Air Balance Calculator
Compare design CFM against measured CFM per branch.
Enter your values
Enter the design airflow in CFM.
Enter the measured airflow in CFM.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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.
% of design = measured / design x 100
At a glance
| What it does | Compare design CFM against measured CFM per branch. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Design airflow, Measured airflow |
| Main output | Percent of design |
| Formula | % of design = measured / design x 100 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Design airflow(CFM)
- Enter the design airflow in CFM.
- Measured airflow(CFM)
- Enter the measured airflow in CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Design airflow150 CFM
- Measured airflow118 CFM
Results
- Percent of design78.7%
- Difference-32 CFM
- ActionOpen damper or check restriction
- Target band135–165 CFM
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.
What do I need to enter into the Air Balance Calculator?
Just 2 values: design airflow, measured airflow. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Air Balance Calculator work out the answer?
A branch is considered balanced when measured airflow sits within 10% of its design value. It applies the formula % of design = measured / design x 100 and shows the working so you can check each step by hand.
Is the Air Balance Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.