Duct Leakage Calculator
CFM25 leakage against code limits.
Enter your values
Enter the duct blaster result in CFM25.
Enter the conditioned area in sq ft.
Enter the code limit in CFM25/100 sq ft.
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 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)
At a glance
| What it does | CFM25 leakage against code limits. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct blaster result, Conditioned area, Code limit |
| Main output | Leakage rate |
| Formula | leakage rate = CFM25 / (area / 100) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Duct blaster result(CFM25)
- Enter the duct blaster result in CFM25.
- Conditioned area(sq ft)
- Enter the conditioned area in sq ft.
- Code limit(CFM25/100 sq ft)
- Enter the code limit in CFM25/100 sq ft.
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.
What do I need to enter into the Duct Leakage Calculator?
Just 3 values: duct blaster result, conditioned area, code limit. 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 Calculator work out the answer?
Duct leakage is reported as CFM at 25 Pa per 100 square feet of conditioned floor area. It applies the formula leakage rate = CFM25 / (area / 100) and shows the working so you can check each step by hand.
Is the Duct Leakage 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.