Construction & Materials
Ductwork Sizing Calculator
Determine round duct diameter or rectangular duct dimensions required for an airflow rate.
Your inputs
Change any value to update the result instantly.
Round duct diameter
10.2357 in
Required cross-section area
0.571 ft²
Nearest standard size
12 in
Equivalent rectangular (assume 8 in high)
10.3 in wide
Formula and working
D = √(4 × CFM ÷ (π × velocity × 60))
- 1
Collect the inputs
Airflow rate = 400 CFM; Design velocity = 700 ft/min
- 2
Apply the formula
D = √(4 × CFM ÷ (π × velocity × 60))
- 3
Result
Round duct diameter = 10.2357 in
Frequently asked questions
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
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