Duct Leakage Percent Calculator
Duct leakage rate as a percent of system airflow.
Enter your values
ⓘ Live results
Results
- 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
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.
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.
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.