How to Read a Duct Leakage Test Report: CFM25, Airflow and What the Numbers Mean
If you have a duct leakage test report in front of you, a few numbers tell you most of the story. The leakage test reports how many cubic feet of air per minute (CFM) your ducts leak when pressurized to 25 Pascals, written as CFM25. The airflow test reports how many CFM actually reach each room. As a rule of thumb, under 50 CFM25 is a well-sealed home system, 100–250 is leaky, and over 400 means most rooms are being shortchanged.
Key Takeaways
| Common Question | Short Answer |
|---|---|
| What is a good duct leakage test result? | For a single home system, under 50 CFM25 is well sealed and under 30 CFM25 is excellent. In MaxSeal’s data from 38 systems, the median after sealing was 50 CFM25. See the results study |
| What does CFM25 mean? | Cubic feet per minute of air leakage measured while the duct system is held at 25 Pascals of pressure, which is close to normal operating pressure. Duct sealing glossary |
| How do I turn leakage into a percentage? | Divide the leakage by the total air the system moves. For example, 561 CFM lost ÷ (561 + 432 delivered) = 56% of conditioned air lost. |
| How long does a duct leakage test take? | Usually 30–60 minutes per system. Room-by-room airflow readings add some time depending on the number of registers. Duct leakage test |
| How much does a duct leakage test cost? | At MaxSeal, a duct leakage and air volume test starts at $295. A Manual J load calculation with a full duct system diagnostic starts at $495. See our price list |
| Is a duct leakage test required in California? | When you replace HVAC equipment or alter ducts, Title 24 generally requires duct leakage testing with HERS verification. Check with your contractor or local building department. Title 24 explained |
| What’s the difference between total leakage and leakage to outside? | Total leakage measures every leak in the duct system. Leakage to outside counts only leaks into unconditioned space such as the attic, crawl space or garage. |
1. The 4 numbers on every report
| Number | What it means | What to look for |
|---|---|---|
| Test pressure (Pa) | The pressure the fan holds the ducts at, usually 25 Pa, close to normal operating pressure | Should say 25 Pa so results are comparable |
| Total leakage (CFM25) | Air escaping the whole duct system at 25 Pa | Lower is better. See the scale below |
| Leakage % | Leakage divided by total air the system moves | The share of conditioned air you’re paying for and losing |
| Register airflow (CFM) | Flow-hood reading at each supply vent | Weak rooms show up as low numbers. Compare room to room |
2. Leakage scale: how bad is your number?
| Total leakage (CFM25) | What it typically means for a single home system |
|---|---|
| Under 30 | Excellent, near-airtight |
| 30–50 | Well sealed (MaxSeal’s standard target) |
| 50–150 | Moderate leakage, some rooms affected |
| 150–400 | Leaky, with noticeable comfort and efficiency losses |
| 400+ | Severe. Many rooms are likely starved |
These bands are MaxSeal’s working guide for single residential systems, not a code standard. Code limits, such as California Title 24, are usually set as a percentage of system airflow.
3. How to read your report, step by step
Step 1: Find total leakage
Look for the CFM figure next to “25 Pa.” On a Retrotec gauge it appears as, for example, “561.5 CFM @ 25.0 Pa.”
Step 2: Turn it into a percentage
Divide the leakage by the total air the system moves (leakage plus air delivered to rooms). Example: 561 CFM lost ÷ (561 + 432 delivered) = 56% of conditioned air lost.
Step 3: Scan room-by-room airflow
Find the lowest numbers. In that same example, the kitchen got 10 CFM while the playroom got 68 CFM. That gap is why one room never feels right.
Step 4: Read the forecast or “after” column
A good report shows expected or measured airflow after sealing, room by room. Starved rooms should show the biggest percentage gains.
Step 5: Check the “recovered” math
Recovered leakage = starting leakage − target leakage. Example: 561 − 50 = 511 CFM, or 91% of the loss recovered.
Step 6: Understand runtime
If rooms get more air per minute, the system needs less time to deliver the same amount. Runtime reduction ≈ 1 − (airflow before ÷ airflow after). Example: 1 − (432 ÷ 575) = 25% less runtime for the same delivered air, in theory.
4. Example: a real two-system home (October 2026)
| Downstairs system | Upstairs system | |
|---|---|---|
| Total leakage | 561 CFM25 | 371 CFM25 |
| Share of conditioned air lost | 56% | 49% |
| Airflow delivered to rooms | 432 CFM | 385 CFM |
| Weakest room | Kitchen, 10 CFM | Master bath, 17–18 CFM per register |
| Target after sealing | < 50 CFM25 | < 50 CFM25 |
| Forecast airflow after sealing | 575 CFM (+33%) | 517 CFM (+34%) |
| Forecast runtime reduction | ~25% | ~26% |
Forecasts are estimates, not guarantees. Every system is different.
5. Total leakage vs. leakage to outside
- Total leakage measures every leak in the duct system, wherever it goes. It’s the number most sealing contractors report.
- Leakage to outside counts only leaks into unconditioned space such as the attic, crawl space or garage. It’s measured by also pressurizing the house with a blower door.
Both matter. Leaks to outside waste the most energy, and leaks inside walls still starve rooms and pull in dust.
Conclusion
A duct leakage report comes down to a few numbers: total leakage at 25 Pa, the share of conditioned air lost, and the airflow reaching each room. Read them together and you can see where your air is going and which rooms are losing out.
Use the leakage scale as a guide, look for the weakest rooms, and treat any forecast as an estimate. Then decide on sealing or repair based on what the numbers show.
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Related: System performance · Our results · Seal the leaks