Air Balancing Percent Calculator
Compare measured airflow to design targets.
Enter your values
ⓘ Live results
Results
- Balance percentage
- 92%
- Difference
- -8 CFM
- Status
- Within balance tolerance
Quick answer
A register or branch is considered balanced when measured airflow is within about 10% of its design CFM.
Balance % = measured CFM / design CFM x 100
How to use the Air Balancing Percent Calculator
- 1Measure actual CFM at the register with a flow hood or anemometer and area.
- 2Compare it against the design CFM from the duct design worksheet.
- 3Adjust balancing dampers until every register is within about 10% of its target.
Worked example
Using the values the calculator loads with:
Inputs
- Measured CFM92 CFM
- Design CFM100 CFM
Results
- Balance percentage92%
- Difference-8 CFM
- StatusWithin balance tolerance
Frequently asked questions
Why allow a 10% tolerance?
Perfect balance is impractical to measure and hold; 10% is the accepted industry standard for a well-balanced system.
What if I can't get enough airflow no matter what?
The branch or trunk feeding it may be undersized — check duct sizing before assuming it's just a damper issue.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.