Use the Duct Calculator
Add duct sizes, compare airflow estimates, and understand the undersized warning.
Open Tools → Duct Calculator to estimate airflow for the duct sizes you enter. The result is an estimate, not measured airflow or a duct design.
- Choose Flex, Round, or Rect to match the duct.
- Enter its diameter or rectangular dimensions in inches.
- Tap Add to include the run in the total. Repeat for the other runs.
- If you want a comparison, select Set System Size and choose the equipment capacity in tons.
- Review the total. Use the delete icon to remove a run you entered by mistake.
What does May be undersized mean?
The entered runs have an estimated total below the calculator’s target for the chosen system size. Check that you entered all relevant runs and the correct dimensions.
This warning does not prove the ducts caused the problem. Compare with field airflow and pressure measurements and the manufacturer’s requirements before changing the system.
The calculator does not replace airflow balancing or professional duct design. For other utilities, see field tools.