Duct Leakage Calculator
CFM25 leakage against code limits.
Enter your values
ⓘ Live results
Results
- Leakage rate
- 6 CFM25/100 sq ft
- Code limit
- 4
- Verdict
- Fail — seal and retest
- Allowed total
- 80 CFM25
- Excess
- 40 CFM25
Quick answer
Duct leakage is reported as CFM at 25 Pa per 100 square feet of conditioned floor area.
leakage rate = CFM25 / (area / 100)
How to use the Duct Leakage Calculator
- 1Pressurise the duct system to 25 Pa with a duct blaster and record the flow.
- 2Enter conditioned floor area served by that system.
- 3Seal boots, plenums and joints with mastic, then retest.
Worked example
Using the values the calculator loads with:
Inputs
- Duct blaster result120 CFM25
- Conditioned area2000 sq ft
- Code limit4 CFM25/100 sq ft
Results
- Leakage rate6 CFM25/100 sq ft
- Code limit4
- VerdictFail — seal and retest
- Allowed total80 CFM25
- Excess40 CFM25
Frequently asked questions
Total or to outside?
Total leakage is easier to test; leakage to outside is the number that actually wastes energy.
Typical old system?
Untested existing ducts often leak 15-30% of system airflow.
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.