Calculator guide
Estimate duct cross-sectional area from airflow and a user-selected target velocity. Use the explicitly labelled inputs below; rates, dimensions, and costs are assumptions, not live quotes.
Formula or method
Required area (sq in) = airflow (CFM) ÷ selected velocity (ft/min) × 144. Equivalent round diameter = √(4 × area ÷ π). The displayed whole-inch round size and 2:1 rectangular dimensions round up to meet or exceed the calculated area.
How to use this calculator
- Set Airflow (CFM). The interface starts with 400; replace it with the value for your scenario.
- Set Target Air Velocity (ft/min). The interface starts with 700; replace it with the value for your scenario.
- Compare the result with the worked example, then vary one assumption at a time so you can see what changes.
Worked example
Enter Airflow = 400 CFM; Target Air Velocity = 700 ft/min. With those assumptions, the calculator gives Required Cross-Sectional Area: 82.3 sq in; Equivalent Round Diameter: 10.2 in; Rounded-Up Round Size: 11 in; Approx. 2:1 Rectangular Size: 14 × 7 in. These outputs are calculated from the same implementation as the interactive tool.
Understanding the result
In this example, Required Cross-Sectional Area is 82.3 sq in. Equivalent Round Diameter is 10.2 in. Verify measurements, manufacturer coverage, packaging, local code, and site conditions. Quantity and budget estimates do not certify structural or electrical safety.
Limitations and assumptions
- Verify measurements, manufacturer coverage, packaging, local code, and site conditions. Quantity and budget estimates do not certify structural or electrical safety.
- The worked example demonstrates the stated assumptions, not a recommendation about your personal circumstances.
Frequently asked questions
Which inputs should I check first?
For HVAC Duct Size Estimator, check Airflow, Target Air Velocity. Keep the displayed units and select options consistent; do not treat default values as recommendations.
Why might my result differ from another tool?
Compare the other tool's hvac duct size estimator method, input units, rounding and assumptions. A matching title does not guarantee that both tools answer the same question.
Does a linked reference certify this result?
No. A reference can explain a formula or provide background for HVAC Duct Size Estimator, but it does not endorse this website, validate personal inputs or establish professional suitability.
Sources
Educational and planning information only. Sources are attributed background or formula references, not endorsements. Verify consequential decisions with an appropriate qualified professional.