Duct Leakage Percent Calculator
Duct leakage rate as a percent of system airflow.
Enter your values
Enter the measured leakage in CFM25.
Enter the total system cfm 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.
- Leakage percentage
- 3.8%
- Rating
- Excellent (meets tight code limits)
- Leakage in CFM
- 45 CFM
Quick answer
Duct leakage is reported as a percentage of total fan airflow; most energy codes require total leakage under 4-6% for new construction.
Leakage % = leakage CFM / total system CFM x 100
At a glance
| What it does | Duct leakage rate as a percent of system airflow. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Measured leakage, Total system CFM |
| Main output | Leakage percentage |
| Formula | Leakage % = leakage CFM / total system CFM x 100 |
| Cost | Free — no sign-up, no download |
How to use the Duct Leakage Percent Calculator
- 1Get the leakage CFM from a duct blaster test at 25 Pa (CFM25).
- 2Divide by the system's total design or measured airflow.
- 3Compare against your energy code's target, commonly 4% to total or 6 CFM per 100 sq ft to outside.
Inputs explained
- Measured leakage(CFM25)
- Enter the measured leakage in CFM25.
- Total system CFM(CFM)
- Enter the total system cfm in CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Measured leakage45 CFM25
- Total system CFM1200 CFM
Results
- Leakage percentage3.8%
- RatingExcellent (meets tight code limits)
- Leakage in CFM45 CFM
Frequently asked questions
What is CFM25?
It's the airflow through duct leaks measured at a standardized 25 Pascal test pressure, used to normalize results between systems.
How is leakage reduced?
Mastic sealant on all joints, seams and boots, plus sealing the air handler cabinet itself.
What do I need to enter into the Duct Leakage Percent Calculator?
Just 2 values: measured leakage, total system cfm. 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 Duct Leakage Percent Calculator work out the answer?
Duct leakage is reported as a percentage of total fan airflow; most energy codes require total leakage under 4-6% for new construction. It applies the formula Leakage % = leakage CFM / total system CFM x 100 and shows the working so you can check each step by hand.
Is the Duct Leakage Percent 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.