Air Balance Calculator
Compare design CFM against measured CFM per branch.
Reviewed by James Whitfield · How we check our maths
Your values
ⓘ Live results
Results
- Percent of design
- 78.7%
- Difference
- -32 CFM
- Action
- Open damper or check restriction
- Target band
- 135–165 CFM
Quick answer
A branch is considered balanced when measured airflow sits within 10% of its design value.
The formula
% of design = measured / design x 100
Worked example
Worked example
Using the values the calculator loads with.
Inputs
- Design airflow150 CFM
- Measured airflow118 CFM
Result
- Percent of design78.7%
- Difference-32 CFM
- ActionOpen damper or check restriction
- Target band135–165 CFM
How to use the Air Balance Calculator
- 1Record the design airflow for the branch from the duct drawing.
- 2Measure the delivered airflow with a flow hood or traverse.
- 3Adjust the balancing damper and re-measure.
Frequently asked questions
What tolerance is acceptable?
Most specifications allow plus or minus 10% on each branch and 5% on the system total.
Should I balance with a dirty filter?
No — fit a clean filter first, otherwise every reading shifts once it is changed.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error