesn't replace a static-pressure calculation.
- Total airflow (400 CFM/ton)
- 1,200
- Trunk diameter (round, typical ductulator table)
- 16"
- Branch per outlet (~100 CFM)
- 6"
- Suggested number of outlets
- 12
Estimate. Formula and sources are under “How it works”. Change any input; the result updates as you type.
Formula
How it works
- Total airflow = tons × 400 CFM
- Main trunk = the smallest round duct that carries that airflow at 0.1 inch of water column friction per 100 ft:
| Round duct | 6" | 7" | 8" | 9" | 10" | 12" | 14" | 16" | 18" | 20" |
|---|---|---|---|---|---|---|---|---|---|---|
| Up to (CFM) | 100 | 150 | 200 | 300 | 400 | 600 | 800 | 1,200 | 1,600 | 2,000 |
- Branches = 6" round, about 100 CFM each
- Registers = total CFM ÷ 100, rounded down
This is a simplified version of ACCA Manual D, meant to help you read a quote. It doesn't calculate static pressure, duct length, fittings or flex-duct losses, which a real duct design needs.
Sources: 400 CFM per ton and duct sizing method: ACCA Manual D. Diameters: standard duct calculator (ductulator) chart at 0.1 in. wc per 100 ft.
Worked example
A worked example
A 2-ton system moves 2 × 400 = 800 CFM. On the friction chart, the smallest round trunk that carries 800 CFM is 14". Each supply branch is 6", about 100 CFM, so the system feeds about 8 registers.
Questions
FAQ
How many CFM per ton?
400 CFM per ton is the design standard for most systems. A 3-ton system moves 1,200 CFM, which needs a 16" round trunk and about 12 supply registers; a 5-ton system moves 2,000 CFM on a 20" trunk with about 20.
What size duct for a 4-ton system?
1,600 CFM: an 18" round trunk and about 16 registers on 6" branches.
Round vs rectangular duct equivalent?
This calculator gives round sizes. A rectangular duct carries the same airflow when its equivalent diameter matches; your installer's duct calculator converts it. Don't size a rectangular duct by matching the width to the round diameter.
Does flex duct change the size?
Yes. Flex duct has more friction than smooth metal, especially when it's stretched loosely or bent, so it usually needs a larger size for the same airflow. This calculator assumes smooth round duct.
My installer's numbers are different. Who's right?
Probably the installer, if they ran Manual D with your actual duct lengths and fittings. Use these numbers to ask questions: a 2.5-ton system at 1,000 CFM on a 10" trunk would be undersized by this chart (it calls for 16").
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